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Burning up waste - energy for cement making

Clay Technology magazine
,
17 Apr 2009
Kiln at Ketton Cement in Lincolnshire, UK

The production of cement is an energy intensive process. Cement manufacturers are turning to alternative fuels, just like their clay counterparts. Richard Butcher, Sales and Marketing Director from Solvent Resource Management Ltd, Heysham, UK, reports

It takes the equivalent of one tonne of coal to make nine tonnes of cement. The cost of coal is therefore a significant proportion of a cement works outgoings.

In order to reduce these costs, cement companies have looked at waste materials as alternatives to coal. This began in the early 1990s, and now over 50% of a cement kiln’s energy input is often derived from wastes. The commercial incentive is that manufacturers can sometimes be paid money to use these alternative fuels, with the worst case being they have to purchase the materials for far less than they would pay for coal.

On fire

There are two parts of the cement making process that require heat input. The first is in the calciner tower where, primarily, limestone (along with silica and alumina) is heated to 900°C to drive off CO2 and convert it to calcium oxide. The second heat input is via the main burner in the rotating kiln. With a flame temperature of around 2,000°C, the chemical reactions in the kiln produce the calcium alumina silicate complex that we know as cement.

A cement kiln is an ideal vehicle for using alternative fuels, as the alkaline environment within it provides a natural scrubbing mechanism for any acidic combustion gases. In addition, ash from the combustion of alternative fuels is incorporated into the cement, just as coal ash would be.

The waste materials that tend to be used in the calciner are traditionally non-hazardous, such as tyres, meat and bone meal (MBM), paper and plastics (aka Profuel, refuse derived fuel or solid recovered fuel), wood chips and sewage sludge pellets.

Meat and bone meal is the granular solid residue that is left after the fat (tallow) has been extracted during high temperature animal waste rendering. It has a calorific value equivalent to 70% of that of coal and is delivered to the cement works in powder tankers. Storage at the site is in specially designed silos, and feed systems tend to be pneumatic. It is classified as 100% biomass, bringing with it CO2 emission benefits to the kiln as well.

Paper and plastic wastes tend to be manufactured from municipally derived and industrial wastes, and also have a calorific value equivalent to 70% of that of coal. The manufacturing process involves removing contaminants such as glass and metal and producing similarly sized pieces to provide constant thermal characteristics within the kiln.

Profuel is delivered to the works in tipper wagons or walking floor trailers, and has to be stored in a covered dry warehouse. It can be conveyed into the kiln by mechanical (belt) or pneumatic means. The biomass content of Profuel is typically 50%. The UK is striving to reduce its dependency on landfill to dispose of municipally derived wastes, thus the amount of Profuel available has increased over the last three years.

Tyres have a calorific value comparable to that of coal and can be introduced into the calciner whole or as chips. Whole tyres are more difficult to feed but shredding them is expensive and the chips are difficult to transport pneumatically, as they have a tendency to bind together and block pipework. Therefore mechanical means are normally used for conveying. Due to the high temperatures within the calciner, complete combustion takes place – there is none of the black smoke that is normally associated with tyre burning.

Wood chips are classed as 100% biomass but have a relatively low calorific value – around 40% of the coal equivalent value. The chips are produced by shredding wood wastes, including chipboard and plywood. The bark content needs to be minimal as it does not burn easily and can lead to unstable conditions in the kiln. Although the chips can be stored outside (rain water tends not to penetrate too deeply), conveying into the kiln can again be by mechanical or pneumatic means.

Sewage sludge pellets are produced by drying and pelletising digested sewage sludge. This is expensive and water companies have tended to find other ways of recycling and disposing of their sludge. The relatively low calorific value of the pellets (40% of coal) and expensive storage requirements (specially designed silos with odour abatement) have discouraged many cement works from using them as an alternative fuel. However, their high biomass content means they are seen as sustainable and renewable.

The non-hazardous alternative fuels that are normally used in the main burner are MBM and Profuel. In addition, a hazardous waste liquid fuel made from waste solvents – known as ‘Cemfuel’ (a Castle Cement Ltd trademark), recycled liquid fuel or secondary liquid fuel – can also be used in the main burner. This fuel is made from waste solvents and other organic wastes that cannot be economically recycled by other means, and may contain up to 20% of organic solids. Typical components of Cemfuel include waste paints, adhesives and oily sludges. This fuel can only be used in the main burner because, as a hazardous waste, its use requires temperatures in excess of 1,200°C. As well as having a good calorific value (typically 75% of coal) and a stable flame, Cemfuel is also found to reduce NOx emissions.

This is a key point in the use of any alternative fuel – it must not have a negative environmental impact, regardless of the commercial benefits. Alternative fuels within the cement industry are highly regulated by the UK Environment Agency, with a vast amount of emissions monitoring carried out.

Further Information: Solvent Resource Management Ltd