Effect Of High Speed Secondary Air Jets On The Overall Performance Of A Large MSW Incinerator With A Vertical Shaft

Abstract

This paper discusses a number of proposed design modifications and changes in operational conditions which have a major influence on the overall performance of the Sheffield municipal solid waste incinerator plant. This 35 MW unit incorporates a heat recovery system for district heating. Four different secondary air injection systems and various primary air distribution patterns along the grate were investigated using computational fluid dynamics in an attempt to eliminate the existing slagging problems and at Ihe same time obtain optimum combustion conditions which would minimise potential emissions of toxic pollutants and reduce maintenance costs at the plant. The modelling work showed that the use of four tangential high speed secondary air jets together with optimising the primary air distribution along the grate, produces substantially longer residence times, improves the temperature profile at the exit, reduces the concentration of toxic pollutants and increases the combustion efficiency of the incinerator. The novel feature of the proposed secondary air injection system is the formation of a large size and significantly strong recirculation zone located above the burning refuse bed, in the middle of the furnace shaft. The presence of this strong recirculation zone improves the overall performance of the incinerator due to intensive mixing of the hot gaseous products (CO, volatile matter and hydrocarbons) evolving from the refuse bed with the combustion air supplied as the secondary air, and thereby greatly improves the gas phase combustion and helps to reduce emissions of chlorinated organic compounds; i.e. Dioxins.

Results obtained to date clearly demonstrate that the proposed secondary air injection system has excellent potential to help satisfy all the requirements for the emission levels and gas residence times specified by the EC Directives for municipal incinerators with a vertical radiation shaft. The principal conclusion reached from this modelling study is that upgrading of the existing municipal solid waste incinerators to meet some current environmental criteria can often be achieved with modification of the present conventional secondary air jet system designs together with the optimisation of the plant overall performance as a function of key process operating variables.