Waste segregation presents thermal treatment opportunities

Developments in national and international waste recycling and re-use programs have led to the adoption of processes that segregate waste material into several streams. It is not viable to recycle several of these streams; nevertheless, some contain materials such as wood (biomass), contaminated paper (biomass), plastics and textiles that have greater fuel energy content than the original waste. Furthermore, adding value through pelletisation can provide a convenient form for handling and transportation of this fuel. An objective of this investigation is to achieve greater electrical power generation efficiency than incinerator boiler/turbines through the generation of an intermediate gaseous fuel for use in a combined cycle. Studies of the pyrolysis process that produces char, oil and gas has demonstrated how the rate of heating influences the proportion of each. Complementary studies of the combustion and gasification characteristics of segregated waste materials, and char derived from them, in special pot 'burners' has shown the effect of composition, form, size, packing and air flow rate on the burning and gasification rates. The results demonstrate the validity of our mathematical modelling code for the processes (FLIC) and when coupled with FLUENT this can in turn be used in the design and operation of domestic or industrial scale plants. These investigations have led to the design of a small scale gasifier that avoids the output of tar by its pyrolysis in a bed of hot char. These units are considered to be suitable for use with a Stirling engine for efficient CHP for individual buildings. An interesting feature of this system is that the overall CHP efficiency is independent of the gasifier efficiency. Our alternative configuration suitable for the generation of gas for a large scale CHP plant employs ultra superheated steam at a temperature of ∼1800°C that also produces a tar-free gas containing carbon monoxide and hydrogen.