Sustaining interest in incineration

In their recently promulgated 'London Communique', the leaders of 18 of the world's chemical engineering societies and organisations committed their members to strive towards the worldwide goal of sustainable development. Chemical and process engineers, they believe, can provide the technology to ensure that the world's resources are utilised in the most energy-efficient and environmentally-friendly fashion.

Printed on parchment, adorned with the seal of the IChemE, the Communique is an impressive document, but is it any more than that? Well, an early answer to the sceptics was given in July at the IChemE's 'First International Symposium on Incineration and Flue Gas Treatment Technologies', co-hosted by the University of Sheffield.

Attracting over 230 delegates from 23 countries, this two-day event addressed what is rapidly becoming the accepted 'best practical environmental option' for the treatment of municipal and other wastes. BPEO status for incineration, particularly with energy recovery, has perhaps still to be widely accepted in this country, where landfill remains the predominant disposal route. The introduction of the landfill tax may go some way to changing minds on this issue, but the international gathering at Sheffield clearly did not need convincing that incineration is the answer to municipal solid waste (MSW) disposal.

Environmentalists, on the other hand, remain to be convinced. In his keynote address, Ed Gallagher, chief executive of the Environment Agency highlighted the problem faced by the waste management industry. 'The public must have total confidence in the operation of incinerators,' he said. 'It will only take one bad plant to damn the whole industry.' That said, however, he argued that environmentalists 'do not give proper credence to what technology may achieve in the future.' And just what technology may, and indeed already has achieved, was the theme of the symposium.

Gallagher believes that an important aspect of incineration in the next century will be 'control: of inputs; of outputs; of the combustion process itself.' But to control a process, you first need to understand exactly what is going on. According to Prof Jim Swithenbank, of Sheffield University's Waste Incineration Centre, incineration is such a complex process that in the past the design of incinerators has not been based on a fundamental understanding and modelling of the process, but more on empirical rules.

But now technology is coming to the rescue of the designers in the form of more easily accessible computational fluid dynamics packages. Swithenbank maintains that almost all CFD codes were written originally by combustion technology research groups in their efforts to understand the complexities of the process. CFD should nevertheless be used with care, Swithenbank warns. 'Engineers must still understand the fundamentals of fluid dynamics and exercise their ingenuity, since CFD can only be used to quantify ideas.'

CFD covers the world

In spite of these caveats, the researchers at the Waste Incineration Centre have used CFD to model the gas flows in incinerators and are developing a model for the burning bed itself, in which many processes -drying, ignition, pyrolysis, gasification, and combustion - take place simultaneously.

Other speakers at Sheffield showed just how rapidly CFD has become a standard tool of the international engineering community. From South Korea, for example, Sangmin Choi explained how colleagues at the Korean Advanced Institute of Science and Technology have used the commercial Fluent CFD package to investigate combustion gas flow and its mixing with secondary air on a pilot scale 2ton/h MSW incinerator.

Fluent also features in the development work being carried out in Denmark by the company Ansaldo Volund. The company's parallel flow furnace features a flue gas recirculation system to improve the thermal efficiency of the plant. As well as the CFD modelling, the design has been cold-tested on a 1/12th scale model to show that it is possible to meet forthcoming EU legislation on NOx emissions by controlling the combustion process rather than post-treatment of the flue gas.

Conventionally, however, control of NOx and other emissions still calls for an 'end-of-pipe' approach. Emphasising this were the parallel sessions in Sheffield dedicated to flue gas treatment technologies. In his keynote lecture, Ray Allen of Sheffield's Waste Incineration Centre put the subject into perspective when he estimated that the 'environmental control section' of a modern incinerator plant could account for 15-20 per cent of the plant's total capital cost and have 'substantial operating costs throughout the lifetime of the plant'. As he says, the only way to make incineration acceptable has been to set new standards in gas clean-up. 'Public acceptability issues are driving the state of the art,' he maintains.

Allen's presentation focused on dry sorbent injection - generally of mixtures of activated carbon and hydrated lime - as the preferred clean-up technology, but other speakers put forward the merits of alternative approaches.

For example, most techniques for reducing NOx emissions rely on ammonia-based selective non-catalytic reduction (SNCR), or selective catalytic reduction (SCR) of the NOx. An obvious disadvantage of these processes is the problem of transporting, storing and handling either anhydrous or aqueous ammonia - particularly in the more urban locations of MSW incinerators.

A safer option is to use urea as the reducing agent. A proprietary system, NOxOUT, was discussed by Francois Grimard of the German company Nalco Fueltech. This SNCR process has been successfully applied in over 60 incineration plants worldwide.

The first application of a novel SCR system to a waste incineration plant was described by David Clark of the Shell subsidiary CRI Catalyst. Ammonia-based, the Shell DeNOx system has the advantage over conventional SCR technology of being able to remove NOx from flue gases at temperatures as low as 120 degrees C, compared with the normal 340 degrees to 380 degrees C. Installed last year at the Heeren waste plant in the Netherlands, the DeNOx system is keeping NOx levels below the 70mg/Nm3 stipulated by the Dutch authorities.

A radically different approach to any of the above is the pulsed corona discharge technology developed by AEA Technology (see PE March 1997, p45). AEA's John Carlow reviewed the results from the demonstration unit that was in operation last year at the Chineham municipal incinerator in Basingstoke. Measurements were made on VOCs, dioxins and furans - the emissions of which most influence public perception of incinerators - and NOx. Results were encouraging and the next stage in the development of this plasma-based technology is to test a scaled-up version to see if it is economically feasible.

According to keynote speaker, Lord Ezra, that next stage will be at SELCHP, the South East London Combined Heat and Power plant. Commissioned in 1994, SELCHP is, according to the company's chairman Gerald Atkins, the role model for the Best Practical Environmental Option (BPEO) for municipal solid waste disposal. An advisor to SELCHP, Lord Ezra is also chairman of Sheffield Heat and Power, which operates a CHP scheme based on the city council's MSW incinerator. Now in its fourth phase, this scheme is, says the company's chief executive David Lawrence, 'the largest district heating scheme in the UK, providing heat to homes, hospitals, universities, offices, public buildings, shops and leisure centres throughout the city centre.'

Both these projects - and the 250,000tpa waste-to-energy plant in Nottingham also mentioned by Lord Ezra - should serve as good reference sites if incineration is to penetrate the waste disposal market beyond its current level of less than 5 per cent.