Power from waste
Learning to live with landfill gas1 October 2006
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.
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.
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 (H2S) – react with oxygen to form SO2 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 (HCl) 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 the carbon dioxide can combine with water to form carbonic acid. These acids may attack engine components. There does seem to be a relationship between the amount of water vapour in the fuel and the amount of sulphur, halides and (some species of) siloxanes that enter the engine through the fuel. Lower amounts of water vapour in the fuel have proven to be better.
Silicon crystals. Microscopic silicon (sand) crystals can travel with the landfill gas, agglomerate during combustion, and form larger particles that cling to exhaust valve faces and seats.
Siloxanes. These substances, commonly found in household products like shampoo, cosmetics and detergents, break down during combustion and lead to hard silica (SiO2) and silicate deposits in combustion chambers, exhaust manifolds and exhaust stacks. In the cylinders, deposits on valve faces lead to grinding action and increased valve seat wear. Chipping of the silicate deposits accumulated in the combustion chamber can lead to severe valve damage. This occurs when a thick deposit chips away, leaving a gap through which hot combustion gas flows while the valve is closed. It can also occur when a loose chip from the combustion chamber gets trapped in a closing valve as it exits the cylinder. The resulting blowtorch effect melts part of the valve, a phenomenon commonly referred to as guttering. In many cases where siloxanes are present in the fuel, siloxane buildup on cylinder heads and on pistons physically reduces cylinder volume and increases the compression ratio, driving up cylinder pressures and carbon monoxide emissions.
Impact of fuel contaminants
The effect of contaminants on the engine depends on a number of factors, including engine component metallurgy, exposure time and rate, engine operating temperature, and the brake mean effective pressure (BMEP) of the engine.
With minimal or no fuel treatment, a standard natural gas engine operating on a typical landfill gas can suffer cylinder head life shortened by 50 to 75% or more when compared with a similar engine operating on pipeline natural gas. Lube oil life is also shortened, through acidification and high levels of silicone in the oil from blowby of H2S and silicon into the crankcase.
Most successful landfill operators analyse their oil for silicon in addition to acids and other standard wear indicators of oil condition. In fact, silicon may dictate the oil change interval: Most landfill sites use 100 to 120 ppm silicon in the oil as a condemning limit. Spark plugs in landfill engines also require extra attention, mainly because of silicon deposits, which shield the electrodes and increase the voltage required to fire the plug. A higher level of siloxanes in the fuel typically translates into shorter spark plug life. In extreme cases, plug life can be reduced by as much as 90%.
Because each landfill fuel is different and subject to seasonal and even daily change, operators must customise maintenance programmes – intervals for oil and filter changes, spark plug cleaning and replacement, top end, in-frame and major overhaul, and other tasks. This is best accomplished by collecting site maintenance and operating data, observing wear and degradation trends, and adjusting practices accordingly.
Generator set manufacturers will provide fuel-quality specifications that include recommended maximum levels of contaminants allowable in an engine to maintain optimum operating conditions. Fuel exceeding specified contaminant concentration recommendations would compromise performance and/or service life.
Exceeding the maximum contaminant values by a small amount might simply reduce oil change or spark plug replacement intervals by some small amount. Accepting contaminant levels far in excess of the recommended levels might decrease oil change and spark plug maintenance intervals by quite a bit, and also significantly decrease the time to major service intervals like top end or in-frame overhauls. And not all contaminants have the same effect on an engine life. For example, if only H2S greatly exceeds the recommended value, it might only have a major impact on the oil change interval and not much of an effect on spark plug or overhaul life, while a very high level of siloxanes might lead to decreased oil, spark plug and overhaul schedules.
Not surprisingly, contaminant limits for landfill engines differ from those for standard engines (refer to Table 1). The recommended levels of contaminants in the chart are those that will allow the engine to achieve the designed service life represented in the manufacturers recommended maintenance schedule. Still, these recommended contaminant limits could help remove some trial-and-error from the development of proper maintenance programs and fuel delivery systems.
The chart lists maximum recommended fuel contaminants on a “contaminant mass/fuel heat value” basis rather than on a “contaminant mass/volume” basis. Assuming the same level of contaminant per unit volume of gas, a volume at 600 btu/ft3 (22.4MJ/Nm3) will have 50% of the contaminants of a volume at 300 btu/ft3 (11.2MJ/Nm3). When the contaminants are found in concentrations above the manufacturer recommended levels, higher contaminant levels per unit volume will have a significant impact on the service life of the engine.
As acids naturally form in the engine, it is important to use a type of lube oil in the engine that will “capture” these acids. The total base number (or TBN) is an indicator of the acid absorbing capability of an oil: the higher the TBN number, the better the acid neutralising capability.
But higher TBN is not always better for a gas engine. An ash material is typically the oil additive used to neutralise acids, and high-ash oils are not recommended for natural gas engines. High levels of ash in lube oil will leave carbon deposits when combusted in natural gas engines that can cause premature wear and require an in-frame overhaul to replace the affected components. Therefore, users should look for an oil with the highest TBN recommended by the engine manufacturer, and use a scheduled oil sampling system to optimise the oil-change interval.
In protecting engine performance and longevity, there is no free lunch. Either basic approach – fuel pretreatment or the acceptance of reduced maintenance intervals with specially designed landfill engines – adds capital cost and affects long-term maintenance expense. The question is whichoption is the more economical for a given site.
Fuel pretreatment approach
A front-end gas processing (pretreatment) system can make up a significant share of a project’s capital cost. The components must be chosen for function, reliability, and resistance to corrosive damage from the impurities they remove. In theory, pretreatment could deliver near-pipeline-quality gas, but that is seldom if ever economic. The pretreatment design usually requires a compromise: fuel good enough to enable reliable engine performance under a reasonable maintenance regimen.
Figure 1 shows a typical gas pretreatment system. Every installation should include an inlet scrubber and fuel filter to remove water droplets from the gas and trap solid matter, and a gas compressor to deliver fuel to the engines at the necessary volume and pressure. Other gas treatment steps commonly include:
• Demister. Removes oil from the gas stream in systems where oil is injected into the gas to lubricate the compressor.
• Gas-to-air cooler. Lowers the temperature of the gas after it is compressed, thus reducing moisture and preventing condensation and attendant acid formation later in the fuel delivery system, or inside the engines.
• Gas-to-gas heat exchanger. Pre-cools the gas entering the dryer to reduce dryer power demand. The gas leaving the dryer is reheated later in the process by the gas-to-gas heat exchanger to prevent water from condensing downstream. These heat exchangers are typically made of stainless steel.
• Dryer. An effective way to reduce halogens and H2S in the gas. The device is usually a gas–to–liquid heat exchanger that uses a refrigerant. The gas is dried by chilling to a dew point of 36 to 37ºF (2.2 to 2.8ºC). Because halogens and H2S are water soluble, reducing water content also reduces their concentrations. The dryer also reduces, to a lesser extent, some species of gas-borne siloxanes.
• Coalescing filter. Removes any remaining water or oil droplets, and remaining solid matter down to 0.4 microns in size.
• Condensate drain. Collects water removed from the gas. The water may be treated for discharge to a sewer system or, in some locations, reintroduced to the landfill to stimulate methane production.
An effective fuel treatment system helps reduce special maintenance demands on the engines but does require maintenance of its own. Every component in the treatment train needs service at intervals dictated by the characteristics of the fuel and the equipment make and model. Since the gas recovery project is only as reliable as the weakest link, all fuel pretreatment components need to be selected with the same attention to detail as the gas compressor, generator set or other system component.
Special engine design
In the mid-1980s, engine designers began looking at the demands of low-energy fuel applications (chiefly landfill and digester gas) and seeking ways to “harden” engines against fuel impurities. In essence, the engine designers accepted the realities of the corrosive fuels introduced to their engines and modified the design of critical components and systems to counteract the effects of many of these contaminants.
The resulting low energy fuel engines still require an inlet gas scrubber and gas compressor, and they may need other fuel treatment steps under certain fuel conditions. In general, though, the engine modifications themselves were designed to counteract the effects of fuel-borne contaminants.
While modifications add to the capital cost of the engines, the capital and maintenance cost of pretreatment equipment can be reduced, sometimes significantly. Field experience demonstrates that the engines achieve acceptable maintenance and service intervals, and availability percentages that are highly competitive in the landfill industry. The engine modifications have three basic strategies:
• Keep harmful substances from forming inside the engine.
• Make highly susceptible components corrosion resistant.
• Eject potentially corrosive gases.
Here are the specific modifications:
• Optimised jacket water temperature. A two-circuit cooling system keeps the jacket water at 230ºF (110ºC), optimal for landfill service and well above the 194° to 210°F (90° to 99°C) range typical of standard natural gas engines.
The warmer jacket water temperature inhibits water vapour entrained in the exhaust gas and blow-by gases from condensing on the cylinder liners and on other internal engine surfaces, thus limiting the formation of acids and attendant corrosion. It also helps keep condensation and acid formation from reaching the lubricating oil, further protecting components and helping to extend oil-change intervals.
Tests to date indicate that the elevated jacket water temperature can improve oil life, significantly reduce cylinder liner pitting and the corrosion of other cylinder components, crankshafts, bearings and other critical wear parts without jeopardising expected component life and maintenance intervals. The elevated jacket water temperature is enabled by a higher coolant flow through the cylinder block, and by the addition of heat-resistant jacket water system and pump seals. In addition, the oil cooler is moved from the jacket water circuit to a lower-temperature auxiliary circuit. This keeps the oil temperature from exceeding its allowable limit of 210ºF (99ºC). The oil cooler thermostat begins to open at 200ºF (93ºC) to prevent the oil from being over cooled.
• Corrosion-resistant materials. As further protection against naturally forming acids, the landfill-specific design minimises the use of bright metals (copper and unprotected steel) in components likely to come in contact with the fuel or exhaust gases. For example, the aftercooler cores, made of copper alloys in standard gas engines, are made of stainless steel in the landfill versions to resist attacks from acids of sulphur, chlorine and fluorine.
On precombustion-chambered engines, the ignition body that holds the prechamber into the cylinder head has higher corrosion resistance than in standard engines. Extensive field-testing has shown this component to be susceptible to corrosion unless the material is upgraded.
The balance of landfill cylinder head components are of a similar design to those in standard engines, but materials may have been modified for longer life. The intake and exhaust valve materials, for example, have been modified for improved heat resistance, and the valve seat angles are optimised to minimise the formation of hard deposits. On some engines, a three-angle-shape valve face design combines long life with increased contact forces that combat deposits from higher-ash oils and other silicon deposits typical in landfill engines.
• Crankcase ventilation. An additional line of defence against engine corrosion comes from a low-pressure air pump that draws warm, filtered air into the crankcase and evacuates harmful gases (see Figure 2). Thus when the engine is needed on line or has been shut down for maintenance or repair, the crankcase components are not exposed to condensing corrosive blow-by gases. Engine oil life will be improved as well.
Where ambient air is cool enough so that ventilation air itself might cause condensation in the crankcase, the inlet air can be preheated by passing it through a duct over the exhaust manifold.
Comparing the merits of external fuel pretreatment with those of engines designed specifically for landfill service requires a rigorous but relatively simple cost analysis.
The process begins with a fuel analysis, because that largely dictates the degree of fuel pretreatment that landfill engines will need. If the landfill has been in operation for awhile, multiple samples should be evaluated to understand the minimum and maximum methane content as well as the minimum and maximum contaminant levels expected in the fuel over time. If the landfill is a new installation, a landfill sample with an “educated guess” as to the methane content and expected fuel contaminant levels (based on the expected fill materials) should be evaluated. In general, the more contaminants there are in the fuel, and the larger their concentration, the greater the demands on the pretreatment system, and the greater its cost.
Based on experience, the pretreatment system designer can estimate the capital (installed) cost of the system as well as the expected maintenance cost of the components and the projected total ownership cost. Based on the expected quality of the treated fuel, the generator set manufacturer will be able to project intervals for basic maintenance, as well as expected run times to top-end, in-frame and major overhauls. This allows a calculation of the total cost of maintenance, including service labour, components, and consumables (fluids, filters, etc) over the project life.
Manufacturers with experience in landfill operation may be able to provide long-term maintenance contracts that guarantee a fixed maintenance cost per kilowatt-hour. Such contracts also can include guarantees covering uptime (as a percentage of total available operating hours) and emissions levels.
With these calculations complete, it is relatively simple to compare installed and ownership costs per kWh for the two alternatives. As a general rule – and especially where the cost difference is not compelling – it may be worthwhile to consider various intangibles. These include:
• Familiarity of in-house staff with the equipment they will be asked to maintain.
• The quality and availability of service and technical support (including time to deliver routine and emergency replacement parts)
• The comparative track records of the equipment suppliers (in particular, their direct experience in landfill-gas-to-energy projects).
Elevated jacket water temperature technology has been in use in landfill applications since the mid-1980s, and many of the other landfill modifications have evolved since then. Altogether, these technologies are at work on more than 920 MW of landfill gas Caterpillar engine projects installed since 1996. Some examples are shown in the panel (page 0). These modified landfill versions were developed through the US Department of Energy Advanced Reciprocating Engine Systems (ARES) program and were introduced commercially for the first time in late 2002. They will be available in 12, 16 and 20 cylinder configurations, operating at 50 Hz or 60 Hz, at speeds up to 1500 rpm, with ratings up to 1950 kW. A leaner fuel mix reduces combustion temperatures and drives down NOx formation. NOx ratings as low as 1.0 and 0.5 g/bhp-hr, 60 Hz (250 and 500 mg/Nm3, 50 Hz) are available without exhaust aftertreatment. A low-pressure fuel system (1.5 to 5 psi/ 10 to 35 kPa) is adopted for landfill gas service.