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 Co-generation with Natural Gas

Co-generation involves the simultaneous generation of electrical or mechanical energy and thermal energy from a single fuel source, usually Natural Gas.

An internal combustion engine or a gas turbine burns Natural Gas to generate electricity or produce mechanical energy, for example for an air conditioning compressor. Heat from the water jacket and exhaust of an internal combustion engine, or the exhaust gases from a gas turbine, are recovered and used to generate hot water or steam.

This can be used for heating a building, for generating chilled water from an absorption chiller or for any other industrial process able to utilize heat in this form.


 Electrical Generation

The process of cogeneration produces electricity at an efficiency of around 30% and can be used in the following ways:

  • Base load - balance of electricity supplied from the power grid

  • Total electrical supply - balance fed into the power grid

  • Peak shaving - used to reduce peak load where Peak Demand Tariff exists, can also provide emergency electrical power

  • Generate power for specific plant - also reduces peak demand, and can provide emergency electrical power.

 Internal Combustion Engine

Up to about 50% of the energy from the prime fuel can be recovered from the water jacket and exhaust as thermal energy can be used for:

  • The production of low pressure steam

  • Hot water

  • Production of chilled water from an absorption chiller

  • Space heating - using either hot water or steam

  • Any industrial process able to use this grade of heat.

 Gas Turbines

Gas Turbines produce dry exhaust gases at a temperature of approximately 500C. These gases can be used directly in drying ovens or in waste heat boilers to produce steam or hot water, which can then be used in the same sort of applications as the internal combustion engine, (see above.)


The production of electricity and use of heat, which would otherwise be wasted, generates efficiencies as high as 75-80%. Compare this to a centrally generated grid electrical efficiency of about 30%. An added environmental benefit is the considerable reduction of carbon dioxide emissions.

 Most Economical

Where there is a balanced demand for electricity (or shaft power) and thermal energy, Natural Gas fuelled co-generation is one of the most economical method of generating electricity currently available.


 What Exactly is Co-Generation?

Co-generation is the concept of producing two forms of energy from one fuel. One of the forms of energy must always be heat and the other may be electricity or mechanical energy. In a conventional power plant, fuel is burnt in a boiler to generate high-pressure steam. This steam is used to drive a turbine, which in turn drives an alternator through a steam turbine to produce electric power. The exhaust steam is generally condensed to water which goes back to the boiler.

As the low-pressure steam has a large quantum of heat which is lost in the process of condensing, the efficiency of conventional power plants is only around 35%. In a cogeneration plant, very high efficiency levels, in the range of 75%90%, can be reached. This is so, because the low-pressure exhaust steam coming out of the turbine is not condensed, but used for heating purposes in factories or houses.

Since co-generation can meet both power and heat needs, it has other advantages as well in the form of significant cost savings for the plant and reduction in emissions of pollutants due to reduced fuel consumption.

Even at conservative estimates, the potential of power generation from co-generation in India is more than 20,000 MW. Since India is the largest producer of sugar in the world, bagasse-based cogeneration is being promoted. The potential for cogeneration thus lies in facilities with joint requirement of heat and electricity, primarily sugar and rice mills, distilleries, petrochemical sector and industries such as fertilizers, steel, chemical, cement, pulp and paper, and aluminum.


Landfill Gas Generation

The most common use of landfill gas (LFG) is for on-site electricity generation. There is little difference between an electric generating plant using landfill gas and one using natural gas or diesel fuel, aside from the need for more extensive gas processing and more careful monitoring of equipment because of the potentially corrosive nature of landfill gas.

An LFG-to-electricity system has three basic components: (1) the gas collection system, which gathers the gas being produced within the landfill, (2) the gas processing and conversion system, which cleans the gas and converts it into electricity, and (3) the interconnection equipment, which delivers the electricity from the project to the final user.

 Gas Collection  - Gas is typically collected by a series of wells strategically placed throughout the landfill, as gas from decomposing garbage exists at all levels of the landfill. The number and spacing of wells depend on specific landfill aspects such as volume, density, and geometry.

Wells are constructed by drilling holes into the landfill, to within 5 to 15 feet (2 to 5 meters) from the bottom. Perforated plastic pipes are inserted into the wells. The area around the pipes is filled with large gravel to prevent refuse from plugging the perforations. Horizontal underground trenches can also be used to recover LFG as layers of the landfill are added.

The wells are connected by a series of pipes leading to larger, header pipes that deliver the gas to the processing and conversion stations. The entire piping system is under a partial vacuum created by blowers or fans at the processing station, causing landfill gas to migrate toward the wells.

 Gas Processing  - Once blowers or fans deliver the gas to a central point, it can be processed or converted to another energy form. At a minimum, the gas needs to be filtered to remove any particles and condensate that may be suspended in the gas stream. After moisture removal, additional gas processing may involve the use of refrigerators or absorbers, such as activated carbon filters, to remove trace contaminants.

 Conversion Equipment  - Either internal combustion engines or turbines can be used to power on-site generators, which convert the gas into salable electricity.

 Interconnection with Utilities  - After the gas is converted to electricity, a dedicated line is used to deliver the electricity to utilities. Interconnection usually includes metering equipment necessary to monitor sales and system protection equipment with emergency shutdown capability to prevent either party from damaging the other's equipment, or operations, or injuring personnel.


How Landfill Gas Generation Works

It has long been known that landfills were a potential source of energy because they produce a methane-rich gas (approximately 55% methane, 45% CO2) soon after startup and for up 25 years after closure. This energy source was first tapped in the early 1980s and had grown significantly since that date. In some cases, LFG is piped to a nearby user where it is burned as fuel in place of pipeline natural gas or other energy sources. Production of electric power by burning LFG in engine or turbine driven generators is also popular. In a few cases LFG has been upgraded to pipeline quality gas and sold to local gas companies for addition into their pipelines. Tax credits are currently available under Section 29 of the IRS code for LFG to energy projects. This feature has encouraged development of many LFG to energy projects.

The major problem with LFG to energy projects is trace components contained in LFG. In addition to methane and CO2, typical gas contains heavy hydrocarbons (both aliphatic and aromatics such as benzene) as well as numerous chlorinated hydrocarbons. These trace compounds are, in some cases toxic or hazardous and also cause rapid failure or engine and turbine components. There are now federal statutes which cover landfill emissions.

The best solution to this problem appears to be the use of the Kryosol Process developed by Kryos Energy Inc. This process is patented and has been proven in two long term applications in the United States. It utilizes refrigerated methanol as a solvent for removal of the toxic/hazardous compounds as well as the CO2 in the LFG. The principal application of this process has been for upgrading LFG to pipeline gas where both cleanup and CO2 removal are required.

As of 1996 about 250 landfill in the U.S. had or will soon have energy recovery. About 70% produce electric power through combustion of LFG. The average developed LFG output is approximately 2.5 MMSCFD which is equivalent to about 1.4 MMSCFD of pipeline natural gas. There are many undeveloped landfills or landfills with only gas gathering and flaring (to minimally meet federal statutes). It is estimated that only 20% per cent of all landfills have energy recovery equipment.

The idea of using the renewable methane generated at landfills, landfill gas (LFG), generally results in much discussion any time the subject is raised in the presence of a few natural gas vehicle (NGV) proponents. The subject then focuses on the use of LFG to expand the NGV industry. It is easy to understand the attractiveness of using LFG. Some of the significant attributes of the concept are:

  1. Landfills are typically located near large metropolitan areas; which make them near the desired points of distribution for vehicular LNG.

  2. LFG provides a renewable, clean-burning energy supply . . . what better public relations could be provided to landfill operators than to show how trash is converted to fuel that helps clean up the air we breathe?

  3. Landfills, because of the acreage required, usually provide adequate area for siting an LNG production facility, even in near urban areas.

  4. Landfills are designed for the ingress and egress of large, trash-hauling trucks, making the logistics of trucking LNG compatible. Many even have on-site scales, further supporting an LNG distribution center.

  5. And, also, LFG, once "cleaned up" and upgraded is pure methane, the most desirable and clean-burning constituent of vehicular LNG.

Tech Power Systems is staying at the front of this and other emerging technologies.  Consult with our knowledgeable staff too see if a Co-Generation System is feasible for your company.


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