The production of bioethanol and biogas from paper sludge
Thesis (MEng)--Stellenbosch University, 2017.
ENGLISH SUMMARY: Recent interest into the production of biofuels such as bioethanol and biogas has increased due to a concern of global climatic change and the depletion of fossil fuels. Sustainable feedstock for the production of these biofuels need to be found. One such source is a waste stream from the pulp and paper industry. Paper sludge, emanating from the primary clarifier in the waste water treatment area of the paper mill, consists of high amounts of cellulose and water which makes it an ideal substrate for biological conversions, such as fermentation and anaerobic digestion. The extensive mechanical and chemical processing during paper making acts as a pre-treatment step by disrupting the biomass structure, making it amenable for enzymatic hydrolysis, the first stage of the biological conversion process. Another advantage is the continuous, localised supply of paper sludge. Two possible biological conversion processes, fermentation and anaerobic digestion, were investigated for the energy yields and economic benefits. The feasibility of these processes has been proven at laboratory scale, with the working volume of experiments conducted in the range of 250 ml, however the scale up to pilot scale still needs to be investigated. The impact of compositional variability in paper sludge from different mills was investigated by comparing process yields from a tissue and printed recycling mill, corrugated recycling mill and virgin fibre pulping mill. Ethanol production through simultaneous saccharification and fermentation was investigated in 20 L reactors, using commercially available enzymes and an industrial Saccharomyces cerevisiae strain. Tissue printed recycle paper sludge yielded a final ethanol concentration and conversion of 27.8 g/L and 70.6%, respectively at a solids loading and enzyme dosage of 33% (w/w) and 15 FPU/g dry substrate, respectively. Corrugated recycle paper sludge yielded an ethanol concentration of 39.4 g/L and a conversion of only 65.7%, with a solids loading and enzyme dosage of 27% (w/w) and 11 FPU/g dry substrate, respectively. Virgin pulp paper sludge had the highest ethanol concentration and conversion of 46.8 g/L and 87.4%, respectively at a solids loading and enzyme dosage of 18% (w/w) and 20 FPU/g dry substrate. Anaerobic digestion for biogas production was tested in 30 L reactors, using mixed inoculum obtained from the waste water treatment facility of a brewery. The methane production for tissue printed recycle, corrugated recycle and virgin pulp paper sludge was 31.6, 54.4 and 37.2 L/kg paper sludge, respectively. The achievable solids loading for tissue printed recycle and corrugated recycle paper sludge was 10% (w/v), while virgin pulp paper sludge was 6% (w/v). The conceptual biofuel plant design for the tissue and printing recycle mills, corrugated recycle mills and virgin pulp mills was based on average paper sludge production rates of 66, 10 and 36 dry tonnes/day respectively. A cash flow analysis was completed for each processing scenario as well as each paper sludge in order to determine what the minimum fuel selling price would be. For the fermentation process, the minimum ethanol selling price for viable investments were R8.34/L, R15.68/L and R5.47/L, respectively at a weighed cost of capital of 12% (real terms). The current market price for ethanol is R8.39/L, showing that virgin pulp paper sludge was the most viable for bioethanol production via fermentation. For the anaerobic digestion process, the minimum methane selling prices were R99.71/kg, R102.39/kg and R146.46/kg, respectively for tissue printed recycle paper sludge, corrugated recycle paper sludge and virgin pulp paper sludge. However, the current market price of methane is R27.26/kg, making the anaerobic digestion process unfeasible. This is due to the low methane yields achieved from paper sludge digestion as well as the high capital investment required to process large volumes of paper sludge. This study proved the feasibility of value addition to paper sludge, which subsequently reduces the amount of waste sent to landfill and benefits the industry revenue with an additional energy stream. Future endeavours are aimed at further reduction of landfill volumes through the anaerobic digestion of the residues after fermentation.