Fundamental studies of the behaviour of biomass and waste materials in gasification systems
The project described in this document is concerned with an experimental study, at laboratory scale, of the characteristics and behaviour of a range of biomass and waste materials, relevant to their utilisation as fuels in gasification systems. The range of fuels studied includes the biomass materials, cereal straw, short rotation coppice wood and pine round wood, and refuse derived fuels (RDF). These materials are increasingly being employed as renewable alternatives to fossil fuels in industrial energy conversion plants in Europe and elsewhere. Chars were prepared from all of the materials at a temperature increase rate of 10 K min-¹ to a final temperature of 1173 K (900 °C), in a N₂ atmosphere. The biomass materials gave yields of char in the range 22-29 % (dry, ash free) and relationships were found which indicated that the char yield values increased linearly both with increasing carbon content and also increasing ash content of the parent material. The isothermal reactivities of these chars were measured in a flowing 0₂ atmosphere over the temperature range 553-713 K. A study of the influence of the ash on char reactivities indicated that the reactivity increased linearly with increasing ash content, the straw char being the sole exception to this. The reactivities of all the chars were found to increase linearly with increasing char CaO concentration. The results of a standard kinetic analysis of the biomass char oxidation data indicated that the reaction is approximately first order with respect to the carbon concentration, and the activation energy values of the chars, derived using the Arrhenius equation, were in the range 91-137 kJ mole⁻¹. These values are typical of these types of highly disordered and impure carbonaceous materials. The influence of CaO content on char activation energy was also investigated and an excellent non-linear relationship was found which indicated that the char activation energy decreasedw ith increasing CaO content. The effects of mineral constituents on the behaviour of biomass materials was further studied by subjecting cereal straw and pine wood samples to acid washing in both 1M HNO₃ and HCl solutions. The ash contents were reduced in all cases and the resultant char yields and char reactivity were significantly lower than those of the parent materials. When plotted on the char reactivity-CaO content curve, the acid washed material values fitted the correlation for the untreated biomass materials. An attempt was made to reproduce the behaviour of the RDF materials by preparing model composites containing mixtures of cellulosic materials and poly(ethene), PVC and Saran. The results indicated that char yields and char characteristics were influenced by component interactions during the pyrolysis process, with the presence of chlorine having a significant effect. The char yields of mixtures containing chlorinated polymers were found to be higher than expected from simple additive behaviour. There was evidence that the char yields and char reactivities were influenced by the behaviour of the polymers during pyrolysis and also by ash components of the mixture.