Simulating hazardous waste InCineration

Rotary kiln incineration in practice: Rechem's Fawley site (photo: Rechem)

ndustry generates a lot of temperature, residence time and waste-US industry alone mixing. Drawbacks of rotary kilns produces about 11 b tiy, of are their high costs- both which 280m t is hazardous. Of capital and maintenance-and this hazardous waste roughly the difficulty of making airtight 11m t is toxic chemical waste. seals between the rotating kiln The challenge to the processing and the stationary end plates. industry is first to reduce the Rotary kilns accept solid, amount of waste it generates slurry, and liquid wastes with and second to dispose safely of heating values between 1000 the remainder, meeting or and 15,000 Btu/lb, with an exceeding ever-tightening optimum value for mixed waste environmental regulations while of about 9500 BtU/lb. It is minimising capital and operating suitable for organic wastes costs. containing phosphorus, silicon, sodium, sulphur, fluorine, Landfill is not a favoured option nitrogen, and halogens. It is not for disposing of hazardous waste, recommended for heavy metals, and in the USA this route is now inerts, inorganic salts or organic banned by the Environmental sludges that become sticky Protection Authority (EPA). Even in during operation. a carefully-designed landfill there The rotary kiln incineration is a risk that leachate will escape, train (Figure 1, page s 12) has carrying with it toxic components four stages: the waste feed such as pentachlorophenol (PCP), system, the rotary kiln itself, the polycyclic aromatic hydrocarbons secondary combustion chamber (PAHs) and polychlorinated and equipment for air pollution biphenyls (PCBs). Once these control (APC). Pre-mixed waste materials have bound themselves enters the kiln, where fuel to soil particles they may take burning in preheated years to biodegrade, meanwhile Ihab H Farag explains how to model a rotary kiln combustion air provides the entering the food chain through temperature needed to groundwater or takeup by plants. incinerate the waste and oxidise the hazardous material. I.nstead, companies who have to dispose of hazardous and The kiln axis is at a slight angle to the horizontal, so the ~ox~c wastes are focusing on thermal treatment, especially rotation causes the hot ashes to move down to the far end of ~nc~neration. Roughly 60% of all hazardous waste can be the kiln. Here the ashes are continuously removed, quenched Incinerated successfully, and for these wastes incineration is if necessary in cold water, and removed for disposal in a often assumed to be the ultimate disposal approach. landfill. The main goal is to achieve 99.99% (or "four nines") The hot gases leaving the kiln are further oxidised in the destruction and removal efficiency (ORE) for hazardous SCC until all the organic material has been completely waste, and "six nines" (99.9999%) ORE for toxic wastes. A converted to carbon dioxide and water vapour. The hot gases properly deSigned and operated incinerator can achieve then pass through a waste heat recovery boiler to improve the these ORE figures, with the added advantages of reducing energy efficiency. Harmful components in the flue gasthe waste volume considerably and allowing the recovery of oxides of nitrogen and sulphur, hydrogen chloride, products Useful heat. of incomplete combustion (PICs) and dust-are treated and I InCineration is also a good way to treat soil contaminated by removed in the APC. The exit gas is discharged to the leak~ge from waste dumps or production plants (excavated atmosphere. andflll material (ELM)), and municipal solid waste (MSW).

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Simulating hazardous waste • • • InCineration

~he EPA estimated in 1989 that t~·ere were roughly 200 bazardous waste incinerators in the USA. They fall into four road classes-liquid incinerators hearth-type furnaces, rotary kilns and fluidised-bed cont~ctors. Of these, rotary kilns ~~e p~obabl~ the most versatile, especially when it comes to ndling solid materials. (S~ rotary kiln followed by a secondary combustion chamber C) can achieve OREs of 99.995%-99.997% in commercial ~~~: Behind these high values lie the rotary kiln incinerator's llity to provide good control over combustion conditions: The Chemical

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The challenge to the industry is to design and build successful incinerators in the shortest possible time and at the lowest cost, including design and operating costs as wei! as capital cost. Unfortunately, because the complete rotary kiln incineration train is a complex system involving several chemical and mechanical processes, design is usually costly and time-consuming. Although experiments are an excellent tool to characterise the process, they are much more costly than calculations. \Vith the availability of advanced process simulators it is obvious that s11

Fuel ~

Feed

Secondary combustion chamber

Waste feed system

Heat recol/ery boiler

to atmospher