TURNING LANDFILL GAS TO ELECTRICITY
SADAT HUSSAIN QURASHE
June 21 - 27, 2010
Landfill methane is an abundant and readily available renewable energy source. It has the potential to fuel 2,700 MW of electric generating capacity in the United States alone and 9,000 MW worldwide. To reduce greenhouse gases, governments around the world are encouraging projects that turn landfill gas to electricity.
Landfill gas is produced naturally as organic waste decomposes in landfills. LFG composes of about 50 percent methane, about 50 percent carbon dioxide and a small amount of non-methane organic compounds. The energy content of landfill gas is 400 to 550 Btu per cubic foot.
A landfill must be at least 40 feet (12 meters) deep and has at least one million tons of waste in place for landfill gas collection and power production to be technically feasible.
LFG develops in landfills in approximately one to three years, depending on the type of waste and environment; peak production of LFG is five to seven years after waste is dumped.
At most municipal solid-waste landfills, the methane and carbon dioxide mixture is destroyed in a gas collection and control system or utility flare. However, to use LFG as an alternative fuel, the gas is extracted from landfills using a series of wells and a vacuum system. Pipes are inserted deep into the landfill to provide a point of release for the landfill gases. A slight vacuum is then applied in the pipe to draw the gases into and through it to a central point, where it can be processed and treated for use in generating electricity, replacing the need of conventional fossil fuels.
LFG must either be thoroughly pretreated or used in engines that are designed to operate on impure fuel. LFG can contain sulfur compounds, halides, water vapor, silicon crystals or siloxanes. In traditional engines, these materials are highly corrosive and damaging.
While fuel pretreatment has a longer history and more popularity in the landfill gas-to-energy market, Cat engine designs that deal with fuel contaminants have a 20-year track record of effectiveness. Engine designs have improved steadily and are available on even the most technologically advanced, high-efficiency gas engines on the market. Treatment System for Landfill Gas
* Gas-to-air coolers lower the temperature of the gas after it is compressed, reducing moisture and preventing condensation and attendant acid formation later in the fuel delivery system or inside the engines.
* Gas-to-gas heat exchangers typically made of stainless steel pre-cool the gas entering the drier to reduce drier power demand. The gas leaving the drier is reheated later in the process by the gas-to-gas heat exchanger to prevent water from condensing downstream.
* Gas driers reduce halogens and hydrogen sulfide in the gas. The device is usually a gas-to-liquid heat exchanger that uses a refrigerant. The gas is dried by chilling to a dew point of 36 to 37 degrees F (2 to 3 degrees C).
* Coalescing filters remove any remaining water or oil droplets and remaining solid matter as small as 0.4 microns.
* Condensate drains collect water removed from the gas. The water may be treated for discharge to a sewer system or, in some locations, reintroduced to the landfill to stimulate methane production.
To meet the needs of customers seeking to use low energy gas as fuel, caterpillar offers the various models and range of low energy fuel generator sets for use with methane fuels found in landfills, coal mines and other environments. For example, The G3520C generator set includes equipment that optimises the performance of engines in parallel-to-grid, continuous-operation applications. This generator set is specially designed to handle fuel with methane concentration variations that are typical of landfill, biogas and CMM operations. The G3520C package is also designed to efficiently and reliably use fuel gas with lower-than-typical methane content. Special approval and engine configurations can be arranged for applications with methane content as low as 25 percent.
Protections like special "hardened" features are also available on the G3520C generator set to offer security against the corrosive nature of biogas and LFG in situations where minimal fuel pretreatment is desirable. For example, the landfill-specific design minimises the use of bright metals such as copper and unprotected steel in components likely to come in contact with the fuel or exhaust gases, which are naturally acidic. The aftercooler cores, made of copper alloys in standard gas engines, are made of stainless steel in Cat's landfill generator sets to resist attack from the acids in sulfur, chlorine and fluorine.
Equipped with a gas engine control module using ADEM III electronic controls, the generator set allows for full engine control from a single source. It can utilise fuel from a range of 25 to 100 percent methane, so the equipment remains efficient throughout the life of a landfill or coal mining project.
The Cat G3520C is available in both 50 and 60Hz models and can be used in LFG, biogas and CMM applications across the globe.
Some power projects using this reciprocating generator set includes a 9.6 LFG power plant in San Antonio, Texas, a 120 MW CMM power plant in Jincheng, China, and a 1.6 MW wastewater treatment facility in Hamilton, Ontario, Canada.
Reciprocating generator sets are far from wasteful or inefficient. They are a mature, proven technology that will meet modern and future energy needs.
CONCLUSIONS AND RECOMMENDATIONS
It is important to also consider the contribution of CHP, utility management of efficiency and conservation programs, and renewable resources to overall energy security.
Energy independence is a topic of great interest globally. The recognition that many highly used resources are finite is leading many countries to explore and expand on renewable resources, and there are persistent concerns for issues like natural disasters, regional conflicts or supply manipulation in countries that provide oil and coal to the rest of the world. Reducing vulnerability to drastic changes in the energy market is important to ensure affordable and reliable power for infrastructure, home and business use.
With the implementation of new policies to drive a cleaner environment, the mitigation of emissions through generator sets is likely to offer opportunities to benefit from alternative fuel investments. Creating power from sustainable resources such as landfill gas is mature and proven technology that is also environmentally responsible.
LFG and biogas may both be used to power fuel-flexible reciprocating generator sets, but low nitrogen oxide-producing natural gas may also be used. The efficiency of the generator sets is proven, with their ability to capture and reuse waste heat for CHP applications while meeting national and local standards for emissions. When utilities are encouraged through decoupling to take advantage of the benefits of renewable fuels and CHP systems, those benefits can be managed from a central source and distributed locally to consumers, allowing communities to be involved with the process of creating and conserving the energy they use. Energy security may only become a reality when all the contributing entities are willing and able to invest in available national resources. Beginning with promoting an existing technology like reciprocating generator sets may lead to essential change in the way power is produced, sold and distributed, leading to a Pakistan that runs on clean, efficient, local power.
The writer is Assistant Manager Projects, Six Sigma Black Belt Allied Engineering & Services Limited, Caterpillar.