Pressurized torrefaction of waste biomass to improve bio coal quality: Synergistic effect between animal waste and wood chips

doi:10.21203/rs.3.rs-4663604/v1

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Pressurized torrefaction of waste biomass to improve bio coal quality: Synergistic effect between animal waste and wood chips

https://doi.org/10.21203/rs.3.rs-4663604/v1

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Perennial global energy demands and the need to cut on emissions from fossil fuels has paved the way for alternative energy sources such as raw biomass and animal waste. In addition, there is limited research in deducing the impact of blending waste material to improve its characteristic properties using pressurized torrefaction. In this regard, this research explored the benefits of blending animal waste with wood chips to produce a bio-coal with improved properties. The process conditions investigated were temperature and pressure intervals of 200°C to 280°C and atmospheric pressure (AP) to 4MPa, respectively. The results showed that a positive adjustment in temperature and pressure improved the fixed carbon content of the blend almost threefold from 19.87–66.90% and the higher heating value (HHV) to 27.32MJ/kg from 13.90MJ/kg at mild torrefaction temperature of 280^oC and gas pressure of 4MPa. Wood chips had an HHV of 27.00MJ/kg upon treatment due to the decomposition of hemicellulose and cellulose which enhanced the thermal stability, fixed carbon content and calorific value. However, animal waste had the least incremental increase in HHV (16.45MJ/kg) due to a high initial content of volatile substances and moisture. Moreover, pressurized torrefaction facilitated the decomposition of cellulose at a lower temperature than the typical range of 315-400^oC if conducted at atmospheric pressure. Furthermore, the study showed that to overcome the drawbacks of having a lower fixed carbon and HHV for animal waste, blending animal waste with wood chips was effective in synthesizing bio-coal with improved properties like some coals.

Animal waste

wood chips

synergistic effect

kinetics

pressurized torrefaction

Bio-coal

Energy is one the most critical elements of human life. It is regarded as the primary input in all areas of life which include transportation, production, and various daily activities. Energy has a direct impact on socio economic issues and living standards. Historically, coal has been extensively used as the primary energy source because it is the most widely abundant available fossil fuel resource. It is the primary source of the world’s electricity supply, providing more than 40% of electricity needs. Southern African Development Community (SADC) countries as exemplified by Zimbabwe, Botswana, Mozambique, and South Africa’s economies which are currently heavily reliant on coal, as the energy source. The global annual consumption of coal is currently 8 billion tonnes per year (IEA, 2022) (Association, 2022) and this is not sustainable due to it being non-renewable and its effects during and after combustion.

Besides the several advantages of coal which include abundance and affordability as a source of energy, it has a myriad of drawbacks as it is detrimental to the environment, impacts public health and contributes to climate change. The production of fuels from raw biomass is widely recognized as a feasible approach to decrease our dependence on finite petroleum resources and address the issue of greenhouse gas (GHG) emissions like carbon dioxide, ultimately leading to carbon neutrality. It is noteworthy that countries such as Zimbabwe, Botswana, South Africa and Mozambique are dependent on agriculture generating a massive amount of raw biomass (Moyo et al., 2022). If this waste is left to fester blindly it becomes a nuisance to healthcare systems, it generates methane a potent GHG when decaying and it becomes a hazard to food security. In this regard, beneficiating raw biomass to produce bio-coal is a sustainable means of waste valorisation leading to a circular economy. However, raw biomass such as animal waste has some intrinsic shortcomings compared to fossil fuels which pose obstacles to its wide utilization due to its physico-chemical properties such as low calorific value, high water content and poor grindability leading to difficulties in transportation and storage (Yang et al., 2024;Pahla et al., 2017). The underlying reason is the high oxygen content of the biomass (O/C ratio). Consequently, to counter limited availability of raw biomass and the drawbacks of sole torrefaction of animal waste, blending animal waste with wood waste is explored in this study. One of the most promising pre-treatment methods for promoting the use of biomass energy is dry torrefaction (usually just torrefaction), so it was considered as a solution to reduce the oxygen content of the blended biomass.

Torrefaction is typically carried out in an inert environment at moderate temperature range of 200–300^oC, where the biomass goes through mild pyrolysis releasing a volatiles such as H₂O, CO₂, and CO along with a few organics. The torrefied biomass has a higher energy density, less moisture and oxygen than raw biomass, and enhanced hydrophobicity and grindability. The residence time, torrefaction temperature and pressure have a significant impact on the quality of bio-coal produced (Ru et al., 2015). Attempts have been made to improve the quality of biomass through optimisation of torrefaction parameters that include pressure and temperature. However, the positive adjustment of these variables has proved too costly as adjusting temperature for example requires energy input whereas increasing pressure will require a vessel constructed with more robust material. Moreover, the economic feasibility of torrefaction of biomass like animal waste today is still not competitive with coal and wood pellets (Cahyanti et al., 2020). The various available torrefaction methods which include dry, wet, microwave and atmospheric torrefaction are effective in removing thermally unstable oxygen from biomass resultantly enhancing energy efficiency. However, the harsh operating conditions and high energy demand of these methods have prevented them from industrial use. An emerging torrefaction method is gas torrefaction which has been deemed relatively simple, has a much more effective oxygen removal efficiency and thermal energy consumption than the aforementioned methods. While biomass is a CO₂-neutral energy source, commercial plants that produce bio-coal may find it attractive to lower their net CO₂ emissions by using a torrefaction technique that simultaneously reduces the evolved CO₂ gas (Lateef and Ogunsuyi, 2021).

Furthermore, new research indicates that the net negative GHG emissions from bioenergy with carbon capture might reach 109 metric tons of CO₂ per year close to 2050, highlighting the significance of reducing CO₂ emissions into the environment (Sarvaramini et al., 2014). This highlights how crucial it is for plants based on bioenergy in the future to intercept CO₂ streams. In this regard, it is imperative that new process configurations such as pressurized torrefaction be explored coupled with using a mixture of feedstock for the torrefaction process.

This is supported by previous research work by Zheng et al ( 2023), where lignin was mixed with maize/corn cobs. Lignin is a biopolymer with a complex nature, which intensifies cross linkage with components existing in the other feed material favouring condensation of volatile components therefore having a positive effect on the yield. In this regard, this study explores blending animal waste material with wood chips which has a high lignin, cellulose and hemicellulose content compared to animal waste. It is anticipated that the synergistic effects of combining these two materials will result in a bio-coal product with improved properties as compared to using animal waste solely.

In this study, the experimental work was conducted to evaluate the characteristics of both solid and gaseous products derived from pressurized torrefaction of wood chips and animal waste alone, and a blend under various conditions. Furthermore, the synergistic effect between wood chips and animal waste was demonstrated by comparing theoretical and actual values, and the effects of blending animal waste and wood chips was deduced by comparing single and combined feedstock final product characteristics.

2.1 Materials

Animal waste and wood chips were used as the biomass source. Both the animal waste as well as the wood chips were randomly sampled from a farm in Bulawayo, Zimbabwe, and from a wood-work workshop, respectively in Bulawayo, Zimbabwe. Unwanted material was handpicked from the biomass and thereafter crushed and sieved to obtain a particle size of less than 1mm.

2.2 Analysis of Raw Materials

A sample of 25 to 50 grams of biomass was weighed which was thereafter dried at 105^ᵒC to drive off all unbound moisture in the sample for 5 hours. Pressurized torrefaction was conducted in a mini-autoclave inert reactor (300ml). The reactor was purged three times with nitrogen to ensure an oxygen free atmosphere. The pressure was adjusted in increments of 1MPa as atmospheric pressure, 1MPa, 2MPa, 3MPa and 4MPa using nitrogen at the preset temperatures. Temperature was adjusted between 200 to 280^oC at intervals of 20^oC. The reactor temperature was achieved by adjusting temperature at a rate of 10^oC/min while keeping the reaction time constant for 30 min. After torrefaction, the samples were thereafter allowed to cool to room temperature and the solid bio-coal sample was collected. The gas produced was collected using a gas bag connected to the gas outlet of the reactor for analysis.

2.3 Analysis of products

The bio-coal produced was characterized by determining the proximate analysis (moisture content, volatile matter, ash content and fixed carbon), the ultimate analysis (C, H, N, O, S) and energy yield. The moisture content of the sample was determined by analysing the sample before and after drying, this is an ASTM standard method (ASTM E871-80). The sample was measured in a crucible, it was then heated for an hour at a temperature of 85–100\(ºC\) in a dying oven. The weight loss obtained is attributed to moisture content. The volatile matter was determined using ASTM standard method (ASTM E872-80): where the sample was measured on a crucible. The sample was then placed in a furnace for 30 minutes at a temperature of 800\(ºC\). When the required time had elapsed, the crucible was removed and allowed to cool in air, it was placed in a desiccator and weighed again afterwards. The weight loss was recorded as volatile matter. The ash content of the sample was determined by analysis using ASTM standard method (ASTM E1755- I), where the sample was measured in a crucible. The crucible was placed in a furnace for 7 minutes at a temperature of 900\(ºC\). The weight loss was recorded as ash content. The fixed carbon of the sample was determined by difference from the moisture content, volatile matter and ash content that was measured. A bomb calorimeter was used to test the biomass samples' gross calorific value (GCV) or higher heating values (HHV). In order to conduct this test, a 1 gram of oven-dried sample briquette was placed in a bomb and filled with oxygen gas at a pressure of 25 to 30 atmospheres (Mugoronji et al., 2022; Mamvura et al., 2018). After placing the sample within the bomb calorimeter, the calorific value was calculated by recording the rise in water temperature.

2.4 Thermogravimetric Analysis

The thermal characteristics of wood chips and animal waste were assessed through TGA analysis. Each material was subjected to a 10 mg sample using a thermo-gravimetric analyzer (Q 500 series, TA Instruments, USA). The investigation on the mass-loss between 50 to 600˚C was carried out with a heating rate of 10^oC/min. A constant inert atmosphere was maintained at a flow rate of 60 ml/min using nitrogen as the carrier gas. All the experiments were performed in duplicate to minimize errors and to ensure precision.

3.1 Raw Materials Analysis

Table 1 shows the percentage composition of fibres in animal and wood waste. Cellulose is the main constituent (20.84–45.48%), followed by hemicellulose (19.98–35.62%) and then lignin (10.6–22.8%), is the least.

Table 1

Component analysis of the biomass (%)
	Cellulose	Hemicellulose	lignin
animal waste	20.84	19.98	10.60
wood chips	45.48	35.62	22.80

3.2 Composition and Thermogravimetric analyses

The thermal stability and volatile components fraction of the materials is determined using a thermogravimetric analyzer (TGA) by observing the weight loss, Fig. 1.

Thermal degradation was critical to determine the temperature range for the disintegration of the animal waste and wood chips prior to pressurized torrefaction. Previous researchers have shown that pyrolysis temperature of hemicellulose, cellulose and lignin range from 200 to 315^oC, 315 to 400^oC and 160 to 900^oC, respectively. Figure 1 illustrates the TG/DTG curves, which indicates occurrence of several processes during the thermal treatment. The TGA curve for animal waste shows a higher mass loss initially than wood chips waste between 50-150^oC which is a result of release of volatile components such as water vapour and low molecular weight organic compounds. Whereas between 200 and about 270^oC there is a sharp increase in the mass loss in both wood and animal waste which is attributed to the degradation of hemicellulose as this is the temperature range for its disintegration. Thereafter, disintegration of cellulose occurs up to a temperature of 360^oC. Finally, degradation of lignin and char produced from cellulose and hemicellulose occurs between 360 and 490^oC which is a gradual process due to the nature of lignin and char. Beyond 490^oC there is a statistically insignificant mass loss because the ash decomposed gradually.

3.3 Factors affecting Pressurized Torrefaction

3.3.1 Effect of Pressure

Similar to research work conducted by (Zheng et al., 2023), a temperature of 240^oC was adopted to investigate the effect of pressure: atmospheric pressure, 1MPa, 2MPa, 3MPa and 4MPa. The pressure increase during the gas pressurized torrefaction was because of the generation of volatiles from the animal waste and wood chips during torrefaction and the thermal expansion of the inert gas with temperature. Figure 2 indicates that there was a significant solid yield at atmospheric pressure for the animal waste, wood chips and the blend, implying that not all volatile matter had been liberated. However, at atmospheric pressure the fixed carbon was the least and it increased with pressure increase to 3MPa, thereafter the increase was statistically insignificant at 4MPa. On the other hand, the volatiles decreased with an increase in pressure indicating that secondary reactions were occurring leading to condensation of volatiles which were then fixated on the retained carbon. Carbon recovery was higher at 3MPa compared to that at atmospheric pressure, this implied that the optimal operating pressure was 3MPa.

3.3.2 Effect of Temperature

One of the key factors influencing the characteristics of biomass during torrefaction is temperature. The variation of temperature during torrefaction highly influenced the calorific value and energy densities of the bio-coal. Thermal treatment of the animal, wood and blended waste, increased the calorific value of bio coal to between 17.0 and 23.7 MJ kg^− 1 as shown in Fig. 3. On the other hand, the energy densities of the bio-coal were comparable with that of low-grade coal, indicating that bio-coal has the potential to be used as an alternative to commercial coals. A sample of coal collected from a coal mining company in Hwange, Zimbabwe was analysed and had a calorific value of 25–26 MJ/kg. Table 2 and Table 3 shows the proximate analysis and ultimate analysis results of animal waste, wood chips and the blend, respectively. With increasing temperature from 200°C to 280°C, the volatile matter was found to decrease significantly yet the fixed carbon content was increasing. At 280°C, the volatile content of the blended mixture (M-280) decreased from 70.54–43.56%, and the fixed carbon content increased from 19.87–63.93%. This showed that temperature played a pivotal role in deoxygenation and dehydrogenation, i.e., removal of volatile matter compounds. Considering the elemental analysis results, the carbon content of the treated bio char was higher than that of the raw materials in all samples investigated. This is also attributed to a reduction in volatile matter and moisture content. Similarly, the HHV after pressurized torrefaction increased for all samples investigated because of the change in C, H, and O content (H/C and O/C ratios). The highest HHV change was noticed for M-280.

3.3.3 Proximate and Ultimate analysis

The proximate analysis results for the three different waste samples under investigation are shown in Table 2.

Table 2

Proximate analysis of biomass samples
Sample	Proximate analysis (wt%)
	Moisture content	Ash Content	Volatiles	Fixed Carbon (FC)
A raw	38.50	19.22	75.28	16.01
Wraw	25.79	60.95	62.51	22.81
A200	3.28	4.29	65.98	46.22
A220	2.05	4.01	62.89	47.16
A240	2.48	3.44	59.56	48.98
A260	2.79	3.48	58.24	49.68
A280	2.66	3.29	57.78	50.65
W200	3.24	5.88	52.62	60.92
W220	3.01	6.18	50.25	62.82
W240	2.75	6.27	48.12	64.26
W260	2.81	6.25	47.98	66.84
W280	2.07	4.85	46.22	67.98
Mraw	29.8	43.01	70.54	19.87
M200	2.94	5.89	60.50	61.54
M220	2.71	5.21	55.76	64.05
M240	2.21	5.51	54.16	65.23
M260	2.08	5.41	50.32	65.92
M280	2.19	5.73	43.56	66.93

M200 is blended biomass at 200^oC, A200 is animal waste at 200^oC and W200 is wood chips at 200^oC.

The ultimate analysis results for the three different waste samples under investigation are shown in Table 3.

Table 3

Ultimate analysis of biomass samples
Sample	Ultimate analysis (wt%)							**HHV (MJ/kg)
	C	H	N	O*	S	O/C	H/C	**HHV (MJ/kg)
A raw	38.50	4.70	0.90	55.74	0.16	0.68	1.30	9.78
Wraw	44.95	6.15	0.52	48.29	0.09	0.79	1.66	9.78
A200	47.82	4.29	0.82	46.99	0.08	0.42	1.31	13.91
A220	48.91	4.01	0.80	46.23	0.05	0.39	1.25	14.01
A240	49.68	3.44	0.72	46.09	0.07	0.37	1.22	13.47
A260	52.72	3.48	0.75	42.98	0.07	0.34	1.20	15.12
A280	55.91	3.29	0.71	40.03	0.06	0.32	1.19	16.45
W200	57.54	5.88	0.44	36.02	0.12	0.46	1.22	21.48
W220	60.49	6.18	0.41	32.78	0.14	0.40	1.23	23.49
W240	66.79	6.27	0.38	26.47	0.09	0.29	0.93	26.89
W260	68.17	6.25	0.37	25.10	0.11	0.23	1.05	27.58
W280	71.46	4.85	0.35	23.24	0.10	0.21	0.80	27.00
Mraw	42.80	5.88	0.78	50.42	0.12	1.20	1.51	13.90
M200	52.45	5.89	0.57	40.99	0.10	0.60	1.20	18.88
M220	58.74	5.21	0.55	35.42	0.08	0.40	1.04	21.02
M240	62.75	5.51	0.51	31.14	0.09	0.36	0.95	23.58
M260	64.01	5.41	0.48	30.03	0.07	0.33	0.92	24.06
M280	69.28	5.73	0.46	24.45	0.08	0.25	0.75	27.32
* Calculated by subtraction
*Calculated by using Dulong equation (HHV(MJ/kg = 0.3381C + 1.4481H-0.1802O))

This analysis is imperative as moisture and ash content significantly affect its HHV, i.e., the higher the moisture content the lower the HHV. This indicates that dry biomass requires little or no energy for drying before it starts to burn. Animal waste, which was composed of cow, pig and goat waste had the highest moisture content as compared to wood and blended waste, it had almost 50% moisture content. This was anticipated since animal waste is collected as wet waste biomass. This implies that to improve the energy properties of animal waste it must be dried and then torrefied.

Blended waste, which is composed of animal waste and wood chips, had relatively a higher moisture content compared to wood chips but less than animal waste. When compared to animal waste and blended waste, wood chips contained a substantial amount of ash content and the least amount of volatile content. Both the amount of ash and moisture limit the heating qualities of fuel as ash does not combust to release heat and it is linked to fouling, and moisture converts to a vapour using the heat released during combustion. The heating values of the three biomass samples were comparable, with wood waste having a marginally higher HHV. The HHV for wood is high due mainly because of lower moisture content, high fixed carbon and low volatile matter even though it has a high ash content.

The ultimate analysis is utilized to assess the material balance and HHV of biomass. The primary elements that were determined include hydrogen (H), carbon (C), nitrogen (N), sulfur (S), and oxygen (O). The contents of C, H, N, S, and O in animal waste, wood chips and the blend are examined. The N and S levels in animal waste and wood chips are relatively low. On the other hand, the C content in wood waste and is high when compared to that for wood chips. The ultimate analysis results demonstrated that both animal waste and wood chips are deemed safe for utilization as fuel in thermochemical processes such as torrefaction.

3.4 Analysis of gas products

The composition of gases produced during pressurized torrefaction are shown in Fig. 3(a) and (b) with a variation in pressure of atmospheric pressure and 3MPa, respectively. A substantial amount of the product gases includes CO and CO₂ and some low molecular weight gases that include CH₄ C₂H₄ and C₂H₆. At 200^oC, lipophilic compounds like saturated and unsaturated fatty acids, sterols start to be released. In addition, structural deformation is initiated as there is sufficient energy to break the bonds between C and H, and C and O, this is the reason a significant amount of CO and CO₂ was noticed as well as traces of CH₄, C₂H₄ and C₂H₆. Moreover, this is attributable particularly to disintegration of carbonyl and carboxyl of xylan units in hemicellulose and cellulose. At 280^oC there was the highest yield of CO and CO₂ both at atmospheric pressure and 3MPa, which is the highest temperature investigated which entailed highest quality of product was produced at this point. At this point there was the most deoxygenation which is the component that interferes with the thermal stability during torrefaction thus the improved bio-coal quality. Whereas methane was mainly produced from methoxy functional groups found in lignin and hemicellulose. Furthermore, C₂H₄ and C₂H₆ concentrations were significant at temperatures between 220-280^oC, respectively which primarily came from aliphatic hydrocarbon branch chains indicating the beginning of poly condensation reactions. A comparison between Fig. 3(a) and (b) indicates that there was a reduction of CO and CO₂, CH₄, C₂H₆ as the pressure was increased from atmospheric pressure to 3MPa. The highest reduction of the gases produced was observed at a temperature of 280^oC with CO₂ reducing from 0.182 ppm to 0.08 ppm. This may be attributed to that the bio-coal already synthesized has enhanced adsorption properties under pressure, consequently the physisorption of the gases under pressure on the solid bio-coal is intensified at elevated pressure and temperatures. It is anticipated that secondary reactions between the volatile alkaline components and solid products at this temperature which is intensified at high temperatures. The prevailing high temperature and pressure ensured retention of more carbon in the solid.

Figure 4 shows that as the carbonization process occurs thermochemical conversion leads to an increasing formation of aromatic carbon. So as torrefaction temperature increases both O/C and H/C ratios tend to decrease and biomass increasing becomes more aromatic and carbonaceous. The ratio reductions are attributed to increasing H and O losses inherently increasing carbon content which is related to volatilization of CH₄ demethylation and CO₂ decarboxylation, respectively.

Figure 5. shows the solid fuel quality derived from wood chips, animal waste and a blend of the materials on a Van Kremelen diagram. It is evident that the raw wood chips and the animal waste are in the biomass region. However, Fig. 5 also indicates that as the raw materials are subjected to thermal conversion of at least 200^oC they move towards the peat coal region. Furthermore, the blend of animal waste and wood chips subjected to torrefaction at 280^oC moved to the lignite coal region as the carbon content improved.

In this work, the physicochemical characteristics of bio coal made from wood chips, animal waste, and a blend of the materials under various conditions were examined, and the influence mechanism was deduced. The application of gas pressure during torrefaction was found to be beneficial for carbon recovery as the solid substrate and simultaneously lowered CO₂ and CO emissions during the torrefaction process. The polycondensation process was enhanced by the increased pressure, which also accelerated the secondary reaction between the volatiles and solid residues. As a result, torrefied bio-coal produced under pressure had a greater fixed carbon content. Furthermore, the synergistic effects of blending wood chips and animal waste was evident in the improved quality of the bio-coal produced as there was a more substantial carbon recovery as a blend as compared to when animal waste and wood chips were used individually. In addition, at elevated temperature and pressure there was a significant reduction in the CO and CO₂ produced indicating that the process was not only effective at improving the quality of the bio-coal but also at simultaneously capturing carbon.

5.0 Funding Source

The authors would like to acknowledge funding from the African Centre of Technology Studies (ACTS) in Kenya in collaboration with the Botswana Digital & Innovation Hub (BDIH) and Fundo Nacional De Investigacao (FNI) in Mozambique.

Author Contribution

N.. Tshuma and L.B. Moyo made substantial contributions to the conception & design of the work as well as drafting the manuscriptT.A. Mamvura and G. Danha made substantial contributions in acquiring, analysis and interpretation of dataG. Simate made a substantial contribution in revising the manuscript critically for important intellectual content

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Pressurized torrefaction of waste biomass to improve bio coal quality: Synergistic effect between animal waste and wood chips

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Abstract

Figures

1.0 Introduction

2.0 Material and Methods

2.1 Materials

2.2 Analysis of Raw Materials

2.3 Analysis of products

2.4 Thermogravimetric Analysis

3.0 Results and Discussion

3.1 Raw Materials Analysis

3.2 Composition and Thermogravimetric analyses

3.3 Factors affecting Pressurized Torrefaction

3.3.1 Effect of Pressure

3.3.2 Effect of Temperature

3.3.3 Proximate and Ultimate analysis

3.4 Analysis of gas products

4.0 Conclusion

Declarations

5.0 Funding Source

Author Contribution

References

Additional Declarations

Status:

Version 1