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Introduction to Gas Engines and their Relevance to Renewable Energy
A gas engine is an internal combustion engine which runs on a gas fuel, such as producergas, biogas or natural gas.
Specifically, the term gas engine refers to a heavy-duty industrial engine capable of running continuously at full load for periods approaching a high fraction of 8,760 hours per year.
The significance of gas engines to renewable energy is that customized gas engines can produce power from biogas and producer gas, two gaseous fuels that are produced from renewable sources (organic waste and biomass respectively).
Gas Engines Working Principle
Using the same principles as any other IC engine, gas engines use gaseous fuels such as biogas, natural gas or producer gas to produce electricity.
For instance, for a biogas engine, the waste of 2,500 cows, 15,000 pigs or 300,000 chickens can create enough biogas to power an engine with electrical output of 500 kW, is enough energy to supply more than 1000 homes in developed countries and over 5000 homes in developing countries such as India.
Types of Gas Engines
Gas engines can be classified according to the fuel they are most suited to use:
- Biogas Engines
- Natural Gas Engines (including landfill gas engines)
- Producer Gas Engines
Of the above, gas engines from renewable energy sources are biogas engines and producer gas engines.
Capacity Range of Gas Engines
Gas engines start from very small capacities (10 kW) to capacities as large as 4 MW.However, the usual capacities of gas engines used are in the range of 50 kW to about 1.5 MW.
Applications for Gas Engines
Typical applications are baseloadpower, including combined heat and power.
Gas engines are rarely used for standby applications, which remain largely the province of diesel engines. One exception to this is the small (
The natural gas engines (LNG) are getting more into the marine market, as the lean-burn gas engine can meet the new emission requirements without any extra fuel treatment or exhaust cleaning systems.
Biogas engines are typically for power production in a variety of industries that have abundant access to organic waste – animal farms, large factories that generate significant amounts of human and animal waste, vegetable markets etc.
Companies Making Gas Engines
While there are a number of companies making gas engines worldwide, prominent manufacturers include GE Jenbacher, Caterpillar Inc., Perkins Engines, MWM, Cummins, Dresser-Waukesha (now part of GE), Guascor, Deutz, MTU, Rolls-Royce with the Bergen-Engines AS, Kawasaki Heavy Industries, MTU Friedrichshafen, Wärtsilä, MAN, Doosan, and Yanmar.
Efficiencies ofGas Engines
Gas engines typically have a mechanical efficiency between 35-45%. The best engines can achieve amechanical efficiency of slightly more than 48%.
Large engines are more efficient than small engines. Gas engines running on biogas typically have a slightly lower efficiency (~1-2%) and syngas reduces the efficiency further still. GE Jenbacher's recent J624 engine is the world's first 24 cylinder gas engine with a high efficiency running on methane.
Whyare Gas Engines Preferred over Combustion based Steam Turbines for Biomass Power Production?
Biomass power plants typically follow one of the two processes: Combustion or Gasification. The general rule of thumb has been that for power plants smaller than 2 MW, gasification route is considered most appropriate. For those power plants beyond 2 MW, combustion using a steam rankine cycle is considered appropriate.
The reasons are not difficult to fathom. While gas engines typically are not available beyond 2 MW, their efficiencies can be as high as 40% for this capacity range. Steam rankine cycles perform poorly at small scales, and offer reasonable efficiencies only beyond 5 MW (in fact their efficiencies increase significantly only beyond 10 MW).
As a result, the preferred route (combustion or gasification) for biomass power plants is dependent usually on the capacity of the biomass power plant.
Where gasification is the preferred route, gas engines naturally come into the picture. These gas engines are typically those that can run on producer gas, which is the resulting fuel from biomass gasification.
Renewable Energy Sectors that can use Gas Engines
Biogas
Industrial sectors producing the following can use biogas engines for power production
- Livestock manures
- Waste feed
- Food-processing wastes
- Slaughterhouse wastes
- Milkhouse wash water
- Fresh produce wastes
- Industrial wastes
- Food cafeteria wastes
- Sewage sludge
In addition to the above, other agricultural operations afford opportunities for biogas production. For example, cassava-processing plants, which produce starch, are common in India and may utilize biogas for electric power.
Producer Gas
The following segments are suited to use producer gas engines for power production
- Agricultural Farms – Jute sticks, cotton stalks, coconut shells
- Mills -Rice Mills, Oil Mills (groundnut shells)
- Cotton Mills (cotton stalk)
- Companies with Access to Large Amounts of Waste Biomass such as Juliefloraand other Woody Biomass
Cost of Gas Engines
Gas Engine prices vary from one manufacturer to the other. Many India gas engines cost much less than European gas engines, but the European gas engines provide much higher efficiencies than do gas engines by Indian companies.
Typical German / European gas engines cost about Rs 2.5 crores per MW ($500,000), while Indian gas engine could cost about 20% less - about Rs 2 crores per MW ($400,000). The European gas engines can provide efficiencies as high as 40% while most Indian-made gas engines typically have sub-30% efficiencies.
Minimum Capacities Available for Gas Engines
Gas engines less than 5 kW will be hard to find. Typically, the minimum capacities of gas engines used are in the 20-30 kW range.
Differences between Gas Engines for Producer Gas, Natural Gas and for Biogas
While the operating principles are the same, gas engines need to be customized for producer gas, natural gas and biogas respectively to produce optimum output.
Let’s take two of the fuels – natural gas and biogas. Natural gas are designed and built for high BTU natural gas fuel. Operating those engines on biogas requires downgrading the engine and generator rating, and the output is in fact much less in comparison to the performance that could be achieved by using the intended natural gas fuel source. Using a natural gas engine for a biogas plant has many disadvantages that are very often overlooked. The engines don’t perform well in biogas configuration, usually they don’t last very long and require a substantial amount of repairs, elevating your service and maintenance cost. Further, natural gas engines only achieve very low levels of efficiency when operated on biogas. That translates into higher fuel consumption and low energy output. These factors have a downward effect onthe bottom line.
For biogas applications, companies offer exclusively designed biogas power generation units, engineered for low BTU and corrosive biogas. These engines are specifically built for the purpose of combusting biological gaseous fuels. That includes thermodynamically optimized combustion technologies, as well as an overall design that's most adequate for this type of application.
Lifetime of Gas Engines
While there are some experts who say that the useful lifetime for gas engines is only about 10 years, a number of prominent manufacturers claim that with suitable, periodic maintenance and repairs, gas engines have a useful lifetime of about 20 years.
CHP and itsRelevance to Gas Engines
Engine reject heat can be used for building heating or heating a process. In an engine, roughly half the waste heat arises (from the engine jacket, oil cooler and after-cooler circuits) as hot water which can be at up to 110 °C. The remainder arises as high-temperature heat which can generate pressurized hot water or steam by the use of an exhaust gas heat exchanger.