Experimental Optimisation and Techno-economic Assessment of a Rice-straw-biochar-based Slurry Used for Fuel Applications

Countries and industries have responded to the environmental impact of fossil fuels by adopting a carbon-neutral strategy. Thermochemical decomposition products of biomass, such as biochar, bio-oil, and non-condensable volatiles, are actively investigated as substitute renewable fuel sources. Although biochar is a potential source for several applications, research on its use as an energy carrier is limited. Biomass, such as rice straw, derived from the third most abundant grain crop grown in the world (rice), is mainly disposed of by field burning, leading to waste of energy resources, air pollution, and public health deterioration. Alternatively, biochar produced from biomass pyrolysis can be suspended in a liquid medium to produce a slurry liquid biofuel for broader use in transport and energy industries. Research on improving biochar physicochemical properties (such as morphology and surface composition), and biochar production efficiency, is scarce. This research encompasses experimental development, a plant design, and techno-economic analysis to cover knowledge gaps in optimising the preparation process that will enable this fuel’s economic and environmental sustainability. The first stage investigates the effects of varying pyrolysis parameters and biomass conditioning on the physicochemical properties of the produced biochar. Later, for fuel production, researching various suspension media (containing water, ethanol, and surfactants) and biochar with different particle sizes and morphology will help determine their effects on slurry fuel quality regarding stability, flow properties, and energy content. Lastly, a simulation of the slurry production process using ASPEN Plus software and developing a techno-economic analysis tool help evaluate the biochar-based slurries’ feasibility for industrial-scale development. The experimental work included the optimisation of biochar and biochar-based slurry production. When varying the pre-treatment and pyrolysis conditions, higher biochar yields with higher heating values and better morphological structures were obtained. Therefore, the rice straw biochar was produced with a 0.2 M diammonium phosphate pre-treatment at 400°C and 30 min dwelling time for optimum results. The experimental development for biochar-based slurry production focused on improving the fuel’s energy content, stability, and flow properties for better transportation, storage, and combustion. The mechanisms of dispersion of biochar particles in liquid media containing water, ethanol, and surfactants were also assessed. It was found that slurries produced with fine biochar particles from treated rice straw had the highest stability and shear-thinning behaviour, favourable properties in slurry processing. Biochar morphology and chemical surface constitution significantly influenced slurry stability, while particle size distribution greatly impacted rheological results. For the experimental work using water and ethanol-water mixtures as the suspension media, the lowest viscosity, highest HHV, and lowest stability were found for the biochar slurry prepared with pure ethanol, thus making it a preferable fuel if combusted shortly after preparation. Contrarily, a water-based slurry is preferable if the combustion occurs after prolonged storage due to its improved stability, higher viscosity, and lower HHV. The techno-economic assessment and plant simulation results showed that if the minimum selling price increases from A$ 1443.3 to A$ 1566.63 /tonne, the payback period becomes eight years at an internal rate of return of 22 %. This cost (A$ 1566.63 /tonne) reduces the project’s vulnerability to increasing prices in operating costs, utilities, and raw materials and gives a probability of a net present value of A$140.90 million. This project had the ambition to serve as an innovative investigation searching alternatives to the traditional pathway of liquid (bio-oil) and gas fuel production via fast pyrolysis and gasification. Also, it aimed to provide a guideline indicating the possible process variations/improvements (for both experimental and process design stages) which could help achieve the financial viability of biochar-based slurry fuels and future commercialisation.