Kinetics of biomass and sewage sludge pyrolysis: Thermogravimetric and sealed reactor studies

Biomass and sewage sludge decomposition is studied by a competitive model consisting of weight loss and char formation reactions. Non-linear optimization technique is used to simulate the experi­ mental curves obtained by TGA by minimizing the sum of the squares functions. Pre-exponential factors and activation energy are obtained for both the reactions for heating rates of 10, 20 and 40 K/min for acacia wood, bagasse, rice husk, and sewage sludge respectively. The order of the reactions determined for sewage sludge are much higher than that of other biomass materials, whereas the activation energies are lower. Isothermal experimental studies resulted in much higher yields of char for all the four samples, and experiments carried out at various initial nitrogen gas pressures confirm that vapour-solid interactions are the cause of secondary char formation. Rice husk and sewage sludge yield higher amounts of secondary char due to the catalytic effect of the in­ organics present in higher amounts in these samples, Shortages and upward spiral in oil prices have given impetus to research on finding new sources of energy. Biomass and sewage sludge have been found to be such renewable and low sulphur energy resources', Amongst the different processes which have been proposed for the energetic utilization of biomass and sewage sludge, pyrolysis remains one of the most promising methods. It has certain advantages over other forms of waste disposal, since potentially all the products of pyrolysis-the gases, pyrolytic oils and char can be used as fuels to offset the costs of disposal. Various models for pyrolysis of biomass com­ ponents (mainly cellulose) have been reported I - / but there is not a single model which describes the pyrolysis of a wide variety of biomass and sewage sludge successfully. The models for sewage sludge pyrolysis8,9 are either very complicated or report very high reaction orders, of the order of 10 and 15. The present work combines experi­ mental and theoretical studies on acacia wood, bagasse, rice husk, and sewage sludge over a wide range of pyrolysis conditions, under dynamic heating and in a sealed reactor at various temper­ atures. The operative temperature range in TGA is 413-973 K. Heilting rates of 10, 20 and 40 K/ min are employed. The purpose of the model is to predict pyrolysis behaviour over a wide tem