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Production of torrefied fuel pellet from agricultural residues and generation of hydrogen-rich syngas

Date

2022-09-15

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Degree Level

Doctoral

Abstract

The primary goal of this research is to produce high-quality coal-like fuel pellet from agricultural residue and to optimize both torrefaction and pelletization conditions to enhance the physicochemical properties of fuel pellets. Moreover, the produced pellet was gasified to optimize the steam gasification conditions to improve gas yield. Additionally, a comprehensive techno-economic analysis was conducted to assess the economic viability of the integrated torrefaction and pelletization unit. There are seven phases to the work plan. In phase one, three different agricultural biomasses (oat hull, canola residue, and barley straw) were torrefied in a fixed-bed reactor via thermal heating to study the torrefaction mechanism and reaction pathways and to understand how physico-chemical properties of torrefied biomass change with different torrefaction operating conditions. The process parameters investigated include temperature (220 °C, 260 °C, and 300°C), and reaction time (30 and 60 min). The effects of torrefaction on the physico-chemical and structural transformations of biomass were analyzed in terms of their proximate and ultimate analysis, HHV, TGA-DTG, FTIR, XRD, XPS, 13C-NMR, ICP-MS, BET and SEM. The most important factor was found to be temperature which influences the yield (42-87%) of torrefaction as well as properties of torrefied biomass. Among the three biomasses, torrefied canola residue possesses the highest heating value (25.26 MJ/kg) followed by oat hull (23.31 MJ/kg) and barley straw (22.89 MJ/kg) torrefied at 300 °C. Thermal torrefaction possesses several disadvantages, for example high energy consumption, intensive labor, and time consumption. Furthermore, batch reactor can only torrefy a small amount of sample. Therefore, torrefaction of canola residue was performed via microwave irradiation in phase two of this research. For the first time, a response surface methodology based on the Box-Behnken design was used to optimize process parameters during microwave torrefaction while the process parameters were temperature (250-450 W), reaction time (10-20 min) and feed load (70-110 g). With increasing microwave power and torrefaction reaction time, both mass and energy yields decreased by 20.4-43.3% and 84 to 70%, respectively. The results show that the carbon content increased by 9.2-16.5% with the degree of torrefaction, while the oxygen content decreased by 12.7-40.0%, resulting in a significant reduction in the atomic ratio of torrefied biomass. Microwave power had the greatest impact on torrefied biomass properties, followed by residence time and feeding load. In the third phase, the produced torrefied biomass (canola residue) was pelletized in a single pellet unit, and torrefaction operating conditions were optimized based on physical, mechanical and thermal properties of pellet. The highest relaxed density (1090 kg/m3), durability (83 %), and tensile strength (0.55 MPa) of fuel pellets were obtained at 250 W for 10 minutes with 90 g feed load which was the optimum torrefaction conditions. The purpose of this study was to investigate how adding bio-additives (such as lignin, sawdust, mustard meal, and pyrolysis-derived bio-oil) to fuel pellets improve their relaxed density, durability, tensile strength, energy density, and hydrophobicity. The mustard meal, lignin, and bio-oil combination increased density, durability, and tensile strength by 21%, 20%, and 123%, respectively. Raw, torrefied, and torrefied pellets with additives were found to have energy densities of 15.6, 19.4, and 21.2 GJ/m3, respectively. In the fourth phase, canola residue was torrefied at optimum torrefaction conditions and co-pelletized with mustard meal at three different biomasses to biomass ratio (25/75, 50/50, 75/25) and biomass to water mass ratio (1.5, 2, 2.5 and 3). Although 75 wt.% of canola residue was successfully pelletized with mustard meal, however 50:50 ratio of canola hull: mustard meal was optimum based on pellet properties. The optimum biomass to water ratio was found to be 2.5. Durability of pellet was found in the range of 97-100% while the tensile strength of torrefied pellets ranges between 1-2 MPa. Addition of secondary biomass decreased the durability as well as strength of pellet. The produced pellet properties were compared with those for pellet obtained from phase three at optimum conditions. The pellet produced in phase three from torrefied canola residue with binders led to highest durability (100%) as well as lowest moisture uptake (16.9%) compared to pellet from phase four. The best pellets were used for steam gasification for syngas production. In fifth phase, steam gasification of pellets prepared from canola residue (pellets from phase 3) was carried out in a tubular fixed bed reactor at varying temperatures (650-850 °C), and equivalence ratio (0.2-0.4). Based on syngas yield and fuel quality, the optimal gasification temperature and equivalence ratio were discovered to be 800 °C and 0.4, respectively. Syngas yields were significantly improved by torrefaction and densification of canola residue. The gasification of torrefied canola residue pellet at optimal conditions produced the highest total gas yield (24.4 mol/kg), syngas yield (22.1 mol/kg), lower heating value of gases (2741 kJ/Nm3), and gas energy recovery (29.2%). When torrefied pellets were gasified, the tar yield dropped by 82%. The objective of sixth phase was to valorize the liquid effluents obtained from microwave torrefaction of canola residue for energy purposes. Hydrothermal gasification of the liquid effluents (feedstock) was carried out at 375-525 °C for 15-60 min with 1:5 and 1:10 feed/water ratios under 22-25 MPa pressure to optimize the operating parameters of the process. The total gas yield (6.6 mmol/g), highest H2 yields (7.8 mmol/g) and carbon gasification efficiency (51%) were obtained at 525 °C in 60 min with a 1:10 feed/water ratio. Homogeneous catalysts including KOH, K2CO3 and Na2CO3 were used to improve the gas yields from torrefaction effluents in optimal conditions. KOH led to maximum H2 yield, hydrogen selectivity and carbon gasification efficiency of 12.8 mmol/g, 89% and 98.2%, respectively. In the final phase, a comprehensive technoeconomic evaluation and sensitivity analysis for a conceptual design for the production of torrefied fuel pellet via integrated torrefaction and pelletization process with and without external additives and then generation of syngas from torrefied canola residue pellet were performed. A discounted cash flow analysis was used to assess the economic feasibility of hydrogen production. The minimum selling price of pellet at the plant gate was $90.5 and $90.2 per tonne for pellet with additives and pellet without additives, respectively while $0.13/kg was for syngas. The economic analysis suggests that both pellet production and syngas generation are profitable. The cost of torrefaction reactor was discovered to be the most important driver of a combined torrefaction and pelletization system. Sensitivity analysis shows that labor cost among all variable cost has the highest influence on both NPV and MSP of pellet for both scenarios.

Description

Keywords

Lignocellulosic biomass, torrefaction, fuel pellet, syngas, economic analysis

Citation

Degree

Doctor of Philosophy (Ph.D.)

Department

Chemical and Biological Engineering

Program

Biological Engineering

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