UTILISATION OF GREEN CHEMICAL TECHNOLOGIES FOR THE VALORISATION OF THAI AGRICULTURAL WASTE

Biorefinery concepts are important for countries with economies closely linked with agriculture, such as Thailand. The use of agricultural waste streams or residues for the production of fuels, chemicals and materials is vital for the move towards a sustainable circular economy. Two such wastes (rice straw and prawn head waste), were selected for valorisation through the use of supercritical carbon dioxide (scCO2) extraction and microwave-assisted pyrolysis (MAP).
scCO2 extraction was investigated as a suitable technology for lipid recovery from rice straw. Yields obtained from scCO2 extraction of rice straw were comparable to hexane Soxhlet, with yields of 0.7% by dry weight at 65 ˚C and 400 bar. Extracts consists of a valuable mixture of compounds including free fatty acids (FFA) and fatty alcohols, sterols and wax esters. This method also serves as a pre-treatment step to provide enhanced interactions between microwaves and the resulting extracted biomass. Wax removal by scCO2 has an influence on the dielectric property of the rice straw, leading to a constant high heating rate and higher final temperatures. The decomposition of rice straw occurred in MAP at around 120 ˚C, approximately 200 ˚C earlier than in the conventional heating. The influence of moisture content, particle size and also inorganic content was investigated. It was found the K3PO4 present in the straw (also present in fertilisers) had a profound effect on MAP resulting in rapid heating and biomass gasification. Such gases may be used as synthesis gas for the production of fuels and chemicals.
scCO2 extraction was effectively utilised for the recovery of hydrophobic compounds from prawn residue (Litopenaeus vannamei). The optimisation of lipid extraction from cepharothorax (heads) by scCO2 demonstrated yields of 1.14% at 400 bar and 40 °C for 2 hours (40 g min-1). Extraction efficiency strongly correlated density, with the highest yield being obtained at the highest density of CO2, 0.96 g/cm3. The FFA profile of the scCO2 extract differed to that of the hexane Soxhlet in abundance, 20789.8 ± 257.3 μg/g of dry biomass compared to 2819.4 ± 87.8 μg/g of dry plant by scCO2 (40°C and 400 bar for 2 hours). Water content of the biomass adversely affects the efficiency of scCO2 extraction. Autolysis of the waste for 6 hours at 50 ˚C under nitrogen prior to scCO2 extraction was found to significantly improve the yield of PUFA. L.vannamei waste can potentially serve as new source of ω-3 and ω-6 PUFA.