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Learning to live with landfill gas: dealing with fuel contaminants in landfill gas-to-energy systems is essentially a matter of economics--weighing the cost of removal against the benefits of improved service intervals, performance and reliability, and assessing whether removal is a better option than running a contaminant tolerant engine design.

Article Excerpt
Landfill-gas-to-energy system operators have a range of options available to them for dealing with fuel-borne contaminants, in two basic categories--gas pretreatment systems that remove from the methane many of the contaminants to produce fuel that meets the engines' operating requirements, and special engine designs that combat the effects of corrosive fuels. The latter forego all but the simplest pretreatment and incorporate specially modified engines that can burn impure fuel, but which still deliver acceptable component life and maintenance intervals. Both solutions have proven effective in multiple applications worldwide. Operators should analyse both methods as part of project planning. The "right answer" for any one project may well include a combination of fuel treatment options and optimised engine maintenance practices. To forego this analysis is to risk missing an opportunity to achieve the lowest practical capital and maintenance cost for the system.

Installing equipment to reduce fuel contaminants delivers better service intervals but has to be balanced against higher ancillary equipment capital and maintenance costs. The alternative, using specialised landfill gas (low energy fuel) engines, means accepting design innovations in component metallurgy and other techniques that limit acid formation and keep engines operating safely but add to engine maintenance costs. Neither choice is universally better than the other. The most favourable approach for a given application depends on various factors: gas composition, emissions limits at the site, local power market conditions, and the site owner's performance and reliability expectations. In the end, the choice typically boils down to economics. The winning approach is the one that delivers, all things considered, the lower cost per kWh.

While fuel pretreatment has a longer history and more name recognition in the landfill-gas-to-energy market, engine designs that deal with fuel contaminants internally have a 20-year track record of effectiveness. Those designs have improved steadily and are available on even the most technologically advanced, high-efficiency gas engines on the market. Therefore, any landfill-gas-to-energy project developer can benefit from analysing and comparing both approaches in the light of site-specific conditions.

Case studies at North American landfill sites illustrate extensive field experience with engines using internal modifications to limit the effects of corrosive fuel contaminants.

Fuel contaminants

For users considering gas engines for power generation, fuel quality has been a key concern since the dawn of the landfill-gas-to-energy industry. Landfill fuel contaminants, if not dealt with, will cause a wide range of engine problems. Contaminants of greatest concern are:

Sulphur compounds. During the combustion cycle, sulphur compounds--notably hydrogen sulphide ([H.sub.2]S)--react with oxygen to form S[O.sub.2] which combines with available water (a major product of combustion) to form a weak sulphurous/sulphuric acid solution. If left unattended, this acid can severely damage aftercooler cores, bearings, and any copper-containing engine components.

Halides. When halogenated hydrocarbons (chlorofluoro-carbons, or CFCs) are oxidised in engine combustion, they release chlorine and fluorine, which in turn unite with water from the combustion process to form hydrochloric (HCI) and hydrofluoric (HF) acids. These acids, if not taken care of properly, will attack piston rings, cylinder liners, exhaust valve stems, valve guides and other critical wear parts.

Water vapour. Water vapour is not considered to be harmful when entrained as a vapour in the fuel at quantities below the point of saturation. (In fact, about 10% of the engine exhaust is water vapour.) However, water vapour in the fuel can combine with organic compounds common in landfill fuels during the combustion process to form mineral acids like sulphuric, hydrochloric or hydrochloric acid. Even...