Natural Gas Engine - LPG, Field Gas and Biogas
2.4.7.1 Liquefied Petroleum Gas
Liquefied petroleum gas (LPG) is composed primarily of propane and/or butane. While propane and butane ratings are higher than gasoline, most stationary spark ignition engines are designed with higher compression ratios that optimize operation with natural gas and its associated high methane number.
Use of fuels with lower methane numbers like LPG in natural gas engines requires retarding of ignition timing and other appropriate adjustments to avoid detonation (knocking). LPG often serves as a back-up fuel where there is a possibility of interruption in the natural gas supply. Off-spec LPG may require cooling to condense out larger volumes of butane or heavier hydrocarbons that would aggravate engine knock. High butane content LPG is recommended only for low compression, naturally aspirated engines.
2.4.7.2 Field Gas
Field gas often contains more than 5 percent by volume of heavy ends (butane and heavier), as well as water, salts and H2S and usually requires some scrubbing before use in natural gas engines. Cooling may be required to reduce the concentrations of butane and heavier components. Field gas usually contains some propane and normally is used in low compression engines (both naturally aspirated and turbocharged). Retarded ignition timing eliminates detonation.
2.4.7.3 Biogas
Biogases (landfill gas and digester gas) are predominantly mixtures of methane and CO2 with HHV in the range of 300 to 700 Btu/scf. Landfill gas also contains a variety of contaminants as discussed earlier.
Biogases are produced essentially at or somewhat below atmospheric pressure so must be compressed for delivery to the engine. After compression, cooling and scrubbing or filtration are required to remove compressor oil, condensate, and any particulates that may have been entrained in the original gas.
Scrubbing with a caustic solution may be required if acid gases are present. Because of the additional requirements for raw gas treatment, biogas powered engine facilities are more costly to build and operate than natural gas-based systems.
A key contaminant in biogas is a class of compounds called siloxanes, a subgroup of silicones containing Si-O bonds with organic radicals. These compounds are widely used for a variety of industrial processes and are also commonly added to consumer products, including detergents, shampoos, cosmetics, paper coatings, and textiles. Siloxanes in wastewater do not break down in wastewater treatment facilities or in landfills. As sludge undergoes anaerobic digestion, it may be subjected to temperatures of up to 150 o F. At these temperatures, siloxanes volatilize and enter the gas stream. Subjected to the heat of combustion in a reciprocating engine (turbine or microturbine), siloxanes leave behind hard deposits of silica on pistons and valve assemblies causing abrasion and impact damage that reduce the life and efficiency of the engine. Siloxanes need to be removed using refrigeration or sorbents such as activated carbon, alumina, synthetic resins, or liquid sorbents.25
For engines operating on biogas, additional capital investment is required for this fuel clean-up, compression, and sometimes derating of the engine capacity due to the lower thermal energy content of the fuel. For a 1,000 kW reciprocating engine, the added equipment and installation cost is about $600/kW.26 Smaller systems can require nearly the same amount of equipment, so unit costs go up rapidly on smaller installations.
Improved engine design and hardened valve seats reduce siloxane damage on engines, thereby reducing the need for complete removal.
2.4.7.4 Industrial Waste Gases
Industrial waste gases that are common used as reciprocating engine fuels include refinery gases and process off-gases. Refinery gases typically contain components such as H2, CO, light hydrocarbons, H2S, and ammonia, as well as CO2 and N2. Process off-gases include a wide variety of compositions. Generally, waste gases are medium- to low-Btu content. Medium-Btu gases generally do not require significant engine derating; low-Btu gases usually require derating.
Depending on their origin and contaminants, industrial gases sometimes require pretreatment comparable to that applied to raw landfill gas. Particulates (e.g., catalyst dust), oils, condensable gases, water, C4+ hydrocarbons and acid gases may all need to be removed. Process offgases are usually available at pressures of several atmospheres or higher, which are generally satisfactory for delivery to an on-site or nearby reciprocating engine facility.
2.4.8 System Availability
The percentage of time that a system is either up and running or available for use is referred to as its availability. Systems are unavailable during periods of scheduled maintenance or forced outages.
Reciprocating engines are maintenance intensive but, they can provide high levels of availability, even in high load factor applications. While natural gas engine availabilities vary with engine type, speed and fuel quality, Table 2-8 illustrates typical availability numbers based on a survey of natural gas engine gensets in CHP applications.
Some engine manufacturers offer engine exchange programs or other maintenance options that increase the ability to promptly deliver and install replacement units on short notice, typically increasing facility availabilities to greater than 95 percent. The use of multiple units or back-up units at a site can further increase the availability of the overall facility over 99 percent.