Two types of reciprocating engines exist; spark-ignition and compression-ignition. These engines can be used with natural gas by either replacing diesel all together with natural gas or by partial displacement. Engines can be modified with an aftermarket kit to allow the use of natural gas as a fuel or an engine can be purchased that is designed for using natural gas. Original equipment manufacturers should be consulted prior to performing any engine modifications to ensure the warranty will remain valid.
Federal regulations should also be reviewed prior to engine modification. Any change to the original configuration of a certified vehicle or engine, including alternative fuel conversion, is a potential violation of the Clean Air Act section 203(a)(3) prohibition against tampering (42 U.S.C. §7522 (a)(3)). The tampering prohibition is important because poorly designed modifications can increase emissions. However, the EPA has established protocols through which conversion manufacturers may seek exemption from the tampering prohibition by demonstrating that emission controls in the converted vehicle or engine will continue to function properly and that pollution will not increase as a result of conversion. Please see the Final Rule and Information for Clean Alternative Fuel Conversion Manufacturers for detailed information about these protocols.
Spark-ignition engines have a spark plug located by the intake and exhaust valves. The spark plug is sometimes angled to optimize the propagation of the flame. The spark must be carefully timed to maximize efficiency. Engine performance decreases dramatically if a spark plug fails to spark or if it sparks at the wrong time. Regular tune-ups and maintenance ensure that the engine is operating optimally. Natural gas can be used in a spark-ignition engine as a dedicated fuel or by alternating between natural gas and diesel in a bi-fuel engine.
Dedicated Natural Gas Engines
Dedicated natural gas engines use only natural gas as the fuel. These engines allow the operator to maximize on the low cost of natural gas and lower overall emissions produced. However, major challenges exist with dedicated natural gas engines due to a basic property of natural gas. Natural gas has a slower flame speed when compared to diesel which leads to the flame moving through the cylinder slower and causing incomplete combustion if the engine isn’t tuned properly. The slower flame speed also results in the loss of power, reduced engine efficiency, and slower torque at high loads.
There are a range of methods being used to compensate for the slow flame speed of natural gas. The first method involves training personnel on how to handle the slower engine response time. This training also reduces engine wear since personnel are using the engine correctly. The second method that can be used is hydrogen enrichment. By blending hydrogen and natural gas together the flame speed is increased and combustion is more complete. Another compensation method used is battery backup. A series of batteries can be charged during periods that the engine is at low loads or at idle. Then when the engine load is high the power from the batteries can supplement energy that the engine cannot provide.
Bi-fuel engines alternate between diesel and natural gas, using only one fuel at a time. Bi-fuel engines only use diesel when ambient temperatures are below an engine specific temperature, during periods of high load, and when natural gas is unavailable. These engines maximize natural gas usage without losing power, efficiency, or torque. Bi-fuel engines can also operate solely on diesel offering the operator more options for fuel usage. The fuel type being used can be either manually or automatically switched depending on the engine’s design. Some engines have both manual and automatic switch options for flexibility. Most engines will switch seamlessly between fuel types, but some will hesitate and others may require a complete shutdown to switch.
Compression-ignition engines operate by compressing the fuel vapor and air. The heat from compression auto ignites the fuel causing the piston to move from position 1 to position 2. These engines are most commonly used with diesel and are increasingly being used with natural gas. However, natural gas has a significantly high auto ignition temperature when compared to diesel or gasoline and the heat from compression does not reach temperatures high enough to ignite natural gas. To compensate for this, a pilot fuel must be used in quantities of at least 30% of total fuel volume or greater to allow for ignition in compression-ignition engines.
Dual Fuel Engines
Dual fuel engines use a blend of two fuels for combustion, typically diesel and natural gas; however, methanol or esters can be used with natural gas as well. The fuel is mixed at a specified ratio and then injected in the cylinder where it is combusted from the heat of compression without a spark plug. Multiple flame fronts occur throughout the cylinder (spark-ignition has one flame origination point), compensating for the slower flame speed of natural gas. Substitution rates of up to 70% natural gas can be achieved with diesel compression-ignition engines at moderate loads. At high loads the percentage of diesel in the blend will need to be increased to prevent engine knock and performance reductions. Dual fuel engines offer the benefits of natural gas by lowering emissions and reducing the cost of fuel without loss of power at high loads. Another benefit is that the engine can be operated solely on diesel if natural gas is unavailable.
Natural Gas Turbines
Turbines have traditionally been used at power generation plants to produce electricity for the electric grid. Increasingly, they are being considered and used in oilfield applications because of their compact, versatile nature. Aeroderivative turbines are the most common and like their namesake they are derived from jet, airplane, and helicopter turbines. They are capable of operating exclusively on natural gas or any other combustible vapor. Turbines combust fuel at a very high temperature which increases nitrous oxide emissions; to counter this, water is also injected with the fuel to reduce the combustion temperature and emissions. Turbines have been developed that do not require water injection, reducing the amount of water usage significantly. Turbines are known for operating at a high efficiency which makes them ideal; however, water injection will reduce the efficiency slightly. Turbines can also be combined with other turbines in a combined cycle to increase efficiency even further. Turbines emit a lower heat signature and produce lower levels of noise than reciprocating engines; increasing employee safety.
Electric motors can be powered either from the electric grid or from electricity generated from a turbine. Turbines provide a versatile electrical generation option since they can provide power in remote regions and do not put strain on the electric grid. Electric motors are compact and produce less noise than their traditional counterparts. Overall emissions are also lowered because the turbine is the only source of emissions.
For more information contact:
Carolyn LaFleur, clafleur@HARCResearch.org, 281-364-4035