Valorisation of cereal straw in Western Australia: Feedstock characterisation and economic & environmental analyses of an advanced pyrolysis process

Saleha Quadsia

Australia is the 14th highest carbon emitter in the world despite hosting only ~0.33% world’s population. Western Australia (WA) is the only state in the country whose reported annual emissions have continuously increased since 2005. Most of these emissions are due to the heavy reliance on fossil fuels, which has caused unsurmountable environmental damage. Currently, wind and solar are the two widely deployed renewable energy sources in the area; however, the inherently stochastic nature of these technologies has made grid integration difficult. Bioenergy has the potential to play a colossal role as a stabilising element in the renewable power grid. WA produces ~8 million tonnes (MT) of cereal straw per annum, most of which is not utilised commercially. This crop presents a promising opportunity to replace traditional fossil fuels with biofuels derived from cereal straw, lowering carbon emissions from the State’s power generation sector. However, directly using cereal straw as a combustion/co-firing feedstock is challenging due to its undesired fuel properties, such as low volumetric energy density and high contents of alkali metals (e.g., sodium and potassium). Pyrolysis is a promising technology that converts bulky biomass into char and bio-oil, with pyrolytic gases being burned onsite for process heat. We have recently developed an advanced pyrolysis process that co-produces biochar/heavy oil (HO) pellets and light bio-oil. The biochar/HO pellets of high volumetric energy content are an excellent feedstock for co-firing in existing coal-based power stations. Light bio-oil of low water content and viscosity can be blended with diesel and burned in stationary diesel engines. This PhD study aims to valorise cereal straw via the advanced pyrolysis process with the following specific objectives: (1) to characterise the effect of cultivar-specific morphology variations on fuel properties for optimising the feedstock selection process; (2) to estimate the potential of the production of biochar/HO pellets in WA and assess the economic feasibility of the proposed advanced pyrolysis process via a decentralised approach; and (3) to investigate the environmental burden of bioelectricity generation from biochar/HO pellet by estimating the combined lifecycle carbon, energy, and water footprints. Firstly, a detailed spatial characterisation of available cereal straw covering 26% of the WA’s Wheatbelt was performed to assess potential variations of fuel properties of straw biomass produced from different shires and varieties. Different fuel pricing strategies were explored, and a new methodology in straw costing was designed using an alternate residue management method. As a result, a set of equations has been developed for accurately estimating the nutrient content in overall straw cost for open burning and straw retention. Despite the spatial distribution, the collected samples showed similar composition, making them suitable for collective use. Assessment of the vertical distribution of straw nutrients showed that the cutting height selection could support the soil nutrient loads and reduce fertiliser costs for the following crop. Thus, the nutrient contribution in straw pricing can be decreased from 42% to 16% by accurately estimating its overall value, making straw a more economical resource for fuel production on a larger scale. Secondly, we studied the techno-economic feasibility of the proposed integrated mobile pyrolysis unit for processing local biomass into high-energy-density biochar/HO pellets. A detailed thermodynamic-based process simulation was developed using Aspen Plus, HSC Chemistry and integrated modelling codes using Python and validated by experimental data. The estimated minimum fuel selling price (MFSP) for biochar/HO pellets is determined to be A$5.67/GJ without considering the benefits of carbon credits, which is at cost parity with coal prices (A$5.56/GJ) even without the help of carbon credit. If the carbon credits generated from biochar combustion are included, this price further reduces to A$5.32/GJ. We also developed a supply curve for biochar/HO pellets and for comparison for biomass pellets to assist project developers in identifying suitable locations for fuel production. The results indicate that approximately 76% is achieved if all the available biomass in WA is directly utilised. Biochar production provides enough thermal energy to replace up to 96% of coal being burned in the region. Onsite wastewater treatment can generate an additional income stream of nutrient-rich water sold to the farmers as fertiliser which can further reduce the pellet cost by 9%. Sensitivity analysis demonstrated that the feedstock cost is the most influential factor affecting the economic feasibility of the produced product. Lowering the straw cost by 36% will reduce the selling price by 22%, whereas the effect of ACCU’s spot price was relatively inconsequential. Thirdly, we performed a cradle-to-grave life cycle analysis (LCA) of carbon, energy, and water footprints of electricity generation from biomass and biochar/HO pellets at a regional scale in WA for a 40 t/d mobile plant. Biochar/HO-derived electricity substantially reduced lifecycle non-renewable energy input to produce 1 kWh electricity from 3.87MJ (biomass) to 1.54 MJ (biochar/HO). This result yielded a carbon footprint of 0.28 kg CO2-eq/kWh as opposed to 0.45 kg CO2-eq/kWh (biomass). Hence, electricity generated from biochar pellets has an average net energy recovery of 2.8 and can be an alternative renewable to coal. Key factors driving the bioelectricity emission intensity and energy use include crop production, mobile pyrolysis operation and power generation efficiency. It is evident from the water footprint analysis that more than 98% of the water footprint for both biomass (200 L/kWh) and biochar (123 L/kWh) comes from biomass production, mainly nitrogenous fertilisers. Analysis of regional prints indicates huge variabilities in water footprint compared to energy and carbon footprint. The findings from the present study are expected to advance the decision-making knowledge for the employment of advanced pyrolysis systems that may significantly valorise cereal straw and promote the development of rural areas in WA. Despite the current low fossil fuel prices, the economy of biochar/HO pellet production via mobile pyrolyser is attractive. The next step is to collect pilot plant data which is essential for future technological refinements and more reliable economic assessments. The biggest challenge to bioenergy is high feedstock costs. With increasing focus on climate mitigation strategies and rising energy demands, future bioenergy applications are limitless.

Valorisation of cereal straw in Western Australia: Feedstock characterisation and economic & environmental analyses of an advanced pyrolysis process

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