Thermal Systems Considered

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Flaring (direct combustion) is the traditional approach to utilize the collected LFG for reducing the GHGs in a landfill site. The flaring of LFG is an economical approach, and also it reduces the risk of explosion of uncontrolled LFG emissions. The operation principle of landfill gas flare is simple; LFG is ignited by bringing it into contact with a supply of air. Different configurations of conduit and chambers can be used for this purpose. In today's market, open and closed flare types are available. Open flares burn landfill gas as open flames, whereas closed flares burn landfill gas in a vertical, cylindrical, or rectilinear enclosure. Details of these flare types may be found in the report by Environment Agency and Scottish Environment Protection Agency (2002). On the other hand, since the collected gas has a considerable amount of heating value, it may be utilized to produce electricity and/or heat. The most commonly used technology for utilizing LFG is internal combustion engines, followed by gas turbines. Additionally, SOFCs are very promising candidates to be used in landfill sites in the future due to their advantages discussed in the previous and following sections. These technologies are discussed in detail in the following subsections.

Fig. 8.1 Greenhouse gas emission routes in a landfill site with LFG collection system.

8.3.1 Internal combustion engine (ICE)

The internal combustion engine is the most widely used technology for electricity generation from LFG, mainly because of its economical feasibility. These engines are attractive because they are compact and easy to transport. The main disadvantage is the high amounts of NOX and CO emissions produced by these engines as compared with other technologies, which contribute to the air pollution. Lean-burn spark ignition engines are the most common type of ICE used in landfill sites. When these engines are operated using LFG, engine power ratings are commonly reduced by 5-10% (SCS Engineers, 1994) compared to operation using natural gas. It should be noted that before the LFG is fed to the ICE, moisture and particu-lates must be removed according to the tolerance limits of the engine, so as not to reduce the engine efficiency and reliability and increase the necessity for more regular maintenance.

The power output of these engines varies between 300 kW and 3.6 MW for an individual unit (Environment Agency and Scottish Environment Protection Agency, 2004). Generally, many ICEs operate together according to the LFG generated to produce more power. A typical landfill site operating with this type of engine should also include a gas flare to burn any LFG collected in excess of maximum requirements of the engine, to burn LFG when the generated gases are low enough to justify the operation of the engine, and to operate during the maintenance.

Gas turbines are the second most popular technologies that utilize LFG. The majority of gas turbines presently operating at landfills are the simple cycle, single shaft type. They are very similar to natural gas turbines except that, because of the low heating value, the number of fuel regulating valves and injectors are doubled (SCS Engineers, 1997). During its operation, large quantities of air enter the compressor.

After the air is compressed, it mixes with fuel in the combustor, and the combusted gas expands in the turbine where power is produced. Some amount of this power is used to drive the compressor.

Compared to ICEs, gas turbines have lower NOX and CO emissions, and also fewer moving parts. Their exhaust can also be utilized in a cogeneration application. However, if electricity generation is more important in an application, the gas turbine is disadvantageous since it has a lower electrical efficiency than the ICE. Other disadvantages are having a high capital cost, being sensitive to LFG supply loads and ambient air temperature variations, and not being suitable for moderate size landfills. For small-size landfills, microturbines are generally selected instead of gas turbines.

8.3.3 Solid oxide fuel cell (SOFC)

The SOFC is an emerging technology that is expected to replace conventional energy systems like ICEs and gas turbines once it has become economically competitive. The SOFC has higher electrical efficiency, lower emissions, a higher exhaust gas temperature that makes it possible to be used in cogeneration applications, quieter operation, and fewer moving parts compared to conventional systems.

There have been demonstrations of SOFC operation using biogas (News, 2005, 2007). These demonstrations include biogas production from wastewater in a sewage treatment plant and animal waste. It has been recently reported that a planar SOFC unit in Finland, which will produce 20 kW of electric power and 14-17 kW of thermal output, is believed to be the first SOFC in the world that is fueled by LFG (News, 2008).

The SOFC is a very attractive option for LFG application since it can use hydrogen and carbon monoxide as fuels, which are reformed from methane. In high temperature SOFCs, this reformation occurs inside the fuel cell; this is called direct reforming. The reforming mechanism is controlled by water-gas shift and steam reforming reactions, which are shown by Eqs. 8.2-8.3. In some cases, it is preferable to use a dry reformer before the LFG enters the SOFC to prevent the carbon deposition at the anode catalyst. The dry reforming reaction is shown in Eq. (8.4).

It should be noted that the collected LFG must be subjected to an extensive gas cleanup process before it enters to SOFC due to high levels of contaminants in the gas. The tolerance limits of SOFCs to these contaminants may be found in the literature (Sime et al., 2002; Xenergy, 2002).

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