Gasification of Sewage Sludge—A Review

Abstract

The increasing amount of sewage sludge produced demands new methods of its management to minimize socioeconomic and environmental problems related to its current treatment. An effective solution may be the thermochemical conversion of sewage sludge through gasification. First, the most known sewage sludge gasification processes are presented along with the challenges that they face. Then the detailed characteristics of sewage sludge are discussed from the point of view of its use in the gasification process, as well as research on the kinetics of gasification of sewage sludge char using various models. As scientific reports on sewage sludge gasification focus on the influence of process parameters on gas yield and composition (especially H₂ and tar content), the main part of the work is devoted to the discussion on the influence of temperature, type, and amount of the gasifying agent and the presence of a catalyst on these parameters. Moreover, the co-gasification of sewage sludge as well as advanced gasification methods, i.e., supercritical water gasification and plasma gasification, are analyzed. Finally, the possibilities of utilization of sewage sludge gasification process by-products were discussed and the impact of the process on the environment was assessed. The review concludes with indications of directions for further research.

1. Introduction

The rapid rate of urbanization and industrialization, inherently related to the increasing consumption of energy and fuels as well as the growing amount of waste, means that currently, many studies focus on the valorization of waste to effectively use its potential as energy carriers or useful products [1]. This is especially important in the case of cumbersome waste, such as sewage sludge, which is potentially hazardous to people and the environment and impossible to reduce. Sewage sludge, i.e., the by-product of wastewater treatment plant operations, in its raw form resembles slurry with significant water content (about 99%) that has to be reduced before management (disposal or energy recovery) [2]. However, in its dry form (i.e., with a dry matter content above 85%) [3], sewage sludge is typically characterized by high organic matter content (over 50 wt.%) with the rest being inorganic components [4,5,6]; it thus has the potential to be used in waste-to-energy processes [7]. Every year, the amount of sewage sludge produced globally increases due to the growth of the population. (It is estimated that by 2050, the world will have more than 9 billion people, whereas in 2023, it had about 8 billion people [8]). Figure 1 shows that the total annual production of sewage sludge (in millions of tons) is especially high in China (11.2 Mt), followed by the cumulative production in 27 countries of the European Union (9.25 Mt), the US (4.96 Mt), India (4.36 Mt), and Japan (2.2 Mt). Among the European Union countries, the most sewage sludge in 2023 was produced in Poland, and it was almost twice as much as in the second country in terms of sewage sludge production—Romania [9]. When it comes to total global production of sewage sludge, there is no agreement in the literature; therefore, this value needs to be estimated based on the available statistical data. Feng et al. [10] undertook this task by making the assumptions that neighboring regions have the same sewage sludge production rates as regions for which such information is available and that wastewater produced around the globe was treated to a similar level as in the EU. The authors estimated that about 160 Mt/year of dry sewage sludge is currently produced globally. Such high values mean that it is necessary to look for effective and environmentally friendly methods of managing this waste, especially in Asian countries.

Currently, the most common (conventional) methods of sewage sludge management are (i) land application; (ii) landfilling; and (iii) incineration (the share of individual development methods depends on the level of development of the country) [11]. Because sewage sludge contains macronutrients (P, N, K, Mg, Ca) and micronutrients (Fe, Mn, Cu, Zn) [12,13], it is often used in agriculture as an organic fertilizer [14]. This application is inexpensive since sewage sludge is applied as a liquid suspension or dewatered cake, and therefore costs of dewatering are reduced [15]. However, sewage sludge also contains harmful substances such as pathogens, heavy metals, and polycyclic aromatic hydrocarbons (PAH) at different concentrations [16,17], which may gradually accumulate within the soil [18,19], causing serious environmental and health risks. Therefore, Zhang et al. [20] suggested that sewage sludge may be used as a fertilizer for energy crops rather than traditional food crops. In turn, landfilling is a way of disposing of sewage sludge, for which it is difficult to find any advantages. There are, however, several drawbacks, such as the threat of groundwater contamination [20], methane emissions [21], or occupying large areas of land [22]. Finally, incineration enables a significant reduction of sewage sludge volume and destruction of toxic compounds [23], as well as eliminating problems related to odor or land occupation [24]. Nevertheless, it is an expensive choice due to the need to dry the sludge before the process, introduce complex extensive flue gas cleaning equipment, and manage post-process ash containing toxic elements [22,25]. A more interesting conventional method of sewage sludge management is anaerobic digestion, i.e., biological decomposition of organic matter into valuable biogas and a nutrient digestate [20]. However, this process is complex (consisting of several stages, i.e., hydrolysis, acidogenesis, acetogenesis, and methanogenesis), time-consuming (up to approx. 20 days), and requires specific temperature and pH conditions. Moreover, even under optimal conditions, only a small part of the organic matter is converted into biogas [26,27]. Consequently, anaerobic digestion is instead used to stabilize the sludge [20].

Thus, the presented conventional methods facilitate partial nutrient and energy recovery (see Figure 2), but they cannot use the sewage sludge potential to the fullest [28].

Much more effective ways of using sewage sludge are alternative methods (see Figure 2), such as pyrolysis, gasification, or liquefaction (which is still in the laboratory research phase [29]), which enable its conversion into highly valued products. Thermochemical conversion processes, such as pyrolysis or gasification, require thermal drying of the sludge—it is assumed that the moisture content in sewage sludge should be less than 40% so that it can be converted without additional support [3]. Moreover, as the drying process progresses (at a concentration of 40–50% organic dry matter), the sludge turns into the so-called “sticky phase”, which causes problems since it deposits on the equipment walls, resulting in lowering the drying efficiency [30]. To prevent this, sewage sludge subjected to drying should be mixed with the completely dried sludge or with lignin, which minimizes the sticking and agglomeration effect [30,31]. Finally, the drying process is expensive and energy intensive. Despite that, due to the advantages of alternative methods, recently growing interest in these methods, especially pyrolysis, for sewage sludge management is observed [32,33,34]. Literature devotes much less attention to gasification, which has several advantages over pyrolysis [35], e.g., pyrolysis requires comprehensive knowledge of the physicochemical, thermal, and kinetic decomposition characteristics of feedstock [36]. The gasification process, on the other hand, is mature, tolerant of diverse feedstocks, and practical on small scales (which is especially important from the viewpoint of gasifier installations in wastewater treatment plants) [37]. Gasification itself is a thermochemical method to produce syngas by partial oxidation of biodegradable material in an O₂-restricted environment and under high temperatures. The gasifying agent (such as air, O₂, steam, CO_2, or their mixture) greatly affects the composition of the resulting gas; however, its main components are H₂ and carbon monoxide, and it may also contain carbon dioxide, methane, tar, water, and other light hydrocarbons [38]. Such gas has numerous applications, such as power generation, H₂ production (through water-gas shift reaction followed by pressure swing adsorption), and conversion into liquid fuels by the Fischer–Tropsch synthesis [39,40]. The advantages of the gasification process have led to its consistent development in the context of sewage sludge management. Consequently, related supercritical processes, especially supercritical water gasification (SCWG), are becoming increasingly popular [28]. The greatest advantage of supercritical processes is that they do not require drying of the raw material [41]. Nevertheless, very promising supercritical processes are still in the research phase, and the main problem in their application on a larger scale is related to the high investment costs of advanced equipment designed to work in drastic conditions [29].

At present, the most promising approach to sewage sludge management seems to be gasification. This process is usually carried out in fixed-bed reactors [42], fluidized-bed reactors [43], rotary kilns [44], and auger and plasma reactors [45]. Among them, the oldest solution is fixed-bed reactors, in which problems with slag accumulation, pressure drops, and the presence of gas impurities (in the case of a downdraft gasifier) may occur. Fluidized-bed reactors are characterized by good heat transfer rates and production of clean syngas and enable catalyst regeneration. However, high flow rates are required to ensure fluidization, which makes handling and maintenance of gasifiers difficult. In turn, rotary wedges are characterized by lower heat transfer rates (which results from the relatively low rate of particle movement), but their greatest disadvantage is ineffective sealing. Finally, auger and plasma reactors enable obtaining gas characterized by low tar content, which translates into its high quality. Unfortunately, in the case of plasma reactors, high energy consumption makes their use on a commercial scale extremely difficult. Gao et al. [46] presented an interesting comparison of gasification reactors from the point of view of their technological strength and market competitiveness. The results of this comparison showed that the auger reactor is deemed the most attractive, followed by rotary kilns and plasma. The other reactors have less technological strength and market competitiveness for sewage sludge. Nevertheless, on a commercial/pilot scale, fixed-bed and fluidized-bed reactors dominate. Moreover, currently, gasification/co-gasification of sewage sludge is carried out mainly for heat and power production for the electricity consumption of their plants as well as for drying sewage sludge operations. The amount of information in the literature on commercial sewage sludge gasification plants is quite limited [47]; however, available information about selected plants is presented below in

Table 1. Selected commercial gasification facilities currently processing sewage sludge.

Location	Reactor	Fuel	Other Information	Ref.
Denmark, Kalundborg	LT-CFBR	Blended sewage sludge and local residues	Temperature in reactor: 730 °C; Gas temperature: ~650 °C; Capacity: 6 MWth; Ash is used for fertilizer field tests.	[43,48]
Taiwan, Tao-Yuan	BFBR	Blended sewage sludge, paper-mill sludge	Temperature in reactor: 900 °C; Gas temperature: 816 and 858 °C; Capacity: 3 MWth.	[49]
USA, City of Lebanon	Downdraft-bed reactor	Blended sewage sludge and local waste (waste wood, scrap tires)	Temperature in reactor: 1250 °F; Gas combusted in thermal oxidizer; Production of 420 kW of electricity.	[50,51]

LT-CFBR—low temperature-circulating fluidized-bed reactor; BFBR—bubbling fluidized-bed reactor.

.

A schematic diagram of the commercial sewage sludge gasification plan (on the example of an installation in Taiwan, Tao-Yuan) is shown in Figure 3. As can be seen, the plant consists of a feed zone, a bubbling fluidized-bed gasifier, a combustion zone, a heat exchanger, and pollution control devices. As mentioned, the water vapor generated by the heat from the combustion of synthesis gas is used to evaporate moisture from sewage sludge. In turn, the flue gas from the boiler passes through a pulse-jet fabric filter to capture particulate matter and is released into the atmosphere [49].

Some key challenges hinder the wider application of sewage sludge gasification. Firstly, there is high moisture content in sewage sludge, its removal is extremely costly and releases a large amount of CO₂ into the atmosphere (which contributes to the greenhouse effect), and drying is required in the case of conventional gasification. In addition, the high moisture and ash content in this material translates into a final product of relatively low quality (low LHV, CCE) and high content of impurities, especially tar, which creates operational problems and hinders further direct use of the gas. In recent years, a number of studies have been devoted to the issue of selecting the optimal conditions and an effective catalyst (or a component for co-gasification). These studies are promising because they indicate a significant improvement in gas calorific value, increased hydrogen yield (even up to ~50%), reduction of tar content (even by approx. ~95%), and a certain degree of reduction of S-compounds and N-compounds [46]. Such a significant improvement in gas quality may translate into attempts to use syngas from sewage sludge to produce liquid fuels by the Fischer–Tropsch synthesis process and various chemicals such as methanol, ethanol, gasoline, and dimethyl. (The gas would first have to be cleaned, which is also a key issue.) However, selecting the appropriate catalyst is very problematic, and currently pilot, demonstration, and commercial plants in various countries have not applied any catalyst (except calcium oxide) [46]. Finally, the management of ash may also be a challenge for a circular economy, due to environmental and social problems associated with sewage sludge.

Taking the above into account, the current research on the gasification of sewage sludge mainly concerns the selection of optimal conditions for the process (including the selection of a catalyst or component for the co-gasification process) to improve the quality of the obtained gas (i.e., increase the H₂ content while reducing the amount of tar in the gas). Many studies also focus on the analysis of the possibilities of recovering phosphorus from ash. More innovative versions of the gasification process are also being analyzed, i.e., supercritical water gasification and plasma gasification. As mentioned, a major advantage of supercritical water gasification is the lack of the need to dry the sludge, which is very demanding, but there are currently no detailed studies on further heat recovery strategies or solar energy applications (for the drying process) integrated with sludge gasification units.

2. Gasification of Sewage Sludge

To understand the nature of sewage sludge gasification, it is important to know the specific stages that occur during the process. In general, gasification may be divided into three stages [46,52], as can be seen in Figure 4:

(1)

Dehydration—moisture evaporation occurring at 70–200 °C

(2)

Pyrolysis—thermal decomposition of sewage sludge occurring between 350–600 °C

(3)

Char gasification (800–1200 °C)—conversion of char into CO and H₂ due to carbon-steam gasification reaction (C + H₂O → CO + H₂), Bouduard reaction (C + CO₂ → 2 CO), hydrogasification (C + H₂ → CH₄), and partial oxidation (C + 0.5 O₂ → CO). This stage is the slowest and limits the rate of the entire process.

According to the Sikarwar et al. [53], the process may be described by general reaction 1.

SS → CO(g) + H₂(g) + CO₂(g) + CH₄(g) + Tar(l) + H₂O(l) + H₂S(g) + NH₃(g) + C(s) + trace species

(1)

2.1. Sewage Sludge as Feedstock for Gasification

Almost any carbonaceous material can be used in the gasification process; however, material features (i.e., proximate and ultimate composition, the composition of the inorganic substance, and the morphology (specific surface area and the content of surface functional groups)) significantly affect the course of the process.

2.1.1. Proximate and Ultimate Composition of Sewage Sludge

Taking into account the proximate and ultimate analysis, attention should be paid primarily to the content of the following: the C element (which determines the energy content in the syngas); the O and volatile matter content (which affects the thermal stability of the material); and the moisture and ash content (which leads to reduced energy conversion efficiency and may cause equipment problems). Figure 5 presents the proximate (a) and ultimate (b) analyses of sewage sludge reviewed in this study [54,55,56,57,58,59,60,61,62].

As can be seen in Figure 5, the composition of the wastewater sewage sludge across analyzed studies differs, which indicates that these are wastes of high variability and heterogeneity, and therefore their suitability for the gasification process cannot be assessed. However, reviewed sewage sludge has a considerable amount of carbon content (typically between 14.7–56.5%), which translates into rather high energy content (reported heating values ranged from 8.9 to 15.7 MJ/kg), which suggests that gas formed during gasification will be suitable for energy production. The second largest elemental component is O₂, the content of which varies within a wide range from 10.4 to 53%, and a particularly high amount of it may deteriorate the syngas heating value [46]. In turn, the nitrogen content is more uniform (2.1–8.4%) and higher than in biomass or coal [63]. This is due to the undigested proteins and microorganisms in the wastewater stream that contain a large number of organic macromolecules linked by peptide or amide bonds (CONH) [56,64]. In turn, from the point of view of proximate analysis, the dominant component is volatile matter (33.4–66.8%), which indicates the high reactivity of sewage sludge but is also the reason for in-depth analysis of this material in pyrolysis processes [32,33,34]. A characteristic feature of the sewage sludge is also high ash content (ranging between 16.0 and 57.1%), which usually is considered ballast since it may create problems for process efficiency [64]; however, in the case of gasification, the optimal amount of ash with a favorable composition may have a catalytic effect on the process [65]. Finally, the most diverse component in terms of quantity is fixed carbon (0.8–43.4%), which is especially important in gasification, since it reflects the amount of char that will undergo gasification reactions. If the fixed carbon content is at the lower limit, there is essentially no material for the gasification reactions, and such sewage sludge will be more suitable for the pyrolysis process. To sum up, sewage sludge can be a good feedstock for gasification, as long as it contains significant contents of element C and fixed carbon and a certain amount of ash with a favorable composition.

2.1.2. Composition of the Inorganic Substance of Sewage Sludge

In sewage sludge gasification, its inorganic substance, which is transformed into ash during thermochemical conversion, may have a beneficial effect on the process if it contains a high amount of catalytically active compounds, i.e., basic metal compounds such as K, Na, Mg, Ca, or Fe, wherein the activity of the compounds of alkali followed by alkaline earth metals is higher than that of transition metals [66,67]. At the same time, ash should contain a limited amount of acidic compounds, known as process inhibitors (Si- and Al-compounds) [68]. A review of the literature (see

Table 2. The composition of ash and content of heavy metals in sewage sludge reviewed in this study.

Ash Composition (wt.%)		Heavy Metals Content (mg/kg)
SiO2	14.4–36.19	As	5.6–56.1
Al2O3	4.02–14.9	Ba	41.5–1300
Fe2O3	5.43–15.6	Cd	0.83 ± 0.06
TiO2	0.58–1.07	Cr	18.6 ± 2.2
CaO	4.73–49.94	Pb	4.0–429.8
MgO	1.4–6.33	Hg	0.1–1.1
K2O	0.42–2.84	Mo	1.7–75
Na2O	0.2–1.99	Ni	8.6–420
MnO	0.003–0.39	Se	2
P2O5	0.11–26.8	Zn	0.0–7500
SO3	1.3–13.1	Cu	75.8 ± 7.0

[69,70,71,72,73,74,75,76]) shows that ashes from sewage sludge contain catalytically active compounds, of which Ca-compounds predominate (constituting in some cases even ~50 wt.%), followed by Fe-compounds. The ashes also contain compounds of the highest catalytic activity (Na, K), although in much smaller amounts (usually do not exceed ~3%). However, this does not exclude their high effectiveness in the process, because literature studies show that the addition of even low amounts of Na and K can improve the process and the quality of the obtained gas, and the more additive, the greater the effect (this applies to the use of K- and Na-based catalysts in the form of one-component and multi-component catalysts [77,78]). When it comes to inhibitory compounds, the main importance in the case of sewage sludge is Si-compounds—according to the data, they constitute 14–36%, while the Al-compounds content is several percent. However, in the case of the ashes reviewed in this study, usually the higher the Si-compound content (at the upper limit of the range), the less Al-compound was in the ash. Therefore, in an extremely unfavorable ash composition, the inhibitory components do not exceed ~40%, which, given the favorable composition of the rest of the ash, indicates that a positive impact of the ash on sewage sludge gasification can be expected.

Another very important issue when assessing the possibility of using sewage sludge in the gasification process is the fact that it contains significant amounts of inorganic contaminants, i.e., heavy metals, such as zinc, copper, chromium, nickel, lead, and cadmium (see Table 2 [6,46,79,80,81]. Literature reports indicate that during the gasification process, due to the reductive atmosphere, the heavy metals migrate from the fuel to the ash phase [37,46,82,83]. In addition, the concentrations of some heavy metals (especially Zn, Pb, Ni, Cu, Cr, and Cd) increase due to the cumulative effects. According to Li et al., who analyzed the content of heavy metals in ashes using a sequential extraction scheme, this increase was at the following levels: cadmium (from 0.93 to 1.67 mg/kg), chromium (from 80.82 to 247.95 mg/kg), copper (from 580.36 to 922.14 mg/kg), lead (from 78.27 to 125.09 mg/kg), and zinc (from 402.09 to 637.50 mg/kg) [82]. The presence of heavy metals may restrict the use of gasification ash in agriculture and land applications due to danger to the natural environment; however, this is not the rule. Ecotoxicological analyses of gasification ashes showed that most of the heavy metals migrated toward the solid phase, but their bioavailability and leachable toxicity were much lesser (the exception is Cd and Zn) [37,82,83]. This means that the gasification process may enable the reduction of heavy metal release into the environment and control of the form in which these metals occur.

2.1.3. Morphology and Chemical Structure of Sewage Sludge

Specific surface area and the number and type of surface functional groups are further parameters taken into account when assessing the suitability of sewage sludge for thermochemical conversion processes. When it comes to sewage sludge, there are not many reports regarding its specific surface area, and those that do exist indicate very low S_BET values (see

Table 3. The specific surface area and typical functional group of sewage sludge reviewed in this study. Reproduced from [91].

Specific surface area
SBET (m2/g)	~1–18.2
Surface functional groups
Wavelenght (cm−1)	Band assigned	Wavelength (cm−1)	Band assigned
630–726	H2O rocking, C-H bending of alcohol, hydrocarbons, aromatic groups	1450	aliphatic C–H deformation; C–H bond vibration in saccharides, N–H group in amides, and C–H in alkenes
974–1028	C-N stretching of amine	1535	stretching vibration of C–N; deformation vibration N–H of the peptidic bond of proteins; cell wall of G+ and G– bacteria
1030–1080	C–O stretching of polysaccharides or polysaccharide substances	1540	asymmetric stretching of C=O in carboxylic groups; N–H bending in amide or amino group
1030	Si–O stretching in the mineral phase of the sludge (silicate impurities/clay minerals)	1540–1520	NH2 deformation of amide
1060	C–O bond stretching vibration in glycerol	1575	COO– in carboxylic functional groups
1160	C–O–C stretching at the glycosidic linkages	1634	C=O groups of carboxylic acids; C=C in alkenes; O–H group (adsorbed water)
1230	C–N stretching of amide II	1650–1590	N–H bend in amide (I)
1234	cell wall of G+ and G– bacteria	1690–1630	C=O stretch in amide
1235, 1230	vibration of C=O in fats and carboxylic acids	1730, 1720	C=O in carboxylic acids and ketonic carbonyls
1245	deforming vibration of NH+ in peptides and proteins	2008–2258	C–O stretching
1338–1397	C-H bending in aromatic and aliphatic hydrocarbons	2265–2413	C=O asymmetric stretching
1384, 1380	N=O in nitrates	2641–2770	C-H stretching
1410	CH2 in polyalcohol	2925, 2855	aliphatic methylene groups in fats and lipids
1419	vibration of C=O group of carboxylates and carboxylic acids; cell wall of G+ and G– bacteria	3000–2800	aliphatic C–H stretching in saccharides, polyalcohols fats and lipids
1445–1380	deforming skeletal vibration of C–H in saccharides	3300–2800	N–H in amines, proteins, peptides
1440	CH2 deformation in fats	3600–3200	O–H group in polymeric compounds (polysaccharides, phenols, etc.) and water

[84,85,86,87]) since sewage sludge is practically non-porous. This is because this parameter does not have a significant impact on the stage preceding the actual gasification, i.e., the pyrolysis stage. The specific surface area of the char produced during pyrolysis is much more important, as it will be the place where the gasification reaction takes place. In turn, the presence of functional groups (especially O₂-containing groups like O–H, C–O, and C=O) certainly favors the pyrolysis stage and will additionally affect the reactivity of the resulting char (a greater number of O₂-containing molecules translates into a greater number of active sites [88]. The specific surface area and typical surface functional groups [89,90,91] of sewage sludge are summarized in Table 3.

2.1.4. Comparison of Sewage Sludge with Its Chars

To reflect the industrial conditions of sewage sludge gasification as accurately as possible, the research should assume the use of sewage sludge in the process, not its char. However, to illustrate what material undergoes the appropriate gasification reactions during the process, this section compares the properties of sewage sludge with its chars obtained under various temperature conditions (fast pyrolysis; final temperature: 300, 400, 500, 600 °C; atmosphere: 1000 mL min⁻¹ flow rate of purified nitrogen (99.999%)) [85]. Indeed, different pyrolysis conditions or a different input material will result in char with slightly different properties, but

Table 4. Comparison of sewage sludge with its chars produced at various temperatures. Reproduced from [85].

	Sewage Sludge	Chars
Content of:		300 °C	400 °C	500 °C	600 °C	700 °C
A (wt.%)	55.7	65.8	75.5	80.6	83.8	86.8
VM (wt.%)	39.7	27.4	16.0	10.2	8.6	5.5
FC (wt.%)	7.7	4.6	6.8	8.5	9.2	7.6
SBET (m2/g)	11.85	14.37	22.68	24.53	26.66	26.70
K (g/kg)	7.47	7.47	8.99	10.1	13.3	16.6
Mg (g/kg)	5.62	6.19	6.96	7.47	7.86	8.06
Ca (g/kg)	17.4	20.6	22.7	23.9	24.0	25.8
Fe (g/kg)	30	34.5	38.4	40.8	41.7	43.1
Pb (mg/kg)	3740	4410	4900	5120	5250	5200
Zn (mg/kg)	735	875	986	1040	1090	1090
Ni (mg/kg)	72.4	86.3	95.4	97.7	101	103
Cd (mg/kg)	169	197	225	235	229	123
As (mg/kg)	26	27	31	32	35	37
Cu (mg/kg)	172	195	213	215	209	227
Cr (mg/kg)	100	105	118	116	106	103

A—ash, VM—volatile matter, FC—fixed carbon.

indicates general trends in changes that occur during the thermal decomposition of sewage sludge, which is also confirmed by other scientific works [86,92,93].

As shown in Table 4, the sewage sludge is characterized by lower ash content and higher volatile matter content compared to its char, and the difference is greater the higher the pyrolysis temperature. In the case of fixed carbon, there is no clear trend; however, analyzing the temperature range of 300–600 °C, it can be seen that the char formed at the lowest temperature was characterized by the lowest FC content, approximately 40% lower than the sewage sludge. However, with the increase in the pyrolysis temperature, the FC content in the chars increased to such an extent that the chars formed at 500 and 600 °C had more FC compared to raw material. Thus, among the results of proximate composition, the increase in FC (in the case of chars formed at high temperatures) and the significant increase in ash content may affect the gasification process. (What effect this will have will depend on the composition of the ash.) Next, the contents of K, Mg, Ca, and Fe were higher in chars compared to sewage sludge and increased with the rising pyrolysis temperature (indicating that these micronutrients were enriched in the chars [94]). From the point of view of the catalytic properties of the inorganic substance, the increase in the K content is particularly interesting (since its mobility and catalytic effect are higher than most of the other metals present in the ash), which was quite significant—from 7.47 mg/kg in sewage sludge to 16.6 mg/kg in char produced at 700 °C. In the case of sample morphology, it is generally visible that the specific surface area of chars was higher than that of sewage sludge and increased gradually from 14.37 m²/g at 300 °C to 26.70 m²/g at 700 °C. Thus, thermal decomposition as well as an increase in temperature during this process increases the surface area available for the gasification reaction. Moreover, according to Yuan et al. [95] and Fuertes et al. [96], the chars produced at higher pyrolysis temperatures (above 400 °C) were alkaline due to the decomposition of organic acid and carbonate as well as because the alkaline organic anion contained in chars becomes stronger at such temperatures. The alkaline character, in turn, is desirable from the point of view of char conversion in the gasification reactions. Finally, the content of all analyzed heavy metals was higher in char than in sewage sludge (the exception is the char formed at 700 °C, where the cadmium content was lower than in the starting material), and elevating pyrolysis temperature generally intensified this enrichment. This is due to the better thermostability of heavy metal compounds in reductive pyrolytic conditions compared to other components of sewage sludge [97]. Taking into account the above observations and the fact that the differences between raw sewage sludge and its chars are also confirmed by other literature reports [86,92,93], it can be concluded that chars made from sewage sludge have better properties compared to input sewage sludge and are a suitable material for gasification reactions.

2.2. Kinetics of Sewage Sludge Gasification

Kinetic analysis of sewage sludge gasification is performed to provide the theoretical basis of the process. Usually, to analyze the kinetics of the process, the TGA analytical method is used, as it allows specific stages of thermal conversion to be easily distinguished. Based on that, the kinetics parameters can be determined assuming single separate reactions for the different stages [98]. The kinetics of the sewage sludge gasification process is the subject of few scientific works [65,99,100,101,102,103,104,105,106], but two approaches can be distinguished among published studies. As the essential stage in the gasification process is the char gasification (and this stage is the subject of kinetic analyses), the research can be divided into (1) in-situ—when the char formed as a result of heating (in an O₂-free/O² limited atmospheres) is directly gasified during one measurement, and (2) ex-situ—when the char has been previously prepared, cooled, and then gasified during a separate measurement. The second approach is generally easier to interpret, because the only observed stage is the gasification stage, while in the first case, all stages of the process are observed, and often pyrolysis and gasification overlap, which may make interpretation of the results more difficult. Figure 6a shows the TGA curves of the CO₂ gasification stage of sewage sludge char, while Figure 6b shows the TGA curve of the entire sewage sludge gasification process. This juxtaposition is intended to illustrate the differences in both approaches used in the literature.

The in-situ approach, although more demanding, more closely reflects the conditions in industrial gasifiers. Moreover, this approach eliminates the influence of the prior preparation of the char (especially the cooling stage) on its surface, and thus its reactivity. Although there is no literature comparing the reactivity of chars from sewage sludge formed by in-situ and ex-situ methods in the gasification process, the existing literature regarding other materials indicates a decrease in the reactivity of ex-situ chars due to lower porosity and consequently specific surface area [108,109]. Furthermore, the higher reactivity of in-situ chars compared to ex-situ chars formed in an inert atmosphere could also be caused by the inclusion of O₂ (from the gasifying agent), which leads to the formation of new pores [110].

Table 5. Summary of scientific work on the assessment of the kinetics of sewage sludge char gasification.

Atmosphere of Pyrolysis */Gasification	Heating Rate K/min	Approach	Temperature Range (°C)	Ea (Model Used) (kJ/mol)	Ref.
Ar/CO2	2, 5, 10, 15, 20	In-situ	600–1000	285.45 (VRM)	[101]
Steam	40	In-situ	800–940	59.39 (FM) 45.7 (CRM)	[102]
Steam + air + O2	40	In-situ	800–940	51.7 (FM) 46.5 (CRM)	[102]
He/CO2	10	In-situ	800–1100	467.37 (VRM)	[103]
CO2 + N2	20	In-situ	430–570	267.3 (VRM)	[100]
N2/Air	**	In-situ	700 800 900	17 (A-EM) 15 (A-EM) 12 (A-EM)	[104]
Ar/O2	10	Ex-situ	400–600	114 (VRM)	[105]
Ar/CO2	10	Ex-situ	800–1000	227 (VRM)	[105]
Ar/steam	10	Ex-situ	750–950	193 (VRM)	[105]
Ar/steam	10	Ex-situ	620–950	180 (VRM) 177 (VRM)	[99]
Ar/CO2	10	Ex-situ	700–1000	211 (SCM) 234 (SCM)	[99]
He/CO2	10	Ex-situ	517–914	180 (ICM)	[65,106]
He/CO2	10	Ex-situ	827–972	168 (ICM)	[65]

*—if applied; **—isothermal char gasification measurement; (VRM)—volume reaction model; (FM)—Friedman method; (CRM)—Coast and Redfern method; (A-EM)—Avrami–Erofeev model; (SCM)—shrinking core model; (ICM)—integrated core model.

summarizes the results of scientific work on the assessment of the kinetics of sewage sludge char gasification under various process conditions formed by in-situ and ex-situ methods.

As can be seen, the available studies on the kinetics of char gasification were mostly conducted in non-isothermal conditions using carbon dioxide as a gasification agent. Much fewer studies have taken into account other atmospheres, such as steam or O₂/air. Nevertheless, it can be noted that generally gasification should be carried out at ~400–600 °C using O₂ as a gasifying agent; 700–900 °C using air; 800–950 °C using steam (or its blends); and 600–1000 °C using carbon dioxide (whereby the reaction initiation temperature is usually higher than 600 °C) to ensure efficient and complete conversion to produce syngas. This is due to the nature of the reaction that occurred in specific conditions, i.e., partial oxidation is an exothermic reaction, while steam-C and Bouduard reactions are endothermic, wheres the latter reaction occurs at higher temperatures than the former. This can also be observed while analyzing values of activation energy obtained by Nowicki et al. [105], who performed gasification measurements with gasifying agents (the other measurement conditions were the same).

Various gas-solid reaction models were applied to describe the sewage sludge char gasification. Among them, the volumetric reaction model was the most commonly adopted, since it turned out to be the best for predicting the rate of char gasification. However, some researchers argued that the choice of model should be strictly related to the gasifying agent used since gasification in steam occurs by a different mechanism than gasification in an atmosphere of carbon dioxide or O₂. According to these researchers [65,106], the shrinking core model is the most appropriate for describing the rate of char gasification in a CO₂ and O₂ atmosphere, whereas the volumetric model (represents the case for which the chemical reaction is rate controlling and where there are no gradients of solid components inside the particle) was the best for describing steam gasification of sewage sludge char. Nevertheless, the presented data of activation energy follow the literature for lignocellulosic char gasification.

As already mentioned, to the best of the author’s knowledge, no studies are comparing in-situ and ex-situ sewage sludge gasification. However, when comparing data summarized in Table 5 (bearing in mind that these studies were conducted in different conditions), it is generally visible that, with some exceptions, the E_a values for the in-situ method were lower, which is consistent with previously presented literature data [108,109] indicating higher reactivity of in-situ chars.

The data presented above show that TGA is an effective tool for a first and fast assessment of these carbonaceous materials in fundamental research.

2.3. Effect of Gasification Conditions on the Quality and Quantity of the Resulting Gas

Current research on the gasification of sewage sludge focuses on determining the optimal conditions enabling the effective course of the process, i.e., obtaining the maximum gas yield of the best possible quality, which in most cases means gas with the maximum share of H₂ and low tar content (which cause fouling and clogging in end-use application devices as well as other undesired downstream issues). The above syngas parameters are influenced by many factors, such as the properties of sewage sludge and the type of reactor (fixed-bed reactors, fluidized-bed reactors, rotary kilns, and auger and plasma reactors are used for this purpose), whereas the most important are temperature, type and amount of the gasifying agent, equivalence ratio (i.e., actual air to sewage sludge mass ratio), and the presence of an appropriate catalyst.

2.3.1. Effect of Sewage Sludge Properties

In general, for gasification to be efficient, the feedstock should be properly prepared following the operating requirements of the gasifier (in which the process will be carried out). This is particularly important in fluidized-bed gasifiers, where appropriate grinding of the raw material is crucial (to ensure an efficient heat transfer rate) to improve gasification reactions and to ensure, at the same time, high H₂ and CO content and low tar content. Otherwise, with too large grains and short residence time, the problem of incomplete conversion may arise [46]. Interestingly, Sattar et al. [111] proved that changing particle size within the range appropriate for a given gasifier has little effect on gaseous composition.

However, from the point of view of the sewage sludge composition, it has generally been proven that higher content of the main sewage sludge components (especially C and H) increases the LHV of gasification gases [37].

Noteworthy is the work of Gil-Lalaguna et al. [112], who carried out gasification of chars obtained by the in-situ and ex-situ methods in a lab-scale fluidized-bed reactor operating at atmospheric pressure, with continuous feed of a solid (around 2.1 g/min of char) and continuous removal of ash. The authors evaluate how the previous pyrolysis affects the subsequent gasification process, especially the quantity and quality of the resulting gas. Interestingly, the authors observed that the gasification of ex-situ char was more effective, which was explained by the fact that heterogeneous reactions of carbon with H₂O or CO₂ were facilitated in the case of ex-situ char (due to higher concentration of carbon and fixed carbon content). As a consequence, the yield of gas was higher (with an increase in the average CO yield of about 70%), whereas the yield of tar was reduced (with an average decrease of about 45%). These results, which are contrary to the literature presented so far, confirm the complexity of the gasification process of sewage sludge chars.

2.3.2. Effect of Operating Conditions

• Temperature

Temperature is one of the most important parameters influencing sewage sludge conversions (and therefore gas yield) as well as gas composition, because it determines the completion of the steam gasification reaction, combustion, the Boudouard reaction, the water gas shift reaction, methane formation, and dry/steam methane reforming reactions. The achieved conversion degree of sewage sludge increases with temperature, which translates into higher gas yield and is accompanied by a decrease in tar and char yields. This effect is observed regardless of the gasification agent used (see Figure 7). Moreover, since the Bouduard reaction is more endothermic compared to the carbon-steam reaction, the CO₂ gasification proceeded more slowly, and the final conversion degrees achieved were lower (in the same temperature conditions) compared to gasification with steam.

The influence of temperature on the composition of synthesis gas is more complex. The increase in temperature results in enhanced CO and H₂ content (CO mainly due to a heterogeneous reaction, whereas H₂ yield rises largely due to the methane decomposition, steam reforming, and hydrocarbon reforming reactions) [46]. Consequently, an increase in the calorific value of the obtained gas is observed. The increase in LHV is also strongly affected by the fact that an increase in syngas temperature is profitable for heat-cracking reactions, which act to promote high-calorific value hydrocarbon production in the gas [37]. However, too high of a process temperature favors the formation of carbon dioxide, thereby lowering the syngas heating value, and may also lead to problems related to the melting of ashes [113]. It is therefore assumed that temperatures in the range of approximately 800—900 °C will lead to high carbon conversion, ensuring the high yield of gas with high calorific value [53].

• Type and amount of gasifying agent

The type of gasifying agent is considered a critical process parameter that affects both process efficiency and the quality of the resulting gas. Regarding biomass gasification, as well as sewage sludge, air is usually used as the gasifying agent due to its low price. This approach usually ensures a high conversion degree, so a high gas yield, but the main component of the obtained gas is non-flammable nitrogen, which makes its calorific value low. One way to reduce this problem is gasification in an O₂ atmosphere (but it is associated with much higher costs) or a steam atmosphere. Steam, or a mixture of steam with air/O₂, seems to be the most advantageous solution since it enables increased H₂ production (which may be about 60 mol.% of the total product) as well as ensuring high thermal efficiency (most reactions with steam are endothermic and the presence of air/O₂ ensures thermal balance) [114]. Moreover, literature reports indicate that syngas with lower tar content may be obtained by using steam compared to air gasification [46]. What is more, tars produced in steam gasification are easier to eliminate compared to those from air gasification [115]. An interesting solution, from the point of view of reducing the amount of greenhouse gas emissions into the atmosphere, is to use CO₂ as a gasification agent. However, while carbon dioxide is often used in TGA studies, to the best of the author’s knowledge, there are no experimental studies on sewage sludge gasification in CO₂ to assess the gas produced. However, there are works examining the impact of CO₂ addition on the pyrolysis (in an N₂ atmosphere) and gasification (in an H₂O atmosphere) of sewage sludge [116]. The authors proved that the addition of CO₂ into both analyzed processes enabled the reduction of condensable hydrocarbons (tar) in the resulting gas by expediting cracking. Moreover, the presence of carbon dioxide in the processes’ atmosphere leads to increased CO yield (by nearly 200%). These results confirm that it is possible to efficiently conduct the sewage sludge gasification process in an atmosphere of carbon dioxide, thus ensuring environmental benefits.

The amount of gasifying agent is another parameter that significantly influences the sewage sludge gasification process. As can be seen in Figure 8, as the amount of the gasifying agent (both steam and carbon dioxide) increases, the conversion degree achieved in the process increases, which translates into a higher gas yield.

As there are no scientific reports focusing directly on the assessment of the influence of the amount of carbon dioxide on the gas obtained during the sewage sludge gasification process, the mentioned work [116] indicates that increasing the amount of CO₂ in the atmosphere during pyrolysis (in the range of 0–30%) resulted in an increase in CO in the resulting gas. In the case of steam, its low content in the sewage sludge gasification process may result in incomplete conversion of the feedstock, as well as a high content of tar and methane in the resulting gas. As the amount of steam increases, an improvement in conversion, formation of H_2, and tar reforming is observed. It is generally estimated that the amount of steam (expressed as the ratio of steam to sewage sludge) should be in the range of 0–2 [117].

• Equivalence ratio (actual air to sewage sludge mass ratio)

Research shows that too low of an ER value is undesirable because it creates locally anaerobic conditions, favoring the pyrolysis process, which reduces the amount of gas formed. As the ER increases to a certain optimal value, the contents of H₂ and CO increase, and lower tar generation is observed. However, a further increase in ER above the optimal value results in a decrease in H₂ and carbon monoxide yields, while the share of non-flammable carbon dioxide increases because gasification is shifted towards combustion. As a consequence, the calorific value of the obtained gas is lower. Thus, it is necessary to select the optimal value of ER, which, depending on the material and the process conditions, will vary; however, it should be in the range of 0.18–0.35 [118,119]. Figure 9 shows the effect of ER on H₂ and CO yields during the gasification of two various sewage sludge [120].

• Presence of catalyst

Research on catalytic gasification of sewage sludge focuses on assessing the impact of various catalysts on the gasification process carried out at low temperatures and more specifically on maximizing the H₂ content in the gas (by catalyzing the heterogenous and homogenous gasification reactions that promote H₂ formation) and/or tar degradation (which, apart from removing this pollutant, results in more H₂, carbon monoxide, and methane content in the gas). Catalysts most frequently used to increase the H₂ content in the gas (while reducing tar as an additional effect) are alkali and alkaline earth metal catalysts (especially Na, K, and Ca-based catalysts) and transition metal catalysts (especially Ni, Fe) or their mixtures [121,122,123]. The first group of catalysts is especially attractive due to their low price, high effect on gasification efficiency, and, especially, water-gas shift reaction, resulting in high H₂ content. Moreover, alkali and alkaline earth metal catalysts show the ability to inhibit tar formation [124,125]. These advantages are particularly important in the case of sewage sludge gasification, since due to the high content of mineral matter in this material, it is difficult to recycle catalysts. Ni-based and Fe-based catalysts also show activity towards the water-gas shift reaction and tar reduction (via a reforming mechanism), and both these phenomena result in increased H₂ content in the resulting gas. However, nickel-based catalysts show a tendency to deactivate due to coke deposition [126]. To prevent this, nickel is usually used in conjunction with another metal as a multi-component catalyst (e.g., Co–Ni or Ni–Fe showed no coke formation) [127,128]. In turn, in the case of catalytic gasification of sewage sludge, aimed mainly at tar reduction, catalysts such as dolomite, olivine, and alumina but also calcium oxide are mainly used [129,130,131]. Their activity in destroying the tar has been repeatedly confirmed in biomass gasification studies [132,133], which is why they have been used for sewage sludge gasification. Apart from their activity (which, according to research, is more or less similar [133]), their advantage is the fact that they occur naturally, which makes them easily available and cheap catalysts.

The mechanism of catalytic steam gasification in a fixed-bed reactor (in the example of biomass since sewage sludge is considered a biomass resource [25]) is shown in Figure 10 [134]. At the top of the gasifier, due to the high temperature, the fast pyrolysis of feedstock particles occurs. The formed volatiles and tars react with steam and the C–C and C–H bond cleavage of these molecules takes part on the catalyst surface. After that, the steam reforming of the tars and hydrocarbons and the water-gas shift reaction occur [134].

A summary of studies on the impact of the above conditions on the sewage sludge gasification process is presented in

Table 6. Summary of scientific work on the assessment of the process conditions on the sewage sludge gasification.

Reactor	Temperature °C	Other Parameters	Observed Effect	Ref.
Laboratory fixed-bed reactor	700	Gasifying agent: atmospheric air and O2-enriched air; ER: 0.12, 0.14, 0.16, 0.18, 0.23, 0.27; SS1 properties: VM: 44.2%, A: 49%, C: 27.72%; SS2 properties: VM: 36.5%, A: 51.5%, C: 31.79%	Optimal ER value: 0.18, resulting in the highest CO content (31.3% and 26.9% for two analyzed sewage sludge) and the highest LHV (~5 MJ/m3)	[37]
Bench-scale fluidized-bed reactor	850	Gasifying agent: N2/air; ER: 0.1–0.2; SS properties: VM: 54.3%, A: 30.6%, C: 49.16%	An increase in ER reduces tar content in gas (from 37.5 g/Nm3 at 0.1 EA to 29.4 g/Nm3 at 0.2 ER) but decreases its lower heating value (from 12.1 to 5.8 MJ/Nm3)	[135]
Lab-scaled bub-bling fluidized-bed gasifier	700–850	Gasifying agent: air; ER: 0.2–0.35; SS properties: VM: 44.6%, A: 44.6%, C: 40.4%	Optimal temperature: 850 °C (the highest cold gas efficiency was achieved—24%); Optimal ER: 0.35 (the highest LHV of gas was obtained, equal to 3.3 MJ/Nm3)	[136]
Lab-scaled bubbling fluidized-bed gasifier	750–900	Gasifying agent: air; ER: 0.1–0.4; SS properties: VM: 50.1%, A: 40.4%, C: 19.85%	The higher temperature increased the gas yield. Maximum combustible gas yield (H2, CO, and CH4) was obtained at ER = 0.25 and 900 °C.	[137]
Fluidized bed and fixed bed	650, 810, 830	Gasifying agent: air; ER: 0.22–0.5; SS properties: VM: 50.3%, A: 34.7%, C: 29.88%	The highest H2 content in produced gas (29 vol.%) was obtained at the equivalence ratio of 0.35. The highest ER (0.5) strongly decreased impurity contents (tar, NH3, and H2S).	[118]
Fluidized-bed gasifier	800	Gasifying agent: air, steam/O2; SS properties: VM: 48.74%, A: 39.9%, C: 30.64%	Steam gasification resulted in the following: - decreased gas yields (60% compared to 75% for air gasification) - increased H2 content (38–39% compared to ~17–19% for air gasification). - increased LHV of gas (almost twice)	[138]
Two fixed-bed gasifiers (downdraft and updraft)	700, 800, 900	Gasifying agent: air, pure O2; SS properties: VM: 47.53%, A: 39.63%, C: 32.22%	An increase in temperature increased the volumetric percentage of the H2, whereas the effect of the gasifying agent was almost insignificant. The highest H2 was obtained at 900 °C, and it was 42 and 40% for air and O2, respectively (updraft gasifier), and 46% and 45% for air and O2, respectively (downdraft gasifier).	[139]
Fluidized-bed gasifier	800	Gasifying agent: air, air + steam; SS properties: VM: 58.3%, A: 41.7%, C: 29.5%	Steam addition increased the H2 and CO2 content and decreased the content of CO and tar (due to the steam reforming reactions).	[140]
Fluidized-bed gasifier	750, 850	Gasifying agent: air, air + steam; SS properties: VM: 58.3%, A: 41.7%, C: 29.5%	Temperature increase up to 850 °C: - rises the combustible gas production - significantly reduces the tar formation (by 49–65% depending on other process conditions). Steam addition increased the H2 and CO2 content in the synthesis gas and reduced the CO, CH4, and CnHm production.	[141]
Quartz tubular reactor	650–850	Gasifying agent: steam (various flow rates); SS properties: C: 30%	An increase in temperature and steam flow significantly raises the gas yield and carbon conversion, but too high of a temperature reduces H2 content (the highest H2 content was obtained at 750 °C (~57%)).	[111]
Tubular reactor	770–850	Gasifying agent: air, steam + air; SS properties: VM: 50.09%, A: 39.04%, C: 29.5%	Higher temperatures reduce tar content and improve the gas yield (including H2 and CO yields). In turn, the addition and increase of steam in the gasifying agent favor the formation of gas with better heating value and H2/CO molar ratio.	[142]
Laboratory-scale semi-batch scale experimental facility	900	Gasifying agent: steam (various amounts represented by various steam/carbon ratios): 3.05, 5.62, and 7.38; SS properties: VM: 44.3%, A: 33.91%, C: 45.79%	Optimum S/C ratio: 5.62 (the highest syngas and H2 yields). Further increase in S/C decreases the H2 content since an increase in the steam flow rate has a twofold competing effect—(1) the tendency to accelerate steam-reforming reactions and (2) the tendency to decrease the reactants’ residence time. Thus, the time for the reaction between steam and condensable hydrocarbons is decreased.	[143]
Laboratory-scale quartz tubular reactor	700–1000	Gasifying agent: steam; Catalysts: KOH, K2CO3, NaOH, and Na2CO3; SS properties: VM: 48.51%, A: 43.11%, C: 27.69%	The greatest improvement in H2 content at low temperatures was obtained by using NaOH and Na2CO3 catalysts. In turn, K2CO3 enhanced in the greatest extent the total gas yield, whereas Na2CO3 was the most effective for improving energy density for sewage sludge.	[121]
Quartz reactor	700, 750, 800	Gasifying agent: steam Catalysts: Ni-Fe and Ni-Co/Al-MCM48; SS properties: VM: 57.74%, A: 29.58%, C: 27.19%	The temperature of 800 °C resulted in the highest gas yield as well as H2 and CO yields (35.3% and 11.7%). The presence of the catalyst resulted in the enhancement of this effect, especially Ni-Co (the highest H2 content ∼52 vol%) due to the improved Ni dispersion and synergy between catalyst components.	[123]
Laboratory-scale fixed-bed reactor	600–800	Gasifying agent: air, ER: 0.2, Catalysts: dolomite, steel slag, and calcium oxide; SS properties: VM: 66.53%, A: 27.33%, C: 39.98%	The temperature increase results in higher gas yield and lower tar content. Catalysts used additionally reduce tar contents (due to the racking of the hydrocarbon structure), especially dolomite. In turn, calcium oxide results in the highest H2 and CH4 content in the resulting gas.	[129]
Fluidized-bed reactor	750–850	Gasifying agent: air + steam Catalysts: dolomite, olivine, alumina; SS properties: VM: 56.0%, A: 44.0%, C: 27.3%	Dolomite was characterized by the greatest activity in tar destruction, followed by alumina and olivine. The presence of steam and the catalysts increased the H2 content in the gases by nearly 60%.	[130]
Fluidized-bed reactor	750–850	Gasifying agent: steam, ER: 0.2–0.4, Catalyst: alumina; SS properties: VM: 53.3%, A: 46.7%, C: 25.9%	The addition of 10 wt.% of alumina significantly reduces tar production (improvement up to 42%) and increases the carbon conversion and LHV of the gas.	[131]
Batch reactor	450	Gasifying agent: steam (supercritical gasification), catalysts: NaOH, KOH, K2CO3, Na2CO3, and Ca(OH)2; SS properties: VM: 57.4%, A: 42.6%	Most catalysts increased H2 content in the gas (especially K2CO3). The exception was Ca(OH)2 (no catalytic effect on H2 yields) but it affected the CO2 yield.	[122]
Batch reactor	400	Gasifying agent: steam (supercritical gasification), catalysts: NaOH, NaOH + Ni; SS properties: VM: 59.52%, A: 41.19%; C: 25.05	Both catalysts increase the yield of H2 in the resulting gas, whereas the effect of NaOH + Ni was greater (the H2 yield was almost five times as much as without catalyst).	[144]

.

The results presented in the above table indicate that the selection of optimal conditions, especially combined with the addition of an effective catalyst, can significantly increase the hydrogen content in the syngas. The observed H₂/CO ratio ranged between 1 and 3 (sometimes higher values were achieved [123,139,142]), wherein most often it was a value between 1 and 2 [122,130,131,140,141]. Such a gas could potentially find application in the production of liquid fuels in the Fischer–Tropsch synthesis (required H₂/CO: 2), as well as in the production of hydrocarbons and synthetic waxes, which can potentially be used in the production of wax paper, candles, creams, and ointments. In addition, synthesis gas with an H₂/CO ratio of 1 to 2 can be used in oxo-synthesis processes, which involve direct reactions of olefins and synthesis gas to produce alcohols or aldehydes [145,146]. As can be seen, these results gave encouraging results to test in commercial and large-scale plants.

2.4. Co-Gasification of Sewage Sludge

Co-gasification of sewage sludge may be an interesting proposition in the case of sludge whose gasification may be problematic due to high ash and moisture content or low carbon content. Moreover, the main advantage of the co-gasification process is an increased degree of flexibility when it comes to the gas production components (by adjusting the fuel mixture) as well as the possibility of a synergy effect. Scientific literature shows that mainly biomass/waste biomass is selected as a component for sewage sludge co-gasification processes (palm kernel shells [147], energy crops [148], forestry waste [148,149,150,151], horticultural waste [152], waste shiitake substrate [137]). Coal [153,154,155] or other waste, such as paper-mill sludge [49] or waste tire char [76], are much less frequently chosen as components for co-gasification. Peng et al. [149] as well as Ong et al. [150] investigated the impact of co-gasification of sewage sludge with biomass waste. Both research groups observed that the gas yield gradually decreased with increasing sewage sludge content in the blend. The former authors also proved that the addition of sewage sludge resulted in a reduction in the amount of H₂ in the produced gas and that the maximum content of sewage sludge in the blends should not exceed 20% (to ensure efficient gasification and avoid problems related to the formation of agglomerated ash). In turn, the latter researchers observed that with 30% of sewage sludge in the blend, the H₂ content was the highest among all analyzed cases, which was attributed to the synergy effect. Moreover, the authors conclude that the steam generated from the moisture of sewage sludge promoted the concentrations of H₂ and CO in the product gas.

In turn, Hu et al. [152], who conducted the process of co-gasification of sewage sludge with horticultural waste, indicated that, as in the case of wood waste, with the increase in the amount of sewage sludge in blends, the yields of gas decreased, but the H₂ content increased in the entire analyzed range (confirming the synergy effect). This phenomenon was more significant at higher temperatures (probably due to the reduction and steam oxidation of Fe species in sewage sludge). As a result, the optimal process conditions were 80% sludge and a temperature of 900 °C. Interestingly, in the case of using coal as a component for co-gasification of sewage sludge, there is also no consensus in the literature as to its impact on the gas produced. Smoliński et al. [155] proved that steam gasification of hard coal resulted in the highest H₂ content in the resulting gas, while the addition of sewage sludge decreased this value. Garcia et al. [153], who analyzed the co-gasification of sewage sludge with bituminous coal and lignite using a laboratory-scale fluidized-bed reactor, drew the opposite conclusions. According to these authors, the main combustible gas components, i.e., H₂, carbon dioxide, and methane, increased with increasing sewage sludge contents, which was especially visible in the case of sewage sludge-lignite blends due to the synergistic effect. Moreover, gas LVH, carbon conversion, and gas energy efficiencies were improved as a result of adding sewage sludge to coal. The above results indicate that the process of co-gasification of sewage sludge is very complex and depends on many factors; therefore, it is impossible to draw clear conclusions regarding the optimal conditions for conducting this process.

2.5. Supercritical and Plasma Gasification of Sewage Sludge

Promising future approaches to sewage sludge gasification are supercritical waste gasification and plasma gasification processes. Supercritical water gasification is carried out at lower temperatures compared to conventional gasification, i.e., 400–600 °C, but with higher pressure (above 22 MPa) and reaction time (even 60 min) [46,156]. Its main advantages are the lack of necessity to dry the sludge before gasification, as well as the possibility of obtaining gas with an increased amount of H₂ and a greater potential for phosphorus recovery from ash (since such severe conditions favor the concentration of minerals in the ash) in comparison with traditional gasification. This makes it the subject of many studies [157,158,159,160,161,162,163,164]. According to Peterson et al. [160], depending on the SCWG conditions, gases of different compositions may be obtained, i.e., temperatures higher than 500 °C result in H₂-rich gas; catalysts and temperatures between 374–500 °C result in methane-rich gas; whereas catalysts and subcritical temperatures result in mixed gas. A significant part of the research on SCWG uses catalysts (analogous to those in the case of conventional gasification) to increase the value of the final products [157,158,159]. However, as research shows, the quality of gas obtained during supercritical gasification of sewage sludge is mainly affected by temperature, whereas the effect of catalysts is lower and depends on the temperature. Yang et al. [161] analyzed the effect of conditions (temperature: 500–800 °C; pressure: 25–35 MPa; feed concentration: 1–30%) during non-catalytic supercritical sewage sludge gasification on the resulting gas. The authors proved that feed concentrations of 15–20 wt.% and high temperatures resulted in the highest H₂ yields, while the effect of pressure is limited. In turn, Adar et al. [162] examined the catalytic SCWG process and concluded that the gas of the best quality (60% H₂, 22% CH₄, CO, and H₂S below detection) was obtained at the highest analyzed temperature (650 °C) and with the largest amount of tested catalyst (2% KOH). Thus, in this case, the catalyst enhanced the positive effect of the temperature increase on the obtained gas. Similar conclusions were drawn by Chen et al. [163], who observed that maximum process efficiency and H₂ yield (90.1% and 18.13 mol/kg, respectively) were obtained at the highest analyzed temperature (450 °C) and the highest amount of Ni-Co catalyst. On the other hand, Chen et al. [157] and Yan et al. [164] observed that during the non-catalytic process, the temperature increase (in the analyzed ranges) indeed increased the H₂ yield in the resulting gas (up to 20.66 mol/kg at 750 °C and 1.91 mol/kg at 450 °C, in the first and second cases, respectively). However, the addition of catalysts promoted the steam reforming, water-gas shift, and pyrolysis reaction, thus better enhancing the hydrogen formation at the lower reaction condition. Consequently, in the most efficient catalytic processes, the maximum H₂ yield reached 20.03 mol/kg for the former (process conditions: 650 °C, RNi-Mo2) and 1.60 mol/kg for the latter (process conditions: 400 °C, KOH). As can be seen, the use of catalysts resulted in lower H₂ yields than the yields obtained during non-catalytic gasification at the highest analyzed temperatures. Thus, the selection of optimal SCWG conditions, as well as ways to reduce the process costs, are topics that require further research, because at the moment, SCWG is not a viable commercial solution.

Plasma gasification of sewage sludge is also a highly energy-consuming approach. At the heart of this process is the plasma torch, where an intense electric arc is generated between two electrodes through which a gas flows. The plasma supplies the system heat (temperature up to 4500 °C) that enables complete decomposition of the feedstock organic part, whereas the inorganic part is melted in volcanic lava that vitrifies after cooling. The gas temperature at the reactor outlet reaches about 1000–1200 °C [165,166]. The advantage of this process is the ability to produce clean synthesis gas (including the reduction of toxic compounds) and the fact that it has the lowest environmental impact among advanced thermochemical processes [45,167]. Striūgas et al. [45] compared the gasification of sewage sludge with and without plasma and observed that the plasma reactor reduced tar concentration (from 2.03 to 0.09 g/Nm³) and gas calorific value; however, increased gas yield (from 2.63 to 3.68 Nm³/kg) and the total produced energy amount. The significant positive effect of plasma on the quality of the resulting gas was also confirmed by Vishwajeet et al. [47] and Balgaranova et al. [168]. As a result of plasma gasification of sewage sludge, these researchers obtained syngas with calorific values of 7.5 MJ/mN³ and 5.36 MJ/mN³ (excluding and including nitrogen dilution) and 13 MJ/kg, respectively.

Laboratory studies of the above-mentioned processes provide the most accurate data but may be very expensive. Therefore, the most current scientific reports focus on numerical modeling (using specialized software such as Aspen Plus) to plan, optimize, and evaluate these processes [166,169,170,171]. For example, using numerical methods, Ruya et al. [169] found out that higher temperatures (up to 700 °C) and feed concentrations translate into higher system efficiency during SCWG of sewage sludge. In turn, Cvetinović et al. [166] modeled plasma gasification of sewage sludge, which enabled the selection of the optimal process conditions (T: 1200 K, ER: 0.2, steam-to fuel-ratio: 0.1) and estimation of the process costs (72.30 €/t for steam gasification and 88.12 €/t in the case of air gasification).

2.6. Management of Sewage Sludge By-Products

The gasification process is carried out to obtain synthesis gas; however, regardless of the raw material properties or process conditions, certain amounts of by-products, i.e., tar and ash enriched with heavy metals, are also obtained. Tar formed during gasification is particularly undesirable, as it causes problems with corrosion and clogging of equipment due to its viscosity (this especially applies to tars with a high PAH content) [172]. The typical composition of tar from gasification of sewage sludge is as follows [46,140,142]:

∘

N-aromatics (benzo-nitrile, methyl-pyridine, quinoline, phenyl-pyridine, pyridine) state approx. 50%

∘

PAH (naphthalene, methyl naphthalene, biphenylene, biphenyl, phenanthrene, fluorine) state nearly 36%

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Light aromatics state about 6.5%

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S-compounds (2-benzothiophene, propane nitrile) state approx. 4.9%

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O-aromatics (phenol, benzofuran) state about 2.6%.

This composition means that tars can be toxic. A series of studies conducted by Werle et al. [37,173,174] on the toxicity of tars and ashes from gasification of sewage sludge using different methods (Microtox test, Daphtoxkit F survival test) confirms that tars are toxic or highly toxic. Therefore, before potential use, tars should be subjected to deep purification. According to Krzywicka et al. [175], advanced oxidation methods (analogous to those used in cooking wastewater treatment) can be used for this purpose; however, this thesis requires verification. Research on tar from the gasification of sewage sludge focuses on reducing its formation rather than on its management. For this purpose, catalysts are used (see Section 2.3.2.); however, appropriate process conditions such as high temperature, steam addition, and throughput reduction may reduce the amount of tar formed in the process [46,141]. Since biomass contains compounds that promote tar reforming, it would seem that in the co-gasification of biomass with sewage sludge, the amount of tar would be limited [172]. However, there is no detailed research on this topic so far, whereas the available studies on the co-gasification of sludge with coal indicate a negligible effect of the blending ratio on the tar formation [176].

The situation is different when it comes to ash. Non-toxic ash from gasification of sewage sludge can be used as a source of phosphorus or as sorbents. Phosphorus is an essential macronutrient, the use of which is increasing due to increasing global food demand. The main source of this element is phosphate rock—a non-renewable and unevenly distributed worldwide mineral with limited resources. Therefore, the phosphorous-rich sewage sludge ashes seem to be an attractive material enabling the recovery of this element. The literature review indicates that phosphorus recovery from gasification ash is possible. Viader et al. [177] compared ash from the incineration and gasification of sewage sludge from the point of view of the possibility of phosphorus recovery and confirmed that gasification ash can be an interesting alternative to combustion ash. However, it seems that the ashes from the co-gasification of sewage sludge with biomass rich in K- or K-Si-compounds (such as agricultural residues or straw) have a much greater potential [178,179,180]. According to Thomsen et al. [178], ash from gasification and co-gasification of sewage sludge and cereal straw can be used as fertilizer, whereby co-gasification ashes show better fertilizer qualities since they are characterized by the higher content of recalcitrant C, phosphorus, and potassium, a lower content of heavy metals (especially Cd), and an improved P bioavailability compared to the mono-sludge ashes. Similar conclusions were drawn by Hannl et al. [179,180] in studies on phosphorus recovery from ash from co-gasification of sewage sludge with sunflower husks and wheat straw. This is due to the formation of K-bearing phosphates in the blends instead of Ca/Fe/Al phosphates in the sewage sludge ash (about 45–65% of P may be incorporated in crystalline K-bearing phosphates) [179]. Interesting studies were also conducted by Hartmann et al. [181], who assessed the possibility of partially replacing phosphate rock with ash from the gasification of sewage sludge. The results show that 8% of phosphorus from phosphate rock can be replaced by phosphorus from gasification ash while maintaining the high solubility of P (further substitution decreased the total P concentration of the final product).

Ash from the gasification of sewage sludge can also be used as adsorption material for the reduction of toxic organic substances (such as phenol) from the water streams. Comparison of maximum adsorption capacity of the phenol monolayer on various adsorbents indicates that sewage sludge, with capacity q equal to 42.22 mg/g, may be more efficient than some unconventional adsorbents, such as bagasse fly ash (q: 12–13 mg/g), neutralized red mud (q: 5.13 mg/g), olive pomace (q: 4–5 mg/g), and even than activated carbon derived from rice husk (q: 22 mg/g). Moreover, the capacity of commercial activated carbon (q: 47.72 mg/g) was slightly higher than that of ash from sewage sludge gasification [37]. In turn, Gil Lalaguna et al. [182] conducted research on the use of sewage sludge ash for H₂S removal from gas streams at high temperatures, but the results indicated that gasification ash showed worse desulphurization ability than combustion ash.

2.7. Environmental Impact of Sewage Sludge Gasification

Conventional sewage sludge management methods have numerous drawbacks, as mentioned in the introduction section of this review, which is why more and more emphasis is being placed on the development of alternative thermochemical conversion methods, including gasification. Generally, thermal treatment of sewage sludge (which includes not only gasification but also conventional incineration) has advantages from the environmental point of view compared to landfilling, namely complete destruction of microorganisms and pathogens and huge capacity in waste reduction [31]. However, conventional incineration is a process with poor public acceptance due to high pollutant emissions—high-temperature flue gas formed during incineration contains a majority of CO₂, NO_x, and water, as well as CO and SO_x, which cause environmental problems such as acid rain or global warming [183]. Moreover, the flue gases contain compounds harmful to human health, such as particulate matter, polycyclic aromatic hydrocarbons (PAH; their concentration in raw exhaust gases can range from 3.926 to 524.176 μg/m³ [27]), and chlorinated compounds (HCl, HF, dioxins, furans) [31]. Gasification, on the other hand, is more environmentally friendly. First of all, gas produced during gasification consists mainly of H₂ and CO, and its volume is lower compared to flue gas from combustion [120]. As a result, the emission of greenhouse gases into the atmosphere during the combustion of syngas is lower. Additionally, the removal of pollutants from syngas (before its further use) is easier than in the case of exhaust gases, where they are highly diluted. When it comes to other pollutants, due to the reducing atmosphere, gasification prevents the emission of nitrogen and sulfur oxides and heavy metals (especially mercury), as well as significantly reducing emissions of dibenzodioxins and dibenzofurans [120,184]. In addition, according to Fericelli et al. [184], benzene, toluene, naphthalene, and acenaphthalene may be present in syngas in very small amounts; however, after syngas combustion, these compounds are either not detected or present at sub-part-per-billion concentrations in the emitted stack gas. In turn, from the point of view of the impact of process ash on the environment, the situation is not so clear-cut. During incineration, ash with the potential for leaching of hazardous compounds may be obtained [185]. In the case of gasification, there is also such a risk, but studies indicate that these ashes, despite the concentration of heavy metals, are rather non-toxic. Nevertheless, the toxicity of the solid residue should always be checked. There is no doubt, however, that the incineration process produces more ash than the gasification process [186]. Moreover, supercritical conditions during gasification favor an additional reduction in the amount of ash produced. Thus, the conversion of sewage sludge by gasification reduces the problem of managing the solid residue.

2.8. Future Work

Since the high moisture content in sewage sludge is a fundamental problem in its wider use, research into further heat recovery, solar energy application, or the use of pyrolytic char as process feedstock should be investigated. Moreover, a new approach, namely SCWG, should be widely analyzed since it eliminates the need to dry the sludge while providing gas with a higher H₂ content compared to conventional gasification. In addition, research should continue on the selection of the optimal catalyst for the process, taking into account not only its effect on the process but also the issues of its deactivation and regeneration (which are hindered due to the high ash content in the sludge). An aspect that requires further research is also the issue of recovery, primarily of P but also of N and K from ash. Finally, the wider use of numerical methods for modeling sewage sludge gasification processes (especially high-energy-consuming ones, such as SCWG or plasma gasification) is also a key issue for the future, because they allow a lot of important information about a given process to be obtained relatively cheaply and quickly.

3. Conclusions

It may be concluded that despite the high contents of moisture, ash, and heavy metal, the gasification process may be a promising approach for sewage sludge management. This is because they are reactive materials and may contain a significant amount of carbon. Moreover, sewage sludge ash, with a favorable composition, may have a catalytic effect during gasification. This review provides a comprehensive summary of the kinetics analysis of sewage sludge char gasification (performed based on TGA measurements) as well as the effect of the most often analyzed process conditions on the quality and quantity of the resulting gas. The reviewed scientific reports proved that sewage sludge gasification in an atmosphere of steam occurs faster than in an atmosphere of carbon dioxide and that process conditions (i.e., temperature, type and amount of gasifying agent, or equivalence ratio) have a significant impact on the process efficiency and the obtained gas, and their optimization may result in the production of H₂-rich gas with a low tar content. Additionally considered are the use of catalysts and the co-gasification process, which, similarly to process conditions, if properly selected, may result in the production of high-quality syngas. The latest research indicates that advanced gasification methods, such as supercritical gasification (enabling conversion of wet sewage sludge) or plasma gasification, also have great potential. Finally, in addition to the main product of sewage sludge gasification, i.e., syngas, the process also produces a by-product—ash—which seems to be a potential source of phosphorus. To sum up, from the point of view of a circular economy, gasification of sewage sludge may provide valuable, clean syngas, whereas the produced ash may enable the recovery of valuable elements. All this can be achieved in an environmentally friendly way since gasification has a lower impact on the environment than conventional methods of sewage sludge management.