Conversion of Biomass to Energy-Rich Gas by Catalyzed Pyrolysis in a Sealed Pressure Reactor

Abstract

The purpose of this work is to present a technologically feasible method for processing different types of biomass into energy-rich gas with a methane content higher than 60 vol.% or a hydrogen content higher than 65 vol.%. Selected biomass samples with different compositions were tested. Samples with an operational particle size were pyrolyzed under well-defined conditions in a sealed pressure reactor, and the influence of process parameters on the gas yield and composition was evaluated. Different metal catalysts were used. It was found that, depending on the catalyst used, slow catalyzed pressure pyrolysis at a final temperature of 400 °C yields a methane-rich gas with up to 70 vol.% CH₄ or a hydrogen-rich gas with up to 75 vol.% H₂. In addition, by-products (oil and biochar) were analyzed and their uses described. In conclusion, under energy-saving conditions, pressure-catalyzed pyrolysis of biomass provides energy-rich gas and other usable products.

1. Introduction

Biomass is one of the largest sources of renewable energy available. The main sources are undoubtedly waste from logging and wood processing, as well as bio-waste from large-scale agricultural production, but these are currently still underutilized. The advantages of biomass waste are its low cost and good availability in sufficient quantities [1], especially for processing into energy. A recent comprehensive study focusing on the conversion of lignocellulosic biomass into valuable end products for decentralized energy solutions presents pyrolysis as one method to efficiently convert raw biomass into biofuels while minimizing the release of harmful substances [2]. For efficient biomass decomposition, fast and slow pyrolysis are the primary methods considered [3,4] and compared [5]. It can be concluded from these studies that slow pyrolysis promotes a higher gas yield, which further increases with increasing maximum pyrolysis temperature (e.g., up to 870 °C, where the gas yield was 41 wt.% and was the main product) [5]. However, this conclusion was reached for pyrolysis without a catalyst. Under certain conditions, slow, low-temperature catalyzed pyrolysis, which also produces solid, liquid and gaseous products, can be an effective solution for the easy treatment of biomass waste. In this context, one possible method for processing various types of biomass is slow catalyzed pyrolysis in a sealed reactor conducted at a final temperature not exceeding 400 °C. Therefore, this work focuses on catalyzed pyrolysis of biomass in a sealed (pressure) reactor and on two quantitatively important products: pyrolysis gas as an energy-rich gas and biochar.

Pyrolysis gas is a gaseous renewable energy source produced from the above feedstocks, consisting mainly of methane and CO₂. Due to its methane content, this gas can be used for energy purposes. The advantage is that, once the CO₂ has been removed, it can be compressed for further use in the same way as natural gas. Pyrolysis gas can be purified and treated to produce a biomethane-like gas or an energy-rich gas similar to biomethane [6].

Biochar has gained a lot of attention due to its potential uses and environmental benefits. It can be obtained from various types of organic materials such as wood chips, agricultural waste or sewage sludge. One of its production methods is pyrolysis, which results in a porous carbonaceous material with unique properties that make it a promising tool for improving soil fertility, filtering water to improve its quality and preparing catalysts. Above all, biochar has the potential to be used as a biofuel for renewable energy production [7].

This work uses ruthenium on alumina to catalyze the pyrolytic decomposition of biomass. In [8], the authors investigated the effect of ruthenium loaded on alumina on methane-producing reactions and concluded that such a catalyst promotes methane formation. This finding is confirmed in this biomass decomposition study. Furthermore, palladium loaded on charcoal (Pd/C) was used to catalyze hydrogen-yielding reactions. Some aspects of the use of palladium for hydrogen production are presented in [9]. In our work, a Pd/C catalyst was used to increase the hydrogen content in the gas produced by biomass pyrolysis.

The main results of this work are the distribution and composition of the main products—pyrolysis gas and biochar—their energy content and the physicochemical properties achieved by slow, catalyzed pressure pyrolysis. Methane and hydrogen are considered to be the main components of the pyrolysis gas obtained, although hydrogen can also be the main component under the action of a suitable catalyst. This possibility is given special attention in the present work.

2. Materials and Methods

2.1. Materials

Experiments were performed using apricot stones and walnut shells (Central Bohemian Bio-waste Comp., Prague, Czech Republic). The proximate, ultimate and biochemical analyses of the wastes used are given in

Table 1. Proximate analysis and organic elemental analysis of apricot stones (AS) and walnut shells (WS) used (as received, wt.%). VM—volatile matter; FC—fixed carbon.

	Water	Ash	VM	FC	H	C	Sorg	N	O
AS	4.87	2.30	81.53	11.30	7.78	56.08	0.17	6.63	22.17
WS	7.86	2.60	71.53	18.02	5.88	48.50	0.05	1.30	33.81

and

Table 2. Biochemical analysis of apricot stones (AS) and walnut shells (WS) used (dry, wt.%).

	Lignin	Cellulose	Hemicellulose	Extractives
AS	31.33	25.37	29.45	11.44
WS	48.07	25.71	22.07	2.33

.

As catalysts, palladium loaded on charcoal with a Pd loading of 10% (Merck Sigma-Aldrich CZ, s.r.o., powder) (further Pd/C catalyst) and ruthenium dispersed on a porous Al₂O₃ matrix (alumina) with a Ru loading of 5 wt.% (Merck Sigma-Aldrich CZ, s.r.o., powder with moisture content < 2 wt.%) (further Ru/Al₂O₃ catalyst) were used.

2.2. Methods

The biomass samples were pyrolyzed in a laboratory unit consisting of a sealed pressure cylindrical reactor, sample vessel, thermocouple system, pressure sensor, relief valve, measuring panel and control and registration PC (Figure 1). The sealed stainless-steel reactor, with an internal volume of 500 cm³ and designed for pressures up to 50 bar, is closed with a steel lid and a flange with a special polymer seal and equipped with a valve for gas sampling. A 90 cm³ cylindrical stainless-steel vessel containing approximately 25 g of sample (grain size 2–3 mm) and 5 g of catalyst is suspended inside this reactor, with the sample positioned approximately at the center of the reactor. Before the experiment, the reactor is flushed with an inert gas, then a temperature of 550 °C is applied to the wall of the reactor. The sample thus reaches 400 °C (end temperature) in about 30 min, while the pressure of the resulting gas reaches a maximum of 18 bar. Subsequently, the reactor heating is turned off and the reactor is allowed to cool to room temperature. The liquid product is collected in a cooled, calibrated glass vessel at the bottom of the reactor. After pyrolysis, the total gas passes through alumina pellets (desiccant) and a commercial carbon sorbent (CO₂ sequestration) and is analyzed as the gas obtained (resulting gas). The reactor is then disassembled, and the resulting biochar and liquid products are weighed and analyzed; the total gas is determined by difference.

Analysis of the gas obtained was performed on two Agilent Technologies 6890N gas chromatographs. O₂, N₂ and CO were analyzed on a HP-MOLSIV capillary column (40 °C) with helium as the carrier gas (5 cm³ min⁻¹) using TCD; methane and other hydrocarbons on a GS-GasPro capillary column (60 °C) with nitrogen as the carrier gas (20 cm³ min⁻¹) using FID (air = 400 cm³ min⁻¹; H₂ = 30 cm³ min⁻¹; N₂ = 20 cm³ min⁻¹); CO₂ on a GS-GasPro capillary column (40 °C) with helium as the carrier gas (5 cm³ min⁻¹) using TCD; and hydrogen on an HP-5 capillary column (40 °C) with nitrogen as the carrier gas (7 cm³ min⁻¹) using TCD.

Bio-oil composition was determined using the GC-MS method on an Agilent Technologies 6890 chromatograph with an MSD 5973 mass spectrometer. The gas chromatograph was equipped with a DB-XLB capillary column 30 m in length and 0.25 mm in diameter. The carrier gas was helium. For the first minute, the column was maintained at a temperature of 50 °C, after which the temperature was increased to 280 °C or 300 °C with a temperature gradient of 10 °C min⁻¹; the subsequent hold at 280 °C or 300 °C lasted 6 min.

The biochar obtained was analyzed according to ISO standards: ČSN ISO 16948 [10], ČSN ISO 16994 [11], ČSN EN ISO 18122 [12] and ČSN EN ISO 18134-2 [13].

3. Results

3.1. Mass Balance of the Process

First, the mass balance of the process was determined. The results are summarized in

Table 3. Typical mass balance of the slow pyrolysis of apricot stones (AS) and walnut shells (WS), alone and with Ru/Al₂O₃ and Pd/C catalysts, in a sealed reactor (wt.%). The end temperature was 400 °C, WS = AS = 100%.

Waste	Catalyst	Biochar	Bio-Oil	Water	Total Gas
AS	without	36	12	25	27
	Ru/Al2O3	36	13	23	28
	Pd/C	32	12	23	33
WS	without	40	5	28	26
	Ru/Al2O3	43	12	20	25
	Pd/C	35	3	27	35

. The table shows that, quantitatively, the main products are biochar and gas. Although the water yield is also relatively high, it was not considered further because the products with high utility value are biochar, gas and bio-oil. A favorable feature of pyrolysis in a sealed reactor is that it does not exhibit any losses. Based on this mass balance, attention was mainly paid to the composition and properties of the gas obtained and the biochar.

3.2. Composition and Properties of Pyrolysis Gas

At a final temperature of 400 °C, the composition of the gas obtained—after drying and removal of most of the CO₂—was quite favorable using Ru/Al₂O₃ catalysis in both AS and WS pyrolysis, as the methane content was relatively high (63 and 70 vol.%, respectively;

Table 4. Composition of the resulting gas from the pyrolysis of walnut shells (WS) and apricot stones (AS) with catalysts in a sealed pressure reactor (vol.%).

Waste	Catalyst	CH4	C2H4	C2H6	C3H6	C3H8	ΣC4	ΣC5	N2	CO	CO2	H2
	-	21.35	0.02	0.05	0.00	0.01	0.01	0.01	0.28	41.97	0.17	36.13
AS	Ru/Al2O3	62.95	0.05	0.26	0.01	0.02	0.02	0.01	0.09	16.86	6.44	13.29
	Pd/C	1.66	0.02	0.38	0.04	0.17	0.01	0.07	1.33	7.41	6.50	72.41
	-	30.85	0.01	0.02	0.00	0.02	0.02	0.02	3.96	22.50	0.16	42.44
WS	Ru/Al2O3	69.94	0.03	0.11	0.00	0.01	0.01	0.00	0.41	8.22	6.59	14.68
	Pd/C	0.09	0.03	0.08	0.01	0.04	0.01	0.03	0.02	22.67	2.06	74.96

). It can be inferred that this is because a sealed reactor, maintained at a given temperature for a longer period of time, maximizes intermolecular collisions while providing the necessary activation energy for secondary reactions.

However, completely different results were achieved using the second catalyst tested, i.e., in the case of Pd/C catalysis. In this case, the methane content was very low, but the hydrogen content was high, 72 vol.% for AS pyrolysis and 75 vol.% for WS pyrolysis, which is an unexpected finding. Thus, with Pd/C, the utility properties of the gas obtained, namely, higher heating value (HHV), lower heating value (LHV) and gas density (d), were significantly reduced; on the other hand, hydrogen-rich gas was obtained for further utilization.

No oxygen was found in the resulting gas, but in one case it contained almost 4 vol.% nitrogen. Since the input biomass contained nitrogen (Table 1) and the ashes from the ASs and WSs also contained nitrogen (3.09 and 2.16 wt.%, respectively), it is clear that the tested biomass contained N-functionalities. Therefore, the resulting gas was tested for ammonia (using the Nessler test) and methylamine (by GC). Although the results were negative, it can be deduced that the source of the N₂ was N-functionalities heterogeneously dispersed in the feedstock.

The changes in the utility parameters with the catalysts used are shown in

Table 5. Properties of the resulting gas from the pyrolysis of walnut shells (WS) and apricot stones (AS) with catalysts in a sealed pressure reactor. HHV—the higher heating value; LHV—the lower heating value; d—gas density.

Waste	Catalyst	CH4 (vol.%)	H2 (vol.%)	HHV (MJ m−3)	LHV (MJ m−3)	d (kg m−3)
AS	Ru/Al2O3	62.95	13.29	29.22	26.47	0.81
	Pd/C	1.66	72.41	11.48	9.93	0.33
WS	Ru/Al2O3	69.94	14.68	30.88	27.84	0.76
	Pd/C	0.09	74.96	12.65	11.16	0.40

.

3.3. Composition and Properties of Biochar

Biochar can be used in a wide range of applications, from heat and energy production to soil treatment. The properties of charred biomass depend on the feedstock and process conditions. However, the selection of suitable process conditions requires knowledge of the influencing factors, both quantitatively and qualitatively. Production processes include torrefaction and slow pyrolysis at different final temperatures. Extensive evaluation of process conditions has shown that temperature is the most important of all process parameters. Temperature has a dominant effect on the quality of the final biochar; in particular, the temperature range between 200 and 400 °C usually causes key changes in biomass decomposition. Therefore, the heating rate, residence time and, if necessary, the final temperature delay during this interval must be carefully controlled. In this study, the heating rate was always 16.5 ± 1.5 K min⁻¹ within the specified temperature range, the residence time was 30 min and the dwell time at the final temperature (400 °C) was zero. In this way, good or at least acceptable yields were achieved.

Although biochar, as already mentioned, can be used in various ways (biofuel, sorbent, soil additive, catalyst carrier), biofuel is probably its most important use. Therefore, all characterization parameters, including the HHV and LHV, were determined for the obtained biochar (

Table 6. Proximate analysis, organic elemental analysis (wt.%), higher heating value (HHV, MJ kg⁻¹) and lower heating value (MJ kg⁻¹) of the obtained biochar.

Waste	Water	Ash	VM	FC	Stotal	C	H	N	Sorg	O	HHV	LHV
AS	2.18	2.99	8.65	86.18	0.07	91.27	2.80	1.84	0.06	4.03	34.97	34.36
WS	2.05	2.48	9.41	86.06	0.03	90.09	2.76	1.25	0.03	5.87	34.18	33.58

).

Table 6 shows that under the above conditions, a high-quality biofuel was obtained, as it has a low ash content (2 wt.%) and very low sulfur content (0.03 and 0.07 wt.%). On the other hand, it has high a HHV and LHV of 34–35 and 33–34 MJ kg⁻¹, respectively. It therefore has better energy properties than conventional hard coal, whose HHV and LHV are typically 28–30 MJ kg⁻¹ and 27–29 MJ kg⁻¹, respectively. In addition, biochar has a low ash and sulfur content. It can therefore be concluded that under appropriate, well-defined conditions, the biomass under study provides a readily usable biofuel.

4. Discussion

4.1. Composition of Bio-Oil

Although bio-oil yields were relatively low (Table 3), the knowledge gained regarding biogas and biochar should also be supplemented. Therefore, a GC-MS analysis of bio-oils obtained from ASs in the presence of catalysts was carried out. In contrast to the oil obtained with the Pd/C catalyst, the oil yield was higher with ASs (12–13 wt.%, Table 3), so only bio-oils from ASs were analyzed. The composition is presented in

Table 7. GC-MS analysis of the bio-oil from apricot stone pyrolysis in a sealed reactor without a catalyst and with Ru/Al₂O₃ and Pd/C catalysts.

Catalysts	Without	Ru/Al2O3	Pd/C
Compounds	wt.%
Acetic acid	3.47	0.00	7.35
Alkylphenols	1.83	0.00	3.22
Methoxyphenols	8.23	14.51	15.87
Methyl palmitate	0.00	3.52	2.70
Methyl stearate	25.38	27.08	26.83
Methyl arachidate	0.00	0.52	0.00
Alkyl phthalates	0.00	0.01	0.00
Hydroquinone	1.81	1.93	1.67
Aliphatic hydrocarbons C9–C17	31.56	23.09	15.45
Aliphatic hydrocarbons C18–C31	8.02	8.12	7.55
Alkyl cyclohexanes	0.00	0.11	0.00
Alkyl benzenes	0.21	4.96	0.24
Naphthalenes	0.00	0.59	0.00
Sum	80.51	84.43	80.88

.

Table 7 shows two important facts: both tested catalysts promote the formation of methoxyphenols and, conversely, significantly reduce the liquid hydrocarbons content in the obtained bio-oil. Regarding the promotion of methoxyphenol formation, Zhao et al. (2023) [14] reported that Pd-based catalysts promote the conversion of lignin to guaiacols in a hydrogen atmosphere with a very high selectivity of 89%. Thus, our finding is consistent with this recent study. However, another problem is the reduction in C9–C17 hydrocarbons content. This phenomenon must be directly related to the high methane content in the resulting pyrolysis gas in the case of the Ru/Al₂O₃ catalyst and the high hydrogen content in the case of the Pd/C catalyst (Table 4). It is certain that under pressure and at temperatures up to 400 °C, the Ru/Al₂O₃ catalyst can promote the cleavage of lignocellulosic structures to form methane. Although this ability has been demonstrated under extreme conditions [15], it is highly likely that such cleavage can also occur in a sealed reactor under conditions favorable to secondary reactions. Thus, thermal decomposition of lignocellulosic biomass produces C₉–C₁₇ hydrocarbons, some of which further decompose into methane, as shown in Figure 2 using the example of birch wood. This methane formation is likely significantly promoted by the Ru/Al₂O₃ catalyst.

Regarding the high hydrogen content in the obtained pyrolysis gas with the Pd/C catalyst (Table 4), both hydrogen evolution during lignocellulosic biomass pyrolysis (illustrated in Figure 2) and hydrogen adsorption and absorption on palladium [16,17] and charcoal [18,19] should be considered. Charcoal absorbs hydrogen, with the amount and rate depending on temperature and pressure. Thus, charcoal absorbs hydrogen especially at higher temperatures and pressures, but the amount is considered relatively small compared to palladium. On the other hand, its inner surface area can exceed 1000 m³ g⁻¹. Based on this, the mechanism of hydrogen formation can be outlined. During slow pyrolysis, equilibria are continuously established in the primary and secondary reactions. The continuously formed hydrogen is removed from the equilibria due to the sorption properties of the Pd/C catalyst. This catalyst thus supports the re-formation of hydrogen. As a result, a high concentration of hydrogen is in the total gas. The Pd/C catalyst therefore has two functions: it supports the decomposition of lignin to phenols [19] and the formation of hydrogen.

As can also be seen from Table 7, some compounds are represented relatively strongly in the obtained oil: methoxyphenols, methyl stearate and liquid hydrocarbons. Some methoxyphenols can be used in the manufacture of other chemicals, pharmaceuticals, plasticizers and dyestuffs. Methyl stearate can be used as a raw material for surface treatment in fabric spinning, an oiling agent for textiles, a rubber processing agent, a plastic lubricant, an additive for paints and inks and for surfactants. The uses of liquid hydrocarbons are generally known. Thus, even the obtained bio-oils can be useful.

4.2. Possibilities of Reuse or Regeneration of Catalyst Used

Ruthenium catalysts can be regenerated by chemical methods [20,21,22], and it seems that Ru/Al₂O₃ in particular can be reused. In this case, this is possible because the catalyst is located above the sample (Figure 1), so it is not mixed with the charge. The reusability of the catalyst was thus tested by performing WS pyrolysis five times with the same catalyst and determining the mass balance of each experiment. If the catalyst were not reusable, the yields of the individual products would have to change significantly (see Table 3: WS, biochar, bio-oil and water). The achieved result is evident from

Table 8. Mass balances of the pyrolysis of walnut shells (WS) with the repeatedly used catalyst Ru/Al₂O₃ in a sealed reactor (wt.%). SD—standard deviation.

Biomass	Catalyst	Test No.	1	2	3	4	5	Average	SD
WS	Ru/Al2O3	biochar	41.35	44.29	41.71	42.45	45.24	43.01	1.51
		bio-oil	12.46	11.34	12.12	12.73	12.17	12.16	0.47
		water	20.05	20.96	18.06	20.09	23.03	20.44	1.61
		total gas	26.91	25.07	24.87	24.33	25.55	25.34	0.79

.

Since no significant changes in the mass balance were recorded, it can be said that the possibility of reusing this catalyst is real.

The situation is completely different in the case of the Pd/C catalyst. Due to the large inner surface area of charcoal and the specific properties of the Pd lattice, the deactivation of the catalyst is high, and the catalyst must be regenerated. Generally, this catalyst is deactivated due to poisoning, carbon deposition and palladium particle aggregation. Pd/C can be reactivated by surface treatments and extraction followed by impregnation. For example, Pd is extracted from the spent catalyst using aqua regia, then the extracted Pd is re-impregnated onto fresh granular activated carbon using a wet method [23]. Recently, the regeneration of deactivated Pd/C catalysts was investigated by Liu et al. (2019) [24]. The presented method is based on heating the catalyst in an air flow at a temperature of 250 °C for an extended period of time. Although time-consuming, it seems suitable for regeneration under laboratory conditions. After modification, it can likely be considered a viable method to recover the catalytic activity of deactivated Pd/C catalysts used in biomass pyrolysis.