Local Gas Use
The simplest and most cost-effective option for use of landfill gas/ biogas is local gas use. This option requires that the gas be transported, typically by a dedicated pipeline, from the point of collection to the point(s) of gas use. If possible, a single point of use is preferred so that pipeline construction and operation costs can be minimized.
Prior to transporting the gas to the user, the gas must be cleaned to some extent. condensate and particulates are removed through a series of filters and/or driers. Following this minimal level of gas cleaning, gas quality of 35 to 50 percent methane is typically produced. This level of methane concentration is generally acceptable for use in a wide variety of equipment, including boilers and engines.
Although the gas use equipment is usually designed to handle natural gas that is nearly 100 percent methane, the equipment can usually be adjusted easily to handle the gas with the lower methane content.
15.4.2 Pipeline Injection
Pipeline injection may be a suitable option if no local gas user is available. If a pipeline carrying medium quality gas is nearby, only minimal gas processing may be needed to prepare the gas for injection. Pipe line injection requires that the gas be compressed to the pipeline pressure.
· Medium Quality Gas. Medium quality gas will typically have an energy value that is the equivalent to landfill gas with a 50% methane concentration. Prior to injection, the gas must be processed so that it is dry and free of corrosive impurities. The extent of gas compression and the distance required to reach the pipeline are the main factors affecting the attractiveness of this option.
· High Quality Gas. For high-quality gas, most of the carbon dioxide and trace impurities must be removed from the recovered gas. This is a more difficult and hence more expensive process than removing other contaminants. Technologies for enriching the gas include pressure swing adsorption with carbon molecular sieves, amine scrubbing, and membranes and are described in Section 15.4.4.
The schematic of a typical gas filter system is shown in Fig. 15.13.
15.4.3 Electricity Generation
Electricity can be generated for on-site or for distribution through the local electric power grid. Internal combustion engines (ICs) and Gas turbines are the most commonly used for landfill gas/ biogas-to-power generation projects.
· Internal Combustion Engines. Internal combustion engines are the most commonly used conversion technology in landfill gas applications. They are stationary engines, similar to conventional automobile engines, that can use medium quality gas to generate electricity. While they can range from 30 to 2000 kilowatts (kW), IC engines associated with landfills typically have capacities of several hundred kW.
IC engines are a proven and cost-effective technology. Their flexibility, especially for small generating capacities, makes them the only electricity generating option for smaller landfills. At the start of a recovery project, a number of IC engines may be employed; they may then be phased out or moved to alternative utilization sites, as gas production drops.
IC engines have proven to be reliable and effective generating devices. However, the use of landfill gas in IC engines can cause corrosion due to the impurities in landfill gas. Impurities may include chlorinated hydrocarbons that can react chemically under the extreme heat and pressure of an IC engine. In addition, IC engines are relatively inflexible with regard to the airfuel ratio, which fluctuates with landfill gas quality. Some IC engines also produce significant NOx emissions, although designs exist to reduce NOx emissions.
Gas Turbines. Gas turbines can use medium quality gas to generate power of sale to nearby users or electricity supply companies, or for on-site use. Gas turbines typically require higher gas flows than IC engines in order to be
economically attractive, and have therefore been used at larger landfills; they are available in sizes from 500 kW to 10 MW, but are most useful for landfills when they are 2 to 4 MW (USEPA, 1993c). Also, gas turbines have significant
parasitic loads; when idle (not producing power), gas turbines consume approximately the same amount of fuel as when generating power. Additionally, the gas must be compressed prior to use in the turbine.
· Steam turbines: In cases where extremely large gas flows are available, steam turbines can be used for power generation.
· Fuel cells: Fuel Cells, an emerging technology, are being tested with landfill gas. These units, expected to be produced in the 1 to 2 MW capacity range, are highly efficient with relatively low NOx emissions. They operate by converting
chemical energy into usable electric and heat energy.
