Processing of Lignocellulosic Waste Biomass via Fine Cr₂O₃-Catalyzed Pyrolysis in a Sealed Pressure Reactor

Abstract

The aim of the work is to present a technologically feasible method for processing waste biomass into synthesis gas for further use, namely for the synthesis of bio-methanol, which is considered an important renewable fuel. Walnut shells were tested as lignocellulosic waste. Samples with an operating particle size (0.5–3 mm) were pyrolyzed under well-defined conditions in a sealed pressure reactor, using chromium (III) oxide particles and fine particles as a catalyst. The effect of particle size on the yield and composition of resulting synthesis gas and biochar was tested. It was found that slow, catalyzed pressure pyrolysis at a final temperature of 400 °C provides synthesis gas with a H₂/CO ratio of up to 0.9 or up to 1.8 and biochar (35–39 wt.%) useful as biofuel. These products were analyzed and their use described. The residual product was greywater (25–35 wt.%). Overall, under energy-saving conditions (slow pyrolysis, final temperature of 400 °C), pressure and catalyzed pyrolysis of biomass provide usable products and acceptable residuum.

1. Introduction

The catalytic pyrolysis process is one of the key technologies for the sustainable production of fuels and chemicals from renewable sources or waste. The pyrolysis process is usually carried out at temperatures between 400–800 °C in an inert atmosphere without air supply and enables the conversion of organic substances into gases, oils, and a solid carbonaceous residue [1]. Compared to non-catalytic pyrolysis, the use of a catalyst provides greater control over product selectivity (especially oils), lower energy consumption, and a reduction in the formation of undesirable byproducts, such as tars or oxygenated compounds [1,2].

In recent years, the focus has been on the use of transition metal oxidation catalysts, among which chromium (III) oxide (Cr₂O₃) has an important role. Like other transition metal catalysts, chromium (III) oxide is characterized by high thermal stability, strong acid-base properties, and the ability to undergo redox transformations between the oxidation states Cr³⁺ and Cr⁶⁺, which enables the efficient cleavage of chemical bonds in organic molecules [3,4]. It can be used for both hydrocarbon dehydrogenation reactions and as a potential catalyst for pyrolysis processes, where C-C, C-H and C-O bonds are cleaved [5]. Furthermore, thermally modified Cr₂O₃ in the form of nanoparticles can exhibit increased catalytic activity due to a larger surface area, defect density, and tunable surface chemical properties. Such modified nano-Cr₂O₃ can be used for the catalytic oxidation of carbon monoxide at lower temperatures without doping with another metal oxide or a support [6].

Zhang et al. [7] investigated the effect of nine transition metal oxides, namely Co, Cr, Cu, Fe, Mn, Ni, Ti, V and Ce, on the pyrolysis of poplar wood. The results suggest that Ce, Cr, Cu and Fe oxides promote the decomposition reactions, leading to higher yields of some organics, with chromium (III) oxide strongly promoting CO₂ formation in the temperature range of 350–400 °C.

The morphology and particle size of Cr₂O₃ are the main factors influencing its catalytic behavior. Compared to the powdered form, nanostructured forms of Cr₂O₃ exhibit a much higher specific surface area, a higher concentration of defects, and greater availability of active sites, which leads to increased catalytic activity and selectivity [8,9]. It has been demonstrated that nano-Cr₂O₃ catalysts can reduce the activation temperature in pyrolytic reactions of fluorinated hydrocarbons and increase conversion while maintaining high product selectivity [10]. Similar trends can be expected in the processing of biomass and plastics, where the efficient cleavage of macromolecular structures plays a key role.

During pyrolysis of lignocellulosic biomass, Cr₂O₃-based catalysts primarily promote deoxygenation reactions, such as decarboxylation and decarbonylation, leading to the suppression of the formation of oxygen-containing liquid products and, consequently, to an improvement in the quality and stability of the resulting bio-oil [11]. Simultaneously, the yield of gaseous products, especially hydrogen and carbon monoxide, increases, which is important for the production of synthesis gas [12]. In the case of plastic waste, such as polyethylene or polypropylene, catalytic pyrolysis using Cr₂O₃ leads to the effective cracking of polymer chains and increased production of light olefins and aromatic hydrocarbons [13].

Kopač et al. [14] focused on the dehydrogenation of propane and butane using undoped Cr₂O₃/alumina and doped Cr₂O₃. Alkali metals and alkaline earth metals were used as dopants, substituting one active chromium atom site. The adsorption of propane and butane on all doped surfaces is similar to that on undoped Cr₂O₃. These metals act as active sites for all carbon intermediates, while hydrogen is adsorbed on neighboring oxygen atoms. Wegrzyniak et al. [15] achieved a 32 wt.% propene yield at 550 °C when a catalyst containing 20 wt.% Cr₂O₃/alumina was used.

A significant advantage of Cr₂O₃ over other catalysts, such as zeolites, is its relatively high resistance to deactivation by coking. This property is related to its redox abilities and stability at high temperatures, which enables a longer catalyst life in the demanding conditions of pyrolytic processes [16,17]. In addition, nanostructured forms have better thermal conductivity and a more homogeneous distribution of active sites, which further contribute to reducing coke formation [18].

Pressure is another important factor influencing the process of catalytic pyrolysis. Most laboratory studies are carried out at atmospheric pressure; however, a higher pressure can significantly affect both product distribution and reaction kinetics. Higher pressure usually promotes the formation of liquid products and increases conversion, but at the same time, can lead to diffusion limitations, especially with bulk catalysts [19].

Although the amount of research on catalytic pyrolysis is rising, the use of Cr₂O₃ in a fine form remains relatively under-explored compared to traditional catalysts such as zeolites or aluminum oxides. This suggests significant potential for further research, particularly in the areas of catalyst amount optimization, understanding reaction mechanisms at the atomic level, and, maybe, larger-scale applications. Therefore, the aim of this work is to present a technologically feasible method for processing waste walnut shells into synthesis gas and biochar under energy-saving conditions (slow pyrolysis, final temperature of 400 °C). The novelty of this study lies in the combination of three factors—the effect of fine catalyst, CO₂ removal, and the prolongation of retention time of volatile products in the reactor—into a single process that produces synthesis gas with a H₂/CO ratio approaching 2 and biochar as a high-quality fuel. Such gas is suitable for the synthesis of bio-methanol, which is an important biofuel.

2. Materials and Methods

To achieve the research goal, catalytic pyrolysis was carried out in a sealed reactor, which allows a relatively long residence time of obtained volatile products and thus promotes gas formation. Since the obtained gas contained a significant amount of CO₂, a carbon sorbent was added at the reactor outlet to remove CO₂ from the pyrolysis gas or at least reduce its content. By reducing the CO₂ content, a significantly more favorable composition of the pyrolysis gas was achieved with respect to the H₂/CO ratio, which is a key criterion for the quality of synthetic gas. In this case, waste walnut shells with a particle size of 0.5–3 mm (operational particle size) were pyrolyzed (see photo, Figure 1). The pyrolyzed operating fraction was obtained from the supplied waste using a screening machine (AS 200 control, Ekotechnika SRO, Prague, Czech Republic).

2.1. Materials

As mentioned above, the experiments were carried out with waste walnut shells (Central Bohemian Bio-waste Comp., Prague, Czech Republic). The detailed analysis and organic elemental analysis are given in

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

Waste	Water	Ash	VM	FC	H	C	Sorg	N	O
WS	7.86	2.60	71.53	18.02	5.88	48.50	0.05	1.30	33.81

; biochemical analysis of the waste used is summarized in

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

Waste	Ash	Lignin	Cellulose	Hemicellulose	Extractives
WS	2.82	48.07	25.71	22.07	2.33

. The reason for selecting walnut shells is that 4.5 million metric tons of them are produced annually worldwide [20], which is a considerable amount. Another reason was the high lignin content (see below, Section 3.4).

As catalysts, Cr₂O₃ (Merck Sigma-Aldrich CZ, SRO, Prague, Czech Republic) was used. Starting commercial powdered material was first ground on a Retsch (Verder Scientific, Haan, Germany) ZM 200 ultracentrifugal mill (6000 rpm) to a grain size below 0.35 mm, then the obtained fraction was sieved on a screening machine with the finest sieve of 0.15 mm (150 μm). The obtained fraction with a particle size of 0.74–7.25 µm (wet medium, Figure 2A) or 0.76–7.02 µm (wet medium, Figure 2B) was considered to be a fine Cr₂O₃ material. The particle morphology of used nano-Cr₂O₃ is shown in Figure 3 (SEM, magnification 20,000×).

Commercially available activated carbon SC 40 CAS 7440-44-0 (Brenntag CR, SRO, Prague, Czech Republic) was used to capture CO₂. This granular carbonaceous sorbent with a grain size of ~4 mm had a specific surface area of approximately 1000 m²/g (BET).

2.2. Methods

Walnut waste was pyrolyzed in a laboratory unit consisting of a closed stainless-steel reactor, a suspended sample vessel, temperature and pressure sensors, and a gas sampling valve. The internal volume of the reactor was 0.5 L. The waste samples with a grain size of 0.5–3 mm had a mass of 25 g while a layer of 2.5–7.5 g of Cr₂O₃ or fine Cr₂O₃ was placed above the sample layer (see below). The reactor was first flushed with a purge gas and then the reactor wall was heated to 550 °C. The sample thus reached a final temperature of 400 °C in approximately 27 min (no dwell time was applied), while the pressure of the released gas and steam reached a maximum of 22 bar. The pressure profile during pyrolysis is shown in Figure 4. The heating rate was approximately 14–19 °C min⁻¹, so that the pyrolysis proceeded slowly. After cooling the reactor to room temperature, the pyrolysis gas was collected and analyzed—after passing through a column with a commercial carbon sorbent used for CO₂ sequestration—as the resulting gas. The reactor was then disassembled and the obtained biochar and greywater were weighed and analyzed. The total gas was determined from the difference.

The total gas and the resulting gas were analyzed on Agilent Technologies 6890N gas chromatographs, while oxygen, nitrogen, and CO were analyzed on a 40 °C HP-MOLSIV capillary column using TCD. Carbon dioxide was determined on a 40 °C GS-Gaspro capillary column, again using TCD. CH₄ and gaseous hydrocarbons were analyzed on a 60 °C GS-Gaspro capillary column using FID. Hydrogen was analyzed on a 40 °C HP-5 capillary column using TCD. The ammonia content was demonstrated and determined by Nessler’s reagent.

From the composition of gas, its density was calculated. Calculating the density of synthesis gas (syngas) was important because it allowed the determination of the loss due to sorption and removal (or reduction) of CO₂ from pyrolysis gas. If the mass of syngas is

m_syngas,after = (d_syngas,after/d_{syngas,without}) × m_total (wt.%)

(1)

where d_syngas,after is the gas density after CO₂ sorption (kg/m³), d_{syngas,without} is the gas density without CO₂ sorption (kg/m³) and m_total is the total gas determined by difference, the

m_total − m_syngas,after = loss (wt.%)

(2)

Organic impurities in greywater were determined by GC/MS on an Agilent Technologies 6890 chromatograph with a DB XLB capillary column (30 m × 0.25 mm), equipped with an MSD 5973 mass spectrometer. The carrier gas was He. The temperature program was that the column temperature was initially 50 °C (for the first minute), then the temperature increased to 300 °C with a temperature gradient of 10 °C min⁻¹. The subsequent dwell at 300 °C was 6 min.

The biochar was considered as solid biofuel, therefore it was characterized according to ISO standards: ISO 16948—determination of total carbon, hydrogen and nitrogen, ISO 16994—determination of total sulfur, ISO 18122—determination of ash, and ISO 18134—determination of moisture. Further, ISO 18125 was used for determination of higher calorific value (HHV) and lower caloric value (LHV). Inorganic elemental analysis of biochar obtained was performed by XRF method using a Spectro IQ instrument (SPECTRO Analytical Instruments GmbH, Kleve, Germany). A palladium target with the angle set to 90° relative to the central beam was used. The excitation spot measured an area of 1 × 1 mm, with a maximum anode power of 50 W and forced air cooling at 10 cfm. The device was equipped with a HOPG Barkla crystal. Test samples were prepared by the pressed pellet method: 4.0 g of material (particle size 15–20 µm) was thoroughly mixed with 0.9 g of binder (HWC Hoechst wax, Germany) for 10 min. Pellets were pressed at a force of 80 kN.

Particle size analysis of the catalyst used was performed using a Microtrac Sync 5001 particle size analyzer (Microtrac Inc., Montgomeryville, PA, USA) with a measurement range of 0.02–2800 μm. The measurements were carried out in wet mode using the FlowSync module for sample delivery and dispersion in the measuring cell. Prior to analysis, the samples (50–100 mg) were dispersed in water by ultrasonic treatment for 30 s. Each sample was measured three times, and the reported particle size distributions represent the average values of the obtained measurements.

For elemental concentrations in the catalyst used, Energy-dispersive X-ray spectroscopy (EDS) was used on a Quanta 450 scanning electron microscope (FEI, Hillsboro, OR, USA) equipped with a Si(Li) Apollo XL Silicon Drift Detector with a FET preamplifier (EDAX Inc., Mahwah NJ, USA). Data acquisition was performed on EDAX TSL OIM software, v.7.0, with ZAF corrections.

3. Results and Discussion

3.1. Mass Balance of the Process

First, the mass balance of the process was determined. Since the products with high utility value were biochar and synthesis gas (syngas), they were considered as the key ones. Although the greywater yield was high (up to 35 wt.%), it was further considered only as a residue that needed further treatment. An important step was to determine the loss, which was the CO₂ removed by the sorbent. CO₂ must be removed from the pyrolysis gas or at least reduced because it negatively affects the H₂/CO ratio, which is important for the quality of the synthesis gas and its utilization.

An important item related to reducing the CO₂ concentration in pyrolysis gas is loss of the process. In the case of the experiment without using a sorbent, the loss was considered zero; in the experiment using a sorbent, the loss was calculated according to the above Equations (1) and (2). The results are summarized in

Table 3. Mass balance of slow pyrolysis of walnut shells (WS) with/without fine Cr₂O₃ catalyst and with/without sorbent in a sealed reactor (wt.%) (WS = 100%).

WS	Experiment	Biochar	Greywater	Gas	Loss
	without catalyst, without sorbent	36.18	33.58	30.24	0.00
	without catalyst, with sorbent	38.44	31.47	19.32	10.77
	with 20% powdered Cr2O3, without sorbent	35.38	32.24	32.38	0.00
100%	with 20% powdered Cr2O3, with sorbent	37.84	30.96	16.55	14.65
	with 10% fine Cr2O3, with sorbent	36.44	34.01	13.68	15.87
	with 15% fine Cr2O3, with sorbent	38.22	35.04	15.48	11.26
	with 20% fine Cr2O3, with sorbent	37.75	25.17	20.76	16.32
	with 30% fine Cr2O3, with sorbent	38.94	29.60	16.26	15.20

. From this table, it follows that the yields of biochar were satisfied (up to 38 wt.%); however, the synthesis gas yields were not very high when using powdered or fine Cr₂O₃ and sorbent simultaneously (up to 21 wt.%). Moreover, the loss was quite high in this case (up to 16 wt.%). This deficiency is compensated by the high quality of the synthesis gas, given by the relatively high H₂/CO ratio (see below).

3.2. Composition and H₂/CO Ratio of Pyrolysis Gas

Two approaches were used to investigate the properties of pyrolysis gas from a given biomass. First, the composition of pyrolysis gas from pyrolysis without catalyst and gas obtained with commercial chromium (III) oxide powder was compared with the composition of gas obtained with fine Cr₂O₃. Second, the composition of the gases was assessed from the perspective of synthesis gas. The results are summarized in

Table 4. Composition of pyrolysis gas or synthesis gas (syngas) from walnut shell (WS) pyrolysis with/without catalysts in a sealed pressure reactor (vol.%). No C₄ and C₅ hydrocarbons were detected. The amount of added Cr₂O₃ (wt.%) is related to the mass of biomass sample.

Experiment	Cr2O3	CH4	C2H4	C2H6	C3H6	C3H8	N2	CO	CO2	H2	H2/CO
without catalyst, without sorbent (pyrolysis gas)	0	10.77	0.20	1.37	0.17	0.40	0.34	17.93	54.23	14.59	0.81
without catalyst, with sorbent (syngas)	0	12.37	0.02	0.09	0.02	0.02	0.33	37.78	14.26	35.11	0.93
with powdered Cr2O3, without sorbent (pyrolysis gas)	20%	14.89	0.16	0.46	0.07	0.12	1.50	25.25	43.93	13.62	0.54
with powdered Cr2O3, with sorbent (syngas)	20%	12.68	0.02	0.03	0.01	0.01	1.06	37.42	14.16	34.61	0.92
	10%	5.30	0.01	0.04	0.02	0.03	1.21	31.22	8.24	53.93	1.73
with fine Cr2O3	15%	9.02	0.02	0.04	0.02	0.03	0.76	30.50	16.39	43.22	1.42
and with sorbent	20%	4.46	0.02	0.04	0.01	0.02	2.22	26.71	17.59	48.93	1.83
(syngas)	30%	9.83	0.00	0.02	0.02	0.02	4.73	32.20	8.78	44.40	1.38

.

Table 4 shows that the synthesis gas composition with fine Cr₂O₃ catalyst showed a low CH₄ concentration, 5–10 vol.%, but the H₂ concentration was relatively high, 43–54 vol.%, which is significantly higher than that of powdered Cr₂O₃. These values resulted in a H₂/CO ratio approaching 2. As follows from works [21,22,23], syngas with a ratio near to 2 is very suitable for the synthesis of promising biofuels, particularly bio-methanol [23].

Pyrolysis in a sealed reactor can be used to produce syngas even without a catalyst. The resulting syngas will have a H₂/CO ratio of less than 1 (Table 4), but even such syngas is usable. However, this work focuses on syngas approaching 2, suitable for bio-methanol production.

It is true that in only two cases out of four were H₂/CO values close to 2 achieved (Table 4). Such favorable values were recorded for synthesis gas with a low methane concentration, around 5 vol.%, and a higher hydrogen concentration, 49 or 54 vol.%. To explain this phenomenon, the properties of fine Cr₂O₃ need to be considered. In fine Cr₂O₃, compared to powdered Cr₂O₃, there can probably be a significant increase in the specific surface area, the number of defects at the edges and corners of crystallites, oxygen vacancies and surface-available Cr³⁺ ions. This probably can increase the total number of catalytically active sites, including Lewis and Brønsted sites [24,25,26,27]. The increased number of catalytically active sites can probably cause the decomposition of the volatile substances produced by pyrolysis to occur more deeply and to a greater extent, leading to a low concentration of methane and a higher concentration of hydrogen in the resulting gas. In this case, catalyst particles of 0.74–7.25 μm were used. It is likely that the results would be more favorable if smaller particles were used.

A significant advantage is the purity of the obtained syngas. No sulfur compounds or ammonia were detected. Note that H₂S, CH₃S, COS and CS₂, and methylamine were analyzed and in all cases their concentration was 0.00 vol.%. The same applies to ammonia.

3.3. Composition and Properties of Biochar

In this case, biochar as a biofuel was considered. A typical composition, HHV and LHV are summarized in

Table 5. Water, ash and organic elemental analysis (wt.%), higher heating value (HHV, MJ kg⁻¹) and lower heating value (LHV, MJ kg⁻¹) of biochar obtained.

State	Water	Ash	H	C	Sorg	N	O	HHV	LHV
as received	3.49	8.22	3.84	69.73	0.05	0.54	14.13	27.41	26.49
dry	-	8.52	3.98	72.25	0.05	0.56	14.64	28.40	27.53
dry ash free	-	-	4.35	78.98	0.06	0.61	16.00	31.05	30.10

and

Table 6. Inorganic elemental analysis of biochar obtained (XRF, wt.%).

State	Na	Mg	Al	Si	P	S	K	Ca	Zr	Bi	Te
dry	0.08	0.03	0.02	0.06	0.05	0.01	0.37	0.32	0.02	0.08	0.02

. These tables show that the obtained biochar had a low or at least acceptable ash content and a very low organic sulfur content (0.05–0.06 wt.%). Furthermore, high HHV and LHV values were found: 27–31 and 26–30 MJ kg⁻¹, respectively (for comparison, hard coal typically has an HHV and LHV of 28–30 MJ kg⁻¹ and 27–29 MJ kg⁻¹, respectively). Considering the results of inorganic analysis (Table 6), the obtained biochar represents a well-usable biofuel. In addition, there are a number of deashing methods derived from coal processing that can be used to improve both HHV and LHV.

3.4. Further Results. Discussion

To investigate the effect of chromium (III) oxide on synthesis gas production, walnut shells were selected mainly due to their high lignin content (48%) and low extractive content. Lignin was assumed to be a significant source of hydrogen during pyrolysis, which is important for the production of synthesis gas. Also, the low content of extractives (lipids, proteins, carbohydrates, etc.), which are common (although not always) in biomass, is a favorable feature. Moreover, walnut shells are high in cellulose and hemicellulose (in sum), as are other types of biomass (

Table 7. Biochemical analysis of birch wood shavings (BS), apricot stones (AS), peach stones (PS), and walnut shells (WS) (dry, wt.%).

	Lignin	Cellulose	Hemicellulose	Extractives
BS	23.01	44.58	27.27	2.54
AS	31.33	25.37	29.45	11.44
PS	37.11	40.16	18.50	4.23
WS	48.07	25.71	22.07	2.33

, our determinations). The selected biomass was therefore suitable for catalytic pyrolysis study. Zhang et al. [8] also used walnut shells for this purpose.

As above mentioned, the synthesis gas yield with H₂/CO > 1.4 was not very high when using fine Cr₂O₃ and sorbent simultaneously (14–21 wt.%, Table 3). A current study of biomass pyrolysis [28] reports a synthesis gas yield of 52 wt.%, achieved at a pyrolysis temperature of 550 °C and subsequent catalytic reprocessing of pyrolysis gas at 800 °C. The H₂ concentration in the obtained gas was 32.6–35.4 vol.% and CO was 22.0– 22.2 vol%, so the H₂/CO ratio was 1.47–1.61. The yield of solid residue (biochar) was 25 wt.%. The process is quite demanding technically and energetically. The process proposed in this study is energy-saving (pyrolysis temperature of 400 °C) and allows obtaining synthesis gas with a H₂/CO ratio up to 1.8 (Table 4), but its yield is significantly lower than in the work [28]. On the other hand, the yield of biochar is significantly higher, 36–39 wt.% (Table 3). Usually, biochar has an HHV between 25 and 35 MJ kg⁻¹ and LHV ranging from 24 to 33 MJ kg⁻¹ (dry state) [29]. In this work, the HHV of obtained biochar was 28 MJ kg⁻¹ and LHV 27 MJ kg⁻¹ in dry state (Table 5); moreover, very favorable ash composition was found (Table 6). Therefore, it can be used as a high-quality biofuel, comparable to bituminous coal (HHV 27–33 MJ kg⁻¹, LHV 25–31 MJ kg⁻¹).

An important problem is the production of greywater, which is generated in relatively large quantities (25–35 wt.%) during pyrolysis. The obtained water typically contained around 20 wt.% of organic impurities, which included acetic and propanoic acids, phenol, alkylphenols and methoxyphenols, benzoic acid and higher fatty acids and their esters (

Table 8. Organic impurities in greywater (wt.%).

Acetic + Propionic Acids	Phenol	Methyl-Phenols	Dimethyl-Phenols	Ethyl-Phenol	Methoxy-Phenols	Benzoic Acid	Fatty Acids + Their Esters	Total
0.98	1.41	3.75	2.11	1.01	4.87	0.79	5.62	20.54

). Such water can be purified by membrane filtration and using biological treatments (e.g., aerobic bacteria or membrane bioreactors). Then, the water is disinfected with UV light or chlorine to remove the remaining bacteria. Standardized procedures can be used for recycling, such as EN 16941-2, BS 8525-1:2010 and ISO 25446.

Another important issue is the application of the proposed method. The pyrolysis was carried out under its own pressure, which means that the pressure was created in a sealed reactor by the volatile products formed. The primary volatile products formed passed through the catalyst bed and underwent secondary reactions, which were supported by a relatively long retention time. Pyrolysis was slow because, as above mentioned, the heating rate was 14–19 °C min⁻¹. The temperature profiles in the reactor are shown in Figure 5. Such temperature profiles and pressure profile (Figure 4) can be achieved relatively easily for further laboratory research in a sealed reactor with a larger volume, e.g., 1.5 L (the reactor used had a volume of 0.5 L, see above), without major technical difficulties. On the other hand, these conditions have only been demonstrated for biomass with a high lignin content (48%). Therefore, other biomass, including those with higher extractable content, need to be investigated.

Chromium (III) oxide is one of the most commonly used heterogeneous catalysts, mainly due to its resistance to higher temperatures and relatively high specific surface area, which, in the case of fine Cr₂O₃, increases and is probably accompanied by an increase in the number of catalytically active sites. The catalyst used is readily available. It can be reused, which offsets, or at least partially offsets, the cost of the fine material. In this case, the fine material was prepared in-house from a low-cost commercial powder using an ultra-centrifugal mill and a screening machine (see above) and tested for particle size (see above; note: using of findings of [30]). The prepared fine catalyst was completely pure, as evidenced by energy dispersive X-ray spectroscopy (EDS method [31]) (

Table 9. Elemental analysis of fine Cr₂O₃ used (EDS method, K-line), 3 measurements.

	no.1		no.2		no.3		Average
Element	wt.%	Atomic %	wt.%	Atomic %	wt.%	Atomic %	wt.%	Atomic %
Cr	57.01	31.21	55.63	28.01	55.32	28.61	55.99	29.28
O	34.41	61.21	38.30	62.67	35.36	60.94	36.02	61.61
C	2.81	6.65	3.86	8.41	2.34	6.53	3.01	7.19
Cu	0.30	0.13	2.21	0.91	0.26	0.22	0.92	0.42

). Therefore, there is no need to purchase relatively expensive fine material, just the powder and its treatment, which saves the costs. Moreover, Cr₂O₃ is completely harmless to health, which is why it is used, for example, in cosmetics.

Furthermore, closed pressure reactors are commonly used in chemical research, and are affordable and adaptable to given conditions and circumstances. All the above facts indicate that the proposed method can be feasible even with larger laboratory reactors.

The question arises as to how fine Cr₂O₃ actually works. It is necessary to investigate the mechanism of catalyzed decomposition, which is a separate task. In our experience, the inner surface area of fine Cr₂O₃ should be at least 100 m² g⁻¹ to ensure a sufficient extent of decomposition. At such an extent, the cleavage of biomass structure can probably be manifested by increased hydrogen production at the expense of methane. It seems that in the case of biomass with a low extractives content, the problem of the decomposition mechanism will not be so difficult. However, it will be different for biomass with a high extractives content, e.g., apricot stones.

In any case, the production of synthetic gas with a H₂/CO ratio approaching 2 is influenced by three factors: the thermal cleavage of biomass, concentration of CO₂ and the effect of catalyst. This is illustrated by the trend in Figure 6, which was obtained in the presence of fine Cr₂O₃ under the conditions described above. The variable was CO₂ sorption, which was controlled by varying the amount of carbonaceous sorbent. At higher and high CO₂ concentrations (>15–55 vol.%), the resulting synthetic gas had a H₂/CO ratio of around 1; at lower CO₂ concentrations (15–8 vol.%), it was around 2; and at very low CO₂ concentrations (>8–1 vol.%), this ratio increased to as high as 7. Because the effect of thermal cleavage itself (i.e., without catalyst) at a lower CO₂ concentration (14 vol.%) led to the H₂/CO ratio of around 1 (Table 4, second row), it can be inferred that the effect of the catalyst is significant. The possible contribution of catalyst to the cleavage is also shown in Figure 7. From the trend of the hydrogen yield, depending on the amount of catalyst (related to the mass of feedstock), it is clear that without catalyst, the hydrogen yield is 1.95 wt.%, but with catalyst, it increases up to 2.8 wt.%.

Therefore, although the effect of the catalyst cannot be quantified in this experimental setup, it can be said that the contribution of fine Cr₂O₃ is significant in achieving a key H₂/CO ratio approaching 2.

Possibilities of re-use of the fine catalyst used are also important. As mentioned above, the catalyst was placed above the feedstock and was not mixed with it (Figure 8).

Thus, during pyrolysis, the raw gas passed through the catalyst bed. In this configuration, the same catalyst was used eight times and no changes in its composition were observed; practically, its composition was still the same as verified by energy dispersive X-ray spectroscopy. Furthermore, reuse was assessed from the mass balance using the standard deviations of the biochar, greywater and total gas yields (

Table 10. Statistical evaluation of fine Cr₂O₃ reuse (wt.%).

	Biochar	Greywater	Total Gas
mean (n = 8)	37.38	31.47	31.21
standard deviation	1.17	2.91	2.57
standard deviation (%)	3.13	9.25	8.23

).

From Figure 8 and Table 10, it can be concluded that reuse of the considered catalyst is possible because the standard deviation is lower than 10% for all three items. As repeatability of pyrolysis experiments is usually 10% and the standard deviation found was lower than 10%, it can be inferred that the reuse of catalyst did not produce different results in product yields, and thus the catalyst still functioned in the same way.

To compare the achieved results with the literature, it is appropriate to focus on the main product—synthesis gas. Synthesis gas is a key intermediate for the production of ammonia, methanol and synthetic fuels, and is produced mainly from natural gas and coal, but also from biomass. The main methods for producing syngas from biomass include (i) pyrolysis and/or steam reforming of volatiles in the presence mainly of a nickel catalyst; (ii) gasification, in which the feedstock is heated and converted to gas in the presence of partial oxygen or steam; (iii) partial oxidation with limited oxygen. The literature on synthetic gas production [32,33,34,35,36,37] agrees that catalysts play a key role, but the problem is their deactivation. Santamaria et al. [38] studied in detail the effect and deactivation of numerous catalysts on the decomposition of volatiles from lignocellulosic biomass and summarized the causes of deactivation as follows: poisoning, coking and fouling, sintering, component volatilization, inactive compound formation, phase transformation, and particle attrition. The following catalysts were considered: Ni/Al₂O₃ [32]; Ni/SiO₂ [33]; Ni/MgO, Ni/TiO₂ and Ni/ZrO₂ [34,35]; Co/Al₂O₃, Co/MgO, Co/TiO₂ and Co/ZrO₂ [36]; and Rh, Pt, Pd and Ru loaded on CeO₂-SiO₂ [37]. Based on the study of such a number of catalysts, the authors conclude that special attention should therefore be paid to studies addressing the mechanisms of catalyst deactivation and regeneration in order to achieve optimal catalyst discrimination. In this case, no signs of changes were observed in the fine Cr₂O₃ used (

Table 11. Elemental analysis of fine Cr₂O₃ after pyrolysis experiment (EDS method, K-line), 3 measurements.

	no.1		no.2		no.3		Average
Element	wt.%	Atomic %	wt.%	Atomic %	wt.%	Atomic %	wt.%	Atomic %
Cr	56.53	28.39	58.35	29.94	55.70	28.10	56.86	28.81
O	38.30	62.53	39.21	65.39	38.38	62.94	38.63	63.62
C	3.86	8.40	2.02	4.49	3.67	8.03	3.18	6.97
Cu	1.31	0.68	0.42	0.18	2.25	0.93	1.33	0.60

, Figure 9; compare with Table 9 and Figure 3), even when it was reused. On the other hand, experiments were only performed on a laboratory scale and the behavior of the catalyst may not be as favorable on a pilot scale. However, it is worth conducting further research into the effect of the given catalyst on the decomposition of volatiles from different types of biomass.

Finally, biochar has a wide range of potential applications, including its use as a renewable fuel, adsorbent, soil amendment or catalyst carrier. In this study, special attention was paid to its suitability as a biofuel, and therefore biochar was comprehensively characterized (Table 5 and Table 6). In this context, it should be emphasized that the catalyst used did not affect either the composition or HHV and LHV of the resulting biochar.

4. Conclusions

Walnut shells were tested as a lignocellulosic waste material. Samples with particle sizes ranging from 0.5 to 3 mm were pyrolyzed under well-defined conditions in a sealed pressure reactor using chromium (III) oxide particles or fine particles as a catalyst. As observed results, it was found that slow catalytic pressure pyrolysis at the final temperature of 400 °C leads to the formation of synthesis gas with a H₂/CO ratio of 0.9 or 1.8 and biochar, which is usable as a biofuel. The residual product was greywater. Thus, under energy-efficient conditions (slow pyrolysis, final temperature of 400 °C), pressurized catalytic pyrolysis of waste biomass with a high lignin content yields usable products, while the residual product, greywater, can be recycled using standard methods. An effect of the tested fine catalyst on biomass decomposition can qualitatively be characterized as supporting the thermal cleavage of volatile substances released from biomass.

In the near future, the next task will be to determine the mechanism of the decomposition reactions leading to the production of syngas and biochar.

Overall, it can be said that the above declared goal was achieved, as a method of processing waste walnut shells yielding products with high utility value was described. The contribution of the work is the feasible, energy-saving method for processing lignocellulosic waste with a high lignin content using fine Cr₂O₃-catalyzed pyrolysis in a sealed reactor, which provides synthesis gas with a ratio of H₂/CO~2 suitable for bio-methanol production and biochar as a high-quality fuel. A limitation of the proposed method is the way of removing CO₂ from pyrolysis gas using a sorbent, which would be applicable even on a large scale. All process parameters used are applicable on a laboratory scale, but the way of sorption needs further attention. The presented work is an entry into the processing of waste biomass using a sealed reactor, which allows for deeper cleavage of chemical structure and thus more efficient conversion of biomass into synthesis gas and biochar.