Design of a pyrolysis reactor for biochar production for ‘motocharcoal’ briquettes

Abstract

Zimbabwe is an agro-based country and as a result produces a lot of agro-waste ca. 70 000 tons per year. This waste occurs primarily in the form of waste cornstover, bagasse and sawdust which are currently not being utilized. There is potential for conversion of this agriculture waste to charcoal briquettes using the slow pyrolysis technology which increases the waste materials’ heating value. This paper presents a design for a pyrolysis reactor that can be used for conversion of agricultural waste to biochar which can then be used to make, Motocharcoal which is a trade name for the densified charcoal briquettes that are currently being made in Zimbabwe. It is an eco-fuel that can be used for heating and cooking purposes with a heating value of 22.5 MJ/kg. The pyrolysis reactor design was carried out based on an operating capacity of 12 tons/hr for an 8 hour working period based on the availability of the raw material which was mainly cornstover with a density of 200 kg/m3 . Pyrolysis took place under anaerobic conditions to enhance carbonisation. This was done at a temperature of 300 °C and a retention time of 3 hours to increase the calorific value of the biochar before briquetting. A conversion rate of 70% from agro-waste to biochar was achieved. The pyrolysis reactor will be constructed using stainless steel. Temperature and pressure were identified as the critical process parameters that must be controlled as these affect the quality of the biochar which in turn affects the briquettes quality. The pyrolysis reactor volume was 15.3 m3 with atmospheric pressure as the working pressure and a design pressure of 14.9 MPa. Fiberglass will be used as the insulation material to minimise heat losses and water used as the cooling agent. The designed pyrolysis reactor will ensure optimal conversion of the agro-waste to biochar for high quality briquettes