Landfills and wastewater treatment plants emit biogas from decaying waste. To date, the waste industry has focused on controlling these emissions to our environment and, in some cases, tapping this potential source of fuel to power gas turbines, thus generating electricity.
The options now look set to increase following the introduction of technology from US-based Prometheus Energy Company to convert biogas to methane on a commercial scale. In its newly installed California plant, Prometheus converts landfill gas to liquid natural gas (LNG), which is now being used as a diesel substitute in Orange County Transit Authority buses in Southern California.
LNG is simply liquid fuel formed by cooling natural gas to as low as -162°C. LNG is a form of methane - the form that is most useful for use as a vehicle fuel. Methane can also be in the form of compressed natural gas (CNG), liquid compressed natural gas (LCNG) or left as pipeline-quality gas. For the purposes of this article, the terms LNG and methane can be used interchangeably.
Liquefaction enables transportability and storability while transforming the energy source into a superior form of fuel. Moreover, LNG can hold the same energy content in approximately 1/600th of the volume required of methane at room temperature and pressure, and one-sixth of compressed natural gas (CNG). This means longer-distance travel is possible, with smaller fuel tanks on vehicles - cutting down on weight and therefore wear and tear on tyres, brakes and other vehicle parts.
In addition, the most compelling reason for using natural gas fuel grows from an increasing global desire for a cleaner environment. The use of natural gas as a vehicle fuel produces less pollution than gasoline or diesel, biodiesel or ethanol. Natural gas vehicles reduce smog-causing NOx emissions by 50% or greater and particulate matter (soot) by up to 70% compared with diesel fuel. These vehicles produce fewer greenhouse gases, and in many areas (such as Southern California, where Prometheus’ first plant is located, and in all of the United States by 2010) regulations are forcing alternatives to traditional diesel-powered vehicles.
Landfill gas is a gas mixture generally recovered at 1-2 psig from a set of collection wells installed at the landfill.
The primary components of landfill gas are methane (CH4), carbon dioxide (CO2), and nitrogen (N2). (The average concentration of methane specified in the LFG is ~45%, CO2 is ~36% and nitrogen is ~18%.) Other components in the gas are oxygen (O2), water vapour and trace amounts of a wide range of non-methane organic compounds (NMOCs). Overall, landfill gas is a relatively poor-quality gas that is of limited use in its unprocessed state.
The small concentrations of O2 in landfill gas are removed from the process stream by conversion to CO2 and H2O. Essentially all the CO2, H2O, NMOCs, and most of the N2 are rejected during the production of LNG. The trace concentrations of NMOCs in the gas are chemically absorbed, removed as condensates or physically adsorbed in the pre-purifier module.
Prometheus’ California plant uses a combination of purification and cryogenic liquefaction technologies. The proposed process utilizes filters, phase separators, selective reactions, physi-adsorption, freezing and cryogenic refrigeration techniques to purify and liquefy the landfill gas feed stream to produce LNG that has > 97% CH4. While the basic technologies are well understood and have been successfully used commercially for many years, their unique application in this context is the innovation of the company and its key personnel.
The process design incorporates a systematic approach to landfill gas purification prior to and while cooling and liquefying the feed gas mixture. The process also uses a novel method to reject N2 from the LNG. The optimal recapture of work and thermal energy increases the overall thermal efficiency of the system while minimizing its capital costs.
There are no regulated emissions from the landfill gas-to-LNG process except those produced by the natural gas generator for power production. The residual CH4 and other materials contained in the feed gas (N2, CO2, and trace amounts of NMOCs) will be piped to the generator and used as fuel for the plant. Any further remaining CO2 is vented back to the existing flare.
Two main factors set the company’s technology and processes apart from current conventional and alternative fuels, according to Kirt Montague, the company’s Chief Executive Officer.
‘First, the company exploits low-cost, waste, and stranded gas resources that provide feedstock gas at costs significantly less than the market and its competitors,’ he says. In most other biofuels, the feedstock comes from products that already have an economic value and are used to feed people and livestock - corn, soybeans, and various vegetable oils. Even current suppliers of LNG vehicle fuel create the fuel from pipeline gas, a commodity used to heat homes and cook food. And, since pipeline gas is sold at index costs, the cost of the resulting fuel varies with the index cost of its feedstock. By using waste gases for feedstock, Prometheus expects to lower the cost of LNG to the end-user and in some cases can provide fuel in long-term contracts not tied to the index.
The options now look set to increase following the introduction of technology from US-based Prometheus Energy Company to convert biogas to methane on a commercial scale. In its newly installed California plant, Prometheus converts landfill gas to liquid natural gas (LNG), which is now being used as a diesel substitute in Orange County Transit Authority buses in Southern California.
LNG is simply liquid fuel formed by cooling natural gas to as low as -162°C. LNG is a form of methane - the form that is most useful for use as a vehicle fuel. Methane can also be in the form of compressed natural gas (CNG), liquid compressed natural gas (LCNG) or left as pipeline-quality gas. For the purposes of this article, the terms LNG and methane can be used interchangeably.
Liquefaction enables transportability and storability while transforming the energy source into a superior form of fuel. Moreover, LNG can hold the same energy content in approximately 1/600th of the volume required of methane at room temperature and pressure, and one-sixth of compressed natural gas (CNG). This means longer-distance travel is possible, with smaller fuel tanks on vehicles - cutting down on weight and therefore wear and tear on tyres, brakes and other vehicle parts.
In addition, the most compelling reason for using natural gas fuel grows from an increasing global desire for a cleaner environment. The use of natural gas as a vehicle fuel produces less pollution than gasoline or diesel, biodiesel or ethanol. Natural gas vehicles reduce smog-causing NOx emissions by 50% or greater and particulate matter (soot) by up to 70% compared with diesel fuel. These vehicles produce fewer greenhouse gases, and in many areas (such as Southern California, where Prometheus’ first plant is located, and in all of the United States by 2010) regulations are forcing alternatives to traditional diesel-powered vehicles.
Processing the landfill gas
Landfill gas is a gas mixture generally recovered at 1-2 psig from a set of collection wells installed at the landfill.
The primary components of landfill gas are methane (CH4), carbon dioxide (CO2), and nitrogen (N2). (The average concentration of methane specified in the LFG is ~45%, CO2 is ~36% and nitrogen is ~18%.) Other components in the gas are oxygen (O2), water vapour and trace amounts of a wide range of non-methane organic compounds (NMOCs). Overall, landfill gas is a relatively poor-quality gas that is of limited use in its unprocessed state.
The small concentrations of O2 in landfill gas are removed from the process stream by conversion to CO2 and H2O. Essentially all the CO2, H2O, NMOCs, and most of the N2 are rejected during the production of LNG. The trace concentrations of NMOCs in the gas are chemically absorbed, removed as condensates or physically adsorbed in the pre-purifier module.
Prometheus’ California plant uses a combination of purification and cryogenic liquefaction technologies. The proposed process utilizes filters, phase separators, selective reactions, physi-adsorption, freezing and cryogenic refrigeration techniques to purify and liquefy the landfill gas feed stream to produce LNG that has > 97% CH4. While the basic technologies are well understood and have been successfully used commercially for many years, their unique application in this context is the innovation of the company and its key personnel.
The process design incorporates a systematic approach to landfill gas purification prior to and while cooling and liquefying the feed gas mixture. The process also uses a novel method to reject N2 from the LNG. The optimal recapture of work and thermal energy increases the overall thermal efficiency of the system while minimizing its capital costs.
There are no regulated emissions from the landfill gas-to-LNG process except those produced by the natural gas generator for power production. The residual CH4 and other materials contained in the feed gas (N2, CO2, and trace amounts of NMOCs) will be piped to the generator and used as fuel for the plant. Any further remaining CO2 is vented back to the existing flare.
Potential impact of methane
Two main factors set the company’s technology and processes apart from current conventional and alternative fuels, according to Kirt Montague, the company’s Chief Executive Officer.
‘First, the company exploits low-cost, waste, and stranded gas resources that provide feedstock gas at costs significantly less than the market and its competitors,’ he says. In most other biofuels, the feedstock comes from products that already have an economic value and are used to feed people and livestock - corn, soybeans, and various vegetable oils. Even current suppliers of LNG vehicle fuel create the fuel from pipeline gas, a commodity used to heat homes and cook food. And, since pipeline gas is sold at index costs, the cost of the resulting fuel varies with the index cost of its feedstock. By using waste gases for feedstock, Prometheus expects to lower the cost of LNG to the end-user and in some cases can provide fuel in long-term contracts not tied to the index.
 LEFT TO RIGHT California’s Orange County Transit Authority bus fleet is powered by landfill gas-derived LNG • An LNG vehicle being filled up |
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