Cement kilns in the USA, France, Sweden and Norway have been used for many years to destroy hazardous waste. The plants are a potentially attractive means of burning organic residues because they operate at temperatures approaching 1,500oC, have gas residence times of five seconds, and do not need acid gas scrubbing equipment. In contrast, specialist hazardous waste incinerators operate at 1,200oC or less and have residence times of two seconds.
Waste is an attractive fuel to cement producers because it is cheaper than coal, which accounts for a substantial proportion of manufacturing costs. In Britain, household refuse and waste tyres have been used as supplementary fuels by the industry. But companies have been wary of burning hazardous waste because of concerns about adverse public reactions.
In 1991, however, Blue Circle carried out trial burns at a works in Suffolk using waste solvents supplied by a Yorkshire chemical company, Lambsons (ENDS Report 200, pp 11-13). But nothing appears to have come of their plans to expand this venture into a large-scale project.
Meanwhile, Castle Cement has been conducting test burns with CemfuelTM, a fuel formulated from hazardous waste, at its works in Clitheroe, north Lancashire, since last July. The company has just received an authorisation from HM Inspectorate of Pollution (HMIP) to replace 25% of the coal input to two of the site's three kilns with the new fuel.
The works' suppliers are two leading solvent recoverers, Solrec of Morecambe, Lancashire, and CMR in Rye, East Sussex. The fuel is produced from both liquid and solid low flash-point organic wastes, with the solids being ground, pulverised and suspended in a liquid carrier before the fuel is blended to the Cemfuel specification.
Explaining Solrec's decision to move into the business, a spokesman said that the company has latterly been inundated with samples of waste which was either not recyclable or would have no value in the market. The value of recovered solvents has also been declining in real terms for several years.
The fuel specification was developed as the test burns at the Clitheroe works proceeded, with periodic emission measurements being made by external consultants. It has been tailored to ensure that the fuel is compatible with the kiln chemistry, creates no toxicity problems in the product and minimises emissions while delivering energy to the process in a controlled manner. Ceilings have been set on the metal content of the waste together with a 1% chlorine limit, and wastes containing biologically active materials such as pesticides are excluded from the formulation.
According to Castle Cement, the emission tests showed no deterioration in emissions. Indeed, it says that flame stability in the kilns improves when Cemfuel is burned so that some emissions, notably of nitrogen oxides, are reduced. Dioxin emissions are within the 1ng/m3 limit set by HMIP. Modifications to the kilns include a system of interlocks to ensure that the supply of Cemfuel is interrupted if combustion conditions become unstable.
Only one of the kilns authorised to burn Cemfuel is currently in operation. This burns the fuel at a rate of two tonnes per hour, suggesting that total consumption of hazardous waste in the course of a year will be about 12-15,000 tonnes. While not all of the wastes formulated into Cemfuel will necessarily have been incinerated before, this is a sizeable slice of the incineration market, and operators of merchant incinerators may well be concerned about the loss of revenue.
The signs are that the business may expand significantly in the coming years. Solrec is just about to commission an 800 tonnes Cemfuel storage and processing facility, and intends to double its capacity by the end of 1993. Further test burns are under way at Clitheroe with Cemfuel providing up to 50% of the total fuel input.
Castle Cement, which has two other works at Padeswood in north Wales and Ketton, Lincolnshire, says that it is also "looking at extending this opportunity as the project progresses." At Padeswood, it is looking into the feasibility of burning organic wastes such as distillation residues which are less readily pumpable and have higher flash-points than the materials burned at Clitheroe. "Many manufacturers are having to review their disposal methods under the 'duty of care' legislation," a spokesman commented, "and combustion in cement kilns offers advantages over both landfill and conventional incineration." Disposal costs are said to be "significantly lower" with Cemfuel than with merchant incineration.