Thermochemical conversion and upgrading of wheat straw biomass into solid fuel

Thermochemical conversion and upgrading of wheat straw biomass into solid fuel

Herbaceous biomass is a typical agricultural waste produced from leftover crops. Biomass straw has limited uses due to its unfavourable physical characteristics, including bulkiness, varying sizes, varied compositions, and low energy content, but its availability is abundant. This study investigated the wheat straw’s (WS) potential fuel properties and the methods to improve their qualities. Different additives (sawdust: SD, biochar: BioC and bentonite clay: BC) were used to identify optimal pelleting composition. A small-scale pellet mill was used for WS pellet development, and five types of combinations were first investigated (T1: 100% WS, T2: 90% WS + 10% SD, T3: 90% WS + 10% BC, T4: 90% WS + 10% BioC, and T5: 70% WS + 10% BC + 10% BioC + 10% SD).

To compare and improve pellet quality, seven types of pellets (T1, T5 plus T6, T7, T8, T9, and T10) were further considered and analysed with different combinations of additive materials (now including starch and crude glycerol also). Most of these treatments could improve the pellet durability to ≥ 92%, bulk density to ≥ 600 kg/m3 and heating value to ≥18.5 MJ/kg, which meets the pellet ISO 17225-8 standard specification requirements, where the inorganic ash content was all higher than the ISO standard level.

The WS pellet pyrolysis process was studied in a laboratory-scale kiln. The maximum pyrolysis temperature of 600°C was obtained at 60 min for slow pyrolysis. The pyrolysis results demonstrated that additive mixing was particularly useful for pellet (T5) making, resulting in an increased conversion rate, achieving a gas yield of 43.52%, thermal conversion efficiency of 75.67% and syngas production of 46%.

The thermokinetic behaviour of WS pellets (T1 and T5) for both combustion and pyrolysis was determined by thermogravimetric analysis (TGA). The additives, especially biochar added with WS (T5), considerably changed the thermokinetic behaviour of the pellets compared to pellets without additives (T1). Both pellets followed a multistage reaction and the equilibrium chemical reaction behaviour, but the T5 pellet reaction results (E𝛼 and lnA) were significantly higher in pyrolysis and combustion cases.

A CFD model was developed using the ANSYS Fluent 2021R2 for gasification simulation. The study was also performed at a steady state regime considering the non-premixed combustion and species transport models. At the same time, biomass and air flow rates were initially set as 9.0 kg/h and 37.78 Nm3/h, respectively. The model could predict the gasifier's temperature and the composition of the produced gas. With an equivalence ratio (ER) of 0.35 during gasification, the proportions of CO, CO2, H2, and CH4 gas produced were 19.8%, 11.6%, 14.2%, and 0.2% v/v, respectively.

Furthermore, the techno-economic analysis indicated that the cost of pellet production ranged from $232 to $360 per tonne. Compared with the current market price, the profit from pellet production was about 42%. Drying a one-tonne wheat crop (moisture removed from 20 to 12%) would require 20 kg of pellets.

Overall, it was concluded that upgrading the WS biomass into pellets and conversion into energy could provide an efficient alternative fuel source. Additional research is needed to explore alternative additives for reducing ash reduction. Furthermore, modifications are required for the developed CFD model to examine ash and tar production and its validation against data obtained from large-scale gasification.

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