Eternal Flames

Relatively high fuel costs in Europe have encouraged operators of combustion plants to burn alternatives to traditional gas, oil and coal - or at least to investigate this possibility. In the cement industry, fuel costs at one site have actually been eliminated, through the use of landfill gas in combination with chipped tyres and other wastes.

There are many alternative fuels: petroleum coke, sawdust, discarded solvents, and spent lubricants being a few further examples. Some of them even present the prospect of generating revenue.

For any company considering a fuel change, several factors must be considered. The composition, physical properties and calorific value of any fuel are, of course, important factors, but there are many others, some less obvious.

Any alternative fuel must be suitable for the process. Contaminants, such as sulphur or metals, can be particularly problematic. In all combustion processes, they threaten the structure and mechanical soundness of the equipment. Where heat is provided directly - as in cement or lime production - they may affect adversely the medium being processed. Moreover, a particular fuel might prove a poor match to existing plant, possibly reducing output, increasing refractory costs, or posing difficulties in complying with emissions legislation.

Even if your new fuel is suitable, it might not be available. Some fuels are affected by seasonal or other cyclical factors, and may only be available at certain times of the year.

Considering consistency

Some early users of residual fuel oils and refinery bottoms found that the deliveries they received became increasingly viscous, and contained higher concentrations of contaminating metals, as new cracking techniques were developed.

But it's not only the quality of a fuel that might change - the price may also become unstable. Any fuel which gains in popularity will eventually be perceived by the supplier as a marketable resource instead of a waste requiring disposal - especially if there are potential users nearby. This will affect its price.

Regulatory questions must also be considered. The operational standards demanded for alternative fuels are often more stringent than those relating to traditional varieties. For processes regulated under Part I of the Environmental Protection Act, prior consultation and agreement with the relevant authority are necessary.

Alternative fuels are generally more difficult to burn than gas, oil or coal, but regulatory bodies often stipulate their (virtually) complete elimination within the plant. Good fuel/air mixing, long residence times and high temperatures are needed in the combustion vessel to degrade halogen, sulphur, phosphorus and metal compounds.

Fuels with a low proportion of volatile components, such as petroleum coke, can cause problems here, as they can be reluctant to ignite, and burn unevenly. Multi-fuel burners can provide greater flexibility and less vulnerability to temporary problems. These are not dependent on any one source, with the advantage that the most cost-effective fuel at a particular time can be employed. One popular combination is petroleum coke and oil, with the latter used for warm-up and flame stabilisation.

New fuels must be handled and stored carefully. Some sticky solids and liquids can clog pipework, while abrasive residual materials can damage components. Where a fuel is classified as a waste, the procedures for receiving and holding it will be checked by the authorities.

Covering the bases

Some consequences of switching fuels may not be immediately apparent. For example, rotary kilns can be fired with landfill gas. As over half of this fuel can be inert, however, effective fan suction becomes vital. The draught must handle the extra volume of gas which needs to be forced through the kiln to reach the required temperature.

Finally comes the crunch question of cost. The projected cost of buying a new fuel must be compared with the cost of installing the plant necessary to enable its use, plus extra operational expenses. Generally, the cheaper a fuel is, the more must be spent to make it combustible and to burn it, increasing both start-up and running costs.

It is also possible to burn materials which are not fuels - wastes with negligible calorific value, but which need thermal degradation to render them harmless or inert before disposal. If they do not interfere with product quality or production capacity, such materials can provide companies with a source of extra income, as they can levy charges equivalent to merchant incineration rates.

What help is available to companies considering a switch? Initially, the consequences of a fuel change can be evaluated without expensive and inconvenient on-site testing. This service, offered by FCT, involves modelling the combustion plant, and predicts how plant performance, fuel availability, maintenance and safety would be affected by the switch, together with an initial look at costs. Re-commendations are made concerning the optimisation of the plant concerned.

Simon Mather is business development manager of combustion specialist FCT