Bio-oil generation from microwave assisted pyrolysis of stockpiled biosolids

Abstract

This project employed microwave assisted pyrolysis (MWAP) to recover resources from stockpiled biosolids from Victoria, Australia. Biosolids are the stabilised sludge that results from sewage wastewater treatment. The presence of contaminants, unpleasant odours and poor public acceptance make biosolids disposal challenging. Over three million tonnes of biosolids are currently stockpiled in Victoria, having no identified end use. MWAP applies a microwave electromagnetic field to biosolids in a low-oxygen environment, which heats the material, thermally decomposing the organic matter into volatile bio-oils and incondensable gases, leaving behind a biochar. This work focused on assessing the feasibility of generating bio-oil from MWAP of the Victorian biosolids, with a particular focus on analysing the composition of the bio-oil and identifying ways to enhance the process value. A comparative study was also done using local biosolids. MWAP was carried out using a single-mode microwave pyrolysis unit with a nitrogen-gas purge, enabling condensers to trap the resultant bio-oil. Biosolids mixed with a microwave susceptor that absorbed the microwave energy and re-emitted it as heat (activated carbon) were pyrolyzed in sets of experiments where the oil composition and yield were evaluated. As bio-oil produced from MWAP can contain hundreds of organic compounds, a method was developed to analyse the yield of selected compounds with Gas Chromatography Mass Spectrometry and Gas Spectrometry Flame Ionization Detection. To improve the separation of the bio-oil in the chromatography column, samples were first derivatized using N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA). The bio-oil derived from stockpiled biosolids contained a range of compounds, with the largest groups being phenols and carboxylic acids, and had a calorific value similar to that of bio-diesel. Bio-oil yield was low due to the degradation of the biosolids from the extended periods of stockpiling. The MWAP also consumed a large amount of energy per unit mass of biosolids pyrolysed. Under some conditions the MWAP was cost competitive against land application, which costs an average $300/dry tonne. MWAP of stockpiled biosolids cost as little as $218/dry tonne, though >90% of the savings were due to mass reduction and not bio-oil in this case. Larger scale tests are needed to determine whether the costs and technical complexity of the process could be managed. Unidentified components of the bio-oil that were not quantified may improve the economics.