Techno-economic assessment for biochar production from paper mill sludge

Abstract

Sludge generated from the wastewater treatment plant of a pulp and paper factory was used as feedstock for the preparation of biochar using fast pyrolysis followed by chemical activation with NaOH to make activated biochar (ABC). A process utilising 300 kg/day of waste paper mill sludge (PMS) was designed with a plant utilisation of 80%. A pyrolysis temperature of 500 oC and residence time of 2 h was used in a limited oxygen pyrolysis reactor producing biochar at a yield of 40 %. The biochar was activated using NaOH and heated for an hour at 70 oC to enhance the sorption properties of the ABC. The resulting ABC had a BET surface area of 492.02 m 2 /g, had decolourising properties and a pH of 7, hence applicable in wastewater treatment. A capital investment of US$1 860 729.47 is required and the plant life is 20 years, with a net present value of US$ 1 832 445.94. The rate of return is 29.6 %, return on investment is 26.9% and a payback period of 3.7 years. The production of ABC from PMS is economically feasible, with a product selling price of $30.00/kg.