This paper discusses a number of design modifications and changes in operational conditions that have a major influence on the overall performance of the large Coventry municipal incinerator plant (65 MW), which 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 a computational fluid dynamic model, in an attempt to obtain optimum combustion conditions that would minimize potential emissions of toxic pollutants and reduce maintenance costs at the plant. The modeling work showed that the use of four large high-speed secondary air jets firing toward a common center, together with optimizing the primary air distribution along the grate, produces substantially longer residence times and improves temperature profile at the exit. The novel feature of the proposed secondary air-injection system is the formation of a large and significantly strong recirculation zone, located above the burning refuse bed, in the middle of the furnace shaft. This strong recirculation zone improves the overall performance of the incinerator due to intensive mixing of hot gaseous products evolving from the refuse bed with the combustion air supplied as the secondary air, and greatly improves the gas phase combustion and helps to reduce emissions of chlorinated organic compounds. Results obtained clearly demonstrate that the proposed secondary air-injection system has an excellent potential to satisfy most requirements for emissions levels and gas resident times specified by the European Directives (EEC) for municipal incinerators with a vertical radiation shaft.

The value of knowing the gas residence times in large municipal incinerators and the serious error imposed by the traditional use of gas volume flow rate based average residence time with regard to these incinerators are recognized. There is increasing public awareness and concern over emissions from municipal solid waste incinerators. Modelling studies of particle trajectories using computational fluid dynamics shows the utility of simulation for the determination of residence time distribution in incinerators. These studies indicate that residence time distributions contain valuable information, important to the understanding and evaluation of mixing processes in the incinerator overfire region. A number of design modifications and changes in operational conditions (which have a major influence on the mean gas residence times and overall performance of two large municipal incinerator plants (35 MW and 65 MW) are proposed. Specifically, the effect of high speed jets and different internal baffle configurations have been investigated using mathematical modelling, in an attempt to obtain optimum combustion conditions. This would increase the mean gas residence times, minimize the emission of pollutants and improve the temperature profile throughout the system. The utility of having detailed RTD information for the incinerator overfire region is demonstrated by estimating the residence times using numerical simulation of tracer injection into the 3-dimensional reacting flow field of two typical municipal incinerator configurations. The modelling results not only underscore the critical role played by jets in achieving desirable states of mixing in the overfire region but also point to the significance of the geometry effects.This modelling work has yielded important results, all of which clearly could not be experimentally verified at the industrial scale due to practical and cost limitations. Nevertheless, the results guide specific modifications to the design and operation and this new approach will be of considerable use to the incinerator design community.

This paper presents the findings of the modelling studies carried out at Sheffield University in applying the finite difference technique and experimental data to model the Coventry municipal solid waste (MSW) incinerator plant (65 MW). The main objective of this modelling work was to investigate the influence of the design and operating parameters on the overall performance of the incinerator which burns domestic and commercial wastes (approximately 900 tonne/day). A combustion model of the Essenhigh type was employed to model the combustion processes inside the refuse bed on top of the travelling grate. In addition, a mathematical model (FLUENT) was used to predict the three-dimensional reacting flows (gaseous phase) within the incinerator geometry. Experimental measurements of gas composition, temperature and velocity were compared with the model predictions. Modelling results correctly indicated trends and were invaluable for the interpretation of the incinerator performance data. As a result of the test data and mathematical modelling of the whole process, suggestions for design improvements to the Coventry MSW incinerator were made. In particular, as a consequence of this modelling work, it is proposed that the use of four large high speed secondary air jets firing towards a common centre together with optimizing the primary air distribution along the grate can produce substantially longer residence times, improve the temperature profile at the exit, reduce the concentration of toxic pollutants and increase the combustion efficiency of the plant. The principal conclusion reached from this modelling study is that upgrading of most existing MSW incinerators to meet current environmental criteria may be achieved with simple modification of the present conventional designs together with the optimization of the plant overall performance as a function of key process operational variables.