Co-Gasification Performance of Low-Quality Lignite with Woody Wastes Using Greenhouse Gas CO₂—A TG–MS Study

Abstract

The carbon dioxide gasification performance of low-quality lignite-agroindustrial/forest waste blends was investigated in terms of reactivity, conversion, cold gas efficiency, product gas composition and heating value. The experiments were conducted in a fixed bed unit and a thermal analysis mass spectrometer system. Raw materials, chars, liquids and gases were quantitatively analyzed and their energy content was determined. Synergetic effects and the role of minerals were examined, the latter through chemical and fusibility analyses of the ashes. Ahlada lignite (AL) was of low quality, with a calorific value of 8.9 MJ/kg. The biomass materials, ginning cotton waste (GCW) and pine needles (PN) had calorific values 16.6 MJ/kg and 20.1 MJ/kg, respectively. The slagging/fouling propensity of AL ash was low, whereas that of biomass wastes was medium to high. Thermal treatment of the samples and their blends prior to gasification produced upgraded fuels. A Boudourd reaction occurred above 750 °C. Gasification reactivity followed the order: GCW > PN > AL. AL/PN mixtures presented additivity effects; however, AL/GCW mixtures presented synergy effects. When the lignite was blended with the biochars studied, its conversion increased from 90% to 94.5% and its cold gas efficiency from 31.8% to 35%. Generated gas attained a heating value of about 12 MJ/m³.

1. Introduction

Combustion of solid fuels for power generation creates air pollution and is responsible, to a great extent, for the greenhouse gas effect. On the other hand, gasification has been identified as an environmentally friendly process, with higher flexibility of feedstocks and end products [1,2,3]. The product gas of gasification is suitable as an energy source in internal combustion engines, turbines and solid oxide fuel cells, or as source of biofuels and value-added chemicals [3,4,5,6,7]. A two-step process, the first step being the thermal decomposition of the solid feedstock producing liquid and gaseous fuels [2,3,7,8] and the second step, the gasification of char, increases the reactivity of char and the purity of gas, and at the same time, eliminates operational tar problems during the utilization of gas, such as pipe clogging or corrosion [8,9]. The heat required for the second step can be provided by the pyrolysis-combustible products and the coupling of the process with a power plant.

When carbon dioxide from flue gas streams is used as the gasifying agent, the process provides a potential solution to the greenhouse gas problem and carbon dioxide sequestration [1,2,10]. Global net CO₂ emissions must be negative in order to reduce the greenhouse gas effect caused by atmospheric CO₂. This concept is often described as bioenergy with carbon capture and storage (BECSS). Among the various technologies investigated, such as chemical looping combustion of biomass, co-combustion of coal and biomass, synergy between biogas plant and biomass power plant and oxy-combustion with heat accumulation [11], gasification of biomass with carbon dioxide is considered a highly promising technology. The Boudouard reaction (C + CO₂ ↔ 2CO ΔH = 172 kJ/mol) is highly endothermic, and an efficient reductant combined with a strong thermodynamic driving force is required to ensure irreversibility. However, at temperatures above 700 °C the equilibrium shifts towards the formation of CO. This reaction is very important for the gasification of carbonaceous materials, incorporating CO₂ into a valorization cycle for the production of marketable fuels, instead of capturing and sequestration [1,12].

The effectiveness of the gasification process is dependent on many different factors including feedstock properties, type, the concentration of the gasifying agent and operating conditions. A high temperature favors syngas production, while a high specific surface area and a more disordered aromatic structure of char, as well as the presence of alkali or alkaline earth compounds enhance its reactivity [2,3,13,14,15,16,17]. Inherent alkali in fuel ashes can serve as natural catalysts [2,4,14,18,19,20,21]; however, in appliances operating above 1000 °C, they can cause slagging/fouling problems, thus reducing the availability of the systems and increasing the energy cost [22,23,24,25].

Given the recent worldwide energy and climate crisis, global policies have set the targets for transition to a low-carbon economy by recycling and the reuse of wastes as energy sources, in the context of global sustainability goals, a circular economy and reduction of carbon footprints through the promotion of renewables [26]. The European Green Deal [26] and Agricultural and Renewables Policy [27,28,29] are based on decarbonized energy by 2050 and penetration of agricultural wastes in energy markets. The declining of fossil fuel reserves and the environmental restrictions associated with them have led to higher operating costs and lower consumption. Therefore, for power plants to maintain their sustainability, a greater operating flexibility is required. As such, the combined processing of fossil fuels with renewable sources of energy, such as organic wastes showing good synergy with current power plants [30], in a sustainable manner, reducing the potential of global warming, seems an attractive solution. Coal, the principal fossil fuel for electricity generation worldwide (about 40% of global power) [4,31], is the indigenous fuel of many countries. On the other hand, forest and agricultural wastes are readily available in large quantities in most countries. The availability of energy from forest wastes could vary from 10 to 16 EJ/y globally, while that from agricultural wastes could vary from 5 to 27 EJ/y [28].

The carbon dioxide gasification of high-rank coal, mainly of bituminous type, with agricultural residues including sorghum, rice straw, empty fruit bunches, walnut shells [14,18,19,32,33], or forest residues such as pine and oak wood [2,4], has been extensively studied. Past investigations have focused on interactions between fuels, catalytic effects of alkali metals [18,19,32,34] and kinetic analysis of the process [2,4,14,33]. There is limited information on the co-gasification of low-quality lignites with agricultural or forest waste (peanut shells, orange peel, soybean stalk, redwood) [34]. Lignite, which is an important fuel for several national economies around the world, has more proximity with biomass than high-rank coal, and its co-processing with various biomass materials to cover uncertainties in their availability could lead to different kinds of interactions. Therefore, the blending of these fuels for energy generation is challenging. Furthermore, there is lack of data on the characterization of all solid, liquid and gaseous products of the process and evaluation of their energy potential, or their deposition propensity with regard to fuel ashes.

Based on the previous analysis, this work aimed to investigate the carbon dioxide gasification performance of low-quality lignite-agro-industrial/forest waste blends, namely ginning cotton waste and pine needles, in terms of reactivity, conversion, cold gas efficiency, product gas composition and heating value. The experiments were conducted in a fixed bed unit and a thermal analysis mass spectrometer system. Raw materials, chars, liquids and gases were quantitatively analyzed and their energy content was determined. Synergetic effects and the role of minerals were examined, the latter through chemical and fusibility analyses of the ashes.

2. Materials and Methods

2.1. Raw Fuels and Characterization

One lignite and two biomass materials were selected for present study. The lignite (AL) was provided from the open-pit mine of Ahlada, located in Western Macedonia in North Greece. Ginning cotton waste (GCW) was provided from a nearby enterprise and pine needles (PN) were collected from a local forest of the area. Samples were air-dried, riffled and ground to a final particle size below 200 μm (lignite in a jaw crusher and a ball mill, organic wastes in a cutting mill). Blends of lignite with each biomass material were also prepared at weight ratios of 70:30 and 50:50, respectively. These were chosen on the basis of typical ratios used in power plants co-processing such fuels [35], as well as on the low quality of lignite studied requiring a higher-grade material to improve its performance. Fuel characterization was performed in accordance with ASTM standards for the lignite (D5142, D5373, D4239, D5865) and European standards for the solid wastes (CEN/TC335).

Ashes were analyzed using an X-ray fluorescence spectrometer (S2 Ranger EDS, Bruker AXS) to determine their composition of inorganic elements, as well as a heating microscope with a high-definition video camera (Leitz Wetzlar EM201, Hesse Inst., Wetzlar, Germany) to determine their fusibility behavior. Deposition tendency was predicted through common slagging/fouling indices, calculated as follows:

Base-to-acid ratio (B/A) [23]:

B/A = %(Fe₂O₂ + CaO + MgO + K₂O + Na₂O)/(SiO₂ + TiO₂ + Al₂O₃)

(1)

When B/A < 0.5 deposition tendency is low, when 0.5 < B/A < 1 deposition tendency is medium and when B/A > 1 deposition tendency is high.

Babcock index (Rs) [22]:

Rs = (B/A)S

(2)

where S is the percentage of sulphur in dry fuel.

When Rs < 0.6 deposition tendency is low, when 0.6 < Rs < 2 deposition tendency is medium, while when Rs > 2 deposition tendency is high.

Slag viscosity index (Sv) [22]:

Sv = SiO₂.100(SiO₂ + Fe₂O₂ + CaO + MgO)

(3)

If Sv > 72 slagging tendency is low, if 65 < Sv < 72 slagging tendency is medium and if Sv < 65 slagging tendency is high.

Ash fusibility index (Fs) [22]:

Fs = (4IDT + HT)/5

(4)

Fusibility is low when Fs > 1342, medium when 1232 < Fs < 1342 and high when Fs < 1232.

Alkali index (AI) for biomass fuels [23]:

AI = kg (K₂O + Na₂O)/GJ

(5)

When AI = 0.17–0.34 kg/GJ deposition tendency is low, while when AI > 0.34 kg/GJ deposition tendency is high.

2.2. Char Production

For chars production, a lab-scale fixed-bed system was used (Figure 1), as previously described by the authors [8]. The stainless steel reactor (ID = 70 mm, H = 140 mm) was equipped with a stainless steel grid basket to support the sample, a Ni-Cr-Ni thermocouple in contact with the sample bed and a programmable high-temperature furnace with temperature controller of ±3 °C accuracy. After flushing with nitrogen to purge air, the sample was heated up to 600 °C with a rate of 10 °C/min, under a flow of nitrogen of 200 mL/min and retention time 30 min. Solid products of pyrolysis were analyzed with the same standard methods as for the raw fuels. The higher heating value (HHV) of liquid products, obtained after centrifugation of condensable volatiles, was determined by a bomb calorimeter (AC-300, Leco, St. Joseph, MI, USA), while the higher heating value of gases was calculated from the composition of gas and the higher heating values of individual gases, derived after conducting TG–MS (thermogravimetric mass spectrometry) experiments using the same experimental conditions as those of the fixed-bed system. The average heating value of hydrocarbon species measured was 70 MJ/m³. High-purity argon of flow rate 35 mL/min was used for these tests. The transfer line of product gases to the MS was a fused silicon capillary, which was insulated, heated and maintained at 200 °C to avoid species condensation. The secondary electron multiplier operated at 82 eV and 1–400 atomic mass and detected the ions which were separated according to their mass-to-charge ratio. Pyris v3.5 and Quadstar 422 software was used for acquisition of data. Calibration factors were determined from standard gases of high purity in argon. Description of the equipment (TG/DTG Perkin Elmer-MS/SEM QME-200, Balzers, Liechtenstein) and operating conditions can be found elsewhere [8].

2.3. Carbon Dioxide Gasification Experiments

For the gasification of chars and their mixtures in a carbon dioxide atmosphere, the thermal analysis system TG/DTG (sensitivity < 5 μg, temperature precision ±2 °C, balance accuracy 0.2% wt) was used. The weight loss of each sample and the derivative weight loss were measured continuously as functions of temperature from 25 °C up to 1000 °C. The heating rate was 10 °C/min, the flow rate of carbon dioxide 35 mL/min and the flow rate of nitrogen purge gas 45 mL/min. Analysis of gas was carried out using the mass spectrometer coupled online with the thermal analyzer, as mentioned above. Reproducibility of replicate tests was verified by the relative standard deviation (RSD).

The reactivity of each fuel was derived by the following equation:

$$R_G=\frac{R_{\max }}{T_{\max }}\times 100{(1/{\min }^{\circ }\mathrm{C})}_{}$$

(6)

where R_max, T_max the peak reaction rate and temperature, respectively.

Conversion efficiency (dry ash free %) was calculated by:

$$CE=\frac{m_g}{m_c}\times 100(\%)$$

(7)

where m_g and m_c the masses of gas and char, respectively.

The cold gas efficiency [3] was determined as follows:

$$CGE=\frac{m_{g}LH{V}_g}{m_{c}LH{V}_c}\times 100(\%)$$

(8)

where m_g, m_c are the masses of gas and char, respectively (kg), whereas LHV_g and LHV_c are the lower heating values of gas and char, respectively (MJ/kg).

3. Results and Discussion

3.1. Raw Fuel Characterization

Table 1. Proximate and ultimate analysis of raw fuels (% dry).

Sample	Volatiles	Fixed Carbon	Ash	C	H	N	O	S	GCV * (MJ/kg)
AL	36.7	27.5	35.8	27.5	2.1	0.5	32.9	1.2	8.9
GCW	75.8	15.6	8.6	41.9	6.1	0.8	42.4	0.2	16.6
PN	76.8	17.5	5.7	47.8	6.9	0.2	39.3	0.05	20.1

* GCV: gross calorific value.

, presenting the proximate and elemental analyses of the fuels studied, shows that AL was low-quality lignite, highly oxygenated and with a large amount of ash (35.8%), which lowered its calorific value. Its sulfur concentration was also considerable, revealing some emissions of sulfur compounds, such as H₂S, during gasification. On the other hand, both biomass fuels were rich in volatiles (~76–77%), had very low polluting sulfur and nitrogen content, a much smaller amount of ash (5.7–8.6%) and accordingly, an increased calorific value.

From the chemical analysis of ashes expressed as oxides in Figure 2, it can be observed that the ash of lignite contained a large amount of Si and a small amount of alkali Ca, Mg, K and Na, as opposed to the ash of biomass materials, which were rich in Ca, Mg, K and Na, especially the ash of GCW. These alkali metals are reported to present catalytic activity during the gasification process, while Si and Al are known to suppress the reaction rate [3,36].

The characteristic fusion temperatures of ash obtained by the heating microscope are compared in Figure 3. Initial deformation temperatures (IDT), ranging between 1050 °C and 1290 °C, are considered low for systems operating above 1000 °C, implying deposition problems. The fusion temperatures of lignite ash were much higher, reflecting its greater Si content, and lower in alkali species. Furthermore, it is interesting to note that GCW presented the lowest temperature difference between IDT and fluid (FT) temperatures, being known to induce a higher fouling rate in boilers [23].

According to these results and the chemical analysis of ashes of mixtures (

Table 2. Chemical analysis of ashes of mixtures.

Sample	CaO	Al2O3	Fe2O3	MgO	SiO2	K2O	Na2O	P2O5
AL/GCW 70:30	6.6	10.4	8.1	3.9	43.8	14.0	1.3	0.8
50:50	6.5	8.6	5.8	6.4	37.1	20.9	2.0	1.1
AL/PN  70:30	11.2	9.9	8.6	1.2	46.4	2.9	0.4	1.4
50:50	14.1	7.7	6.6	2.0	41.5	2.3	0.7	2.1

), the slagging/fouling indices of all fuels and lignite/biomass blends were calculated (Equations (1)–(5)) and included in

Table 3. Slagging/fouling indices of fuels and deposition tendency.

Sample	AI	B/A	Rs	Sv	Fs	Deposition Tendency
AL		0.32	0.38	74.4	1316	low
GCW	0.19	2.51	0.50	51.6	1076	high
PN	0.006	0.93	0.05	51.9	1222	medium to high
AL/GCW 70:30		0.70	0.63	69.6	1251	medium
50:50		0.96	0.67	66.0	1200	medium
AL/PN  70:30		0.45	0.38	68.5	1296	low to medium
50:50		0.55	0.34	64.1	1272	medium

. As can be seen, the deposition tendency of AL is predicted to be low; however, that of biomass wastes is medium to high. To keep the slagging/fouling propensity low to medium, the biomass percentage in the mixture should not exceed 30%.

3.2. Pyrolysis Products Characterization

The characterization of the solid products, after fuel devolatilization up to 600 °C, is presented in

Table 4. Proximate and ultimate analysis of chars (% dry).

Sample	Organic Matter	Ash	C	H	N	O	S	GCV (MJ/kg)
AL	48.2	51.8	38.7	1.3	0.5	6.4	1.3	14.3
GCW	73.1	26.9	65.3	1.9	0.7	5.2	-	23.9
PN	79.9	20.1	63.5	1.9	0.8	13.7	-	22.4
AL/GCW 70:30	55.0	45.0	46.5	1.6	0.5	5.4	1.0	17.4
50:50	60.9	39.1	52.4	1.6	0.6	5.5	0.8	19.3
AL/PN  70:30	58.1	41.9	46.3	1.4	0.5	9.0	0.9	16.9
50:50	64.6	35.4	51.5	1.6	0.6	10.2	0.7	18.5

. Thermal decomposition resulted in the removal of H- and O- volatile compounds, leaving a char material enriched in carbon and mineral matter. The calorific value was increased in comparison to raw fuels and the higher value was obtained for GCW char. When AL lignite was mixed with the biomass materials, an upgraded char with higher organic matter and lower ash content was produced, resulting in an enhanced heating value.

The composition of the gaseous products of pyrolysis, as measured by the TG–MS system, is shown in Figure 4, whereas the higher heating values of chars, oils and gases from the fuels under study are compared in Figure 5. The principal constituents of evolved light gases were CO and CO₂, with lower quantities of CH₄, H₂ and C_xH_y, which were formed at higher temperatures, from the cracking of stronger aliphatic or aromatic bonds [37]. As seen in Figure 5, the higher heating value of PN gas was greater (13.4 MJ/m³), due to its higher content in CO (~90%), satisfying the requirements in thermal energy for the process [8]. In addition, the calorific value and the yield of bio-oil produced from this fuel were the highest among the samples, reaching values 33.8 MJ/kg and 56.3%, respectively. Nevertheless, the calorific values of both GCW and PN bio-oils, ranging between 25 MJ/kg and 34 MJ/kg, were higher than values reported for other biomass materials [38].

3.3. Gasification Characteristics of Lignite and Biomass Chars and Their Mixture

Figure 6 compares the DTG (differential thermogravimetric) profiles of the lignite and the two biochar materials as a function of temperature and

Table 5. Gasification parameters of fuels and blends (dry basis).

Sample		AL	GCW	PN	AL/GCW 70:30	AL/GCW 50:50	AL/PN 70:30	AL/PN 50:50
Tmax (°C)		860	893	890	923	937	910	917
Rmax × 102 (min−1)		1.4	8.0	3.9	1.7	2.0	2.0	2.4
RG × 104 (min−1/°C)		0.16	0.89	0.44	0.19	0.21	0.22	0.26
Gas (mol%)	CO	93.4	92.3	92.2	93.2	92.9	93.1	93.0
	H2O	5.0	5.9	6.3	5.1	5.3	5.3	5.5
	H2	1.3	1.5	1.1	1.4	1.5	1.3	1.2
	CH4	0.1	0.08	0.1	0.1	0.1	0.1	0.1
	CxHy	0.2	0.2	0.2	0.2	0.2	0.2	0.2
HHV (MJ/m3)		12.2	12.1	12.0	12.2	12.1	12.1	12.1
CE (%)		90.0	100.0	100.0	92.4	94.1	92.0	94.5
CGE (%)		31.8	41.2	40.1	30.9	31.5	33.6	35.0

summarizes the characteristic parameters derived from the processing of these curves, along with the average composition of product gas between 600 °C and 1000 °C and its higher heating value, for all fuels and blends studied. As can be observed, a Boudouard reaction occurred above 750 °C. GCW and PN biochars presented a peak inflection temperature around 890 °C and displayed a much higher rate than the lignite AL. The maximum rate of GCW was 2-fold higher than that of PN and about 6-fold higher than that of AL. The reactivity of fuels followed the order: GCW > PN > AL. Additionally, in the case of AL, the conversion of organic matter to gas was not complete and the cold gas efficiency was lower (31.8%) than the corresponding GCW and PN samples (41.2% and 40.1%), which is in agreement with earlier data [10]. This low-reactivity behavior of AL char is associated with its high-ash content, richness in silicon and aluminum inhibitors to the gasifying agent, as shown in Figure 2. Accordingly, the conversion of AL lignite was reduced compared to the GCW and PN samples, whereas product gas composition was not affected by the mineral matter. On the other hand, the great reactivity of GCW char and consequently, the final conversion are correlated to its enhanced specific surface area (57.4 m²/g) as also demonstrated by past investigations [1,15,36], as well as its high concentration in inherent alkali K and Na (Figure 2), which are known to exhibit a catalytic effect during gasification [1,14]. Finally, Table 5 shows that the higher heating value of generated gas was practically the same for all fuels, because the principal gas component was CO and it was higher in comparison to other gasification processes, such as those using steam as the gasifying agent. Previous research by the authors has shown that the higher heating value of gas generated by steam gasification of various biomass fuels varied between 9.6 MJ/m³ and 11.4 MJ/m³ [39]. The amounts of H₂, H₂O, CH₄ and C_xH_y were minor.

Upon the gasification of coals with biomass materials, synergistic effects are known to take place, when different results are obtained in comparison to those achieved from the weighted average values of the individual fuel components [4,19,34]. These mutual interactions are the result of several complex mechanisms and are favored when the fuels react in the same temperature regime. Blends of feed ratio, gasifying agent, temperature, physical structure and chemical composition of component fuels are the important controlling parameters [20]. During current experiments, it was observed that when the lignite was mixed with the biomass materials at percentages up to 50%, the gasification behavior of the GCW and PN mixtures was different. From Table 5 and Figure 6 and Figure 7, it can be seen that the characteristic parameters of the AL/PN mixtures, such as reaction rate and reactivity, were very close to the weighted average values, implying additivity effects between component fuels. However, for AL/GCW blends, peak position was displayed at a higher temperature than the theoretically expected value and reaction rate and reactivity were much lower than expected from the contribution of each fuel, suggesting synergy effects between AL and GCW materials. Taking into consideration that operating conditions, reaction regime and blending ratios were the same for the chars studied, the reasons behind the behavior of AL/GCW mixtures could be found in the structure and composition of the fuels. Although the higher specific surface area of GCW and its high content in K and Na which exhibited a catalytic effect, as previously shown, were expected to significantly increase the reactivity of the blends, this was only slightly improved and did not show a linear relationship with the amount of blending. It is possible that the increased percentage of silicon and aluminum of lignite ash had a negative effect on alkali catalysts, due to blocking of the metals by forming aluminosilicates, as reported by some previous studies [1,20,34]. Additionally, the O/C molar ratio of AL–GCW char blends (Table 4) was much lower (0.07–0.09) than that of AL–PN blends (0.14–0.15) or the lignite (0.12), indicating a more ordered aromatic structure with greater stability [3], leading to reduced reactivity. Further investigations are needed to provide some insights into such synergistic mechanisms.

Nevertheless, the upgraded chars produced after blending lignite with the biomass fuels and pyrolyzing at 600 °C, having higher organic matter and lower ash than the lignite, presented increased gasification reactivity and enhanced conversion. Additionally, the cold gas efficiency was increased from 31.8% (for lignite) to 35% (blends), mainly due to the larger amount of gas produced, whereas the higher heating value of product gas reflected the higher heating value of CO too, for all mixtures. Generated CO gas can be utilized as a fuel for industrial combustion purposes, or as a reagent for the production of hydrogen via the water–gas shift reaction, of methanol by catalytic processes, of liquid fuels via Fischer–Tropsch synthesis and of various chemicals [1,12,40].

4. Conclusions

AL lignite, having a high amount of ash (35.8%), was of low quality, with a calorific value 8.9 MJ/kg. Biomass materials, GCW and PN, were rich in volatiles and their calorific values were 16.6 MJ/kg and 20.1 MJ/kg, respectively. The slagging/fouling propensity of AL ash was low, whereas that of biomass wastes was medium to high.

Thermal treatment of the samples and their blends prior to gasification produced upgraded fuels in comparison to raw materials. The higher heating value of solid, liquid and gaseous products of this process varied between 14.3–23.9 MJ/kg, 24.7–33.8 MJ/kg and 9.8–13.4 MJ/m³, respectively.

A Boudouard reaction occurred above 750 °C. The maximum rate of GCW was 2-fold higher than that of PN and about 6-fold higher than that of AL. Gasification reactivity followed the order: GCW > PN > AL. AL/PN mixtures presented additivity effects, however AL/GCW mixtures presented synergy effects. When the lignite was blended with the biochars studied, its conversion increased from 90% to 94.5% and its cold gas efficiency from 31.8% to 35%. The generated gas attained a heating value of 12–12.2 MJ/m³, as CO was the main component.