Biogas or landfill gas (LFG) is typically produced from anaerobic decomposition of organic matter in an oxygen-free environment (Saho et al., 2011). It can also be produced through pyrol­ysis and gasification processes. Primary sources include biomass, green waste, plant material, manure, sewage, municipal waste and energy crops. While its composition can vary significantly depending on the source and production process, the main constituents include CH4 (50-75% by volume), CO2 (25-40%), N2 (0-10%), and small traces of H2O, O2, H2, and hydrogen sulfide. It may also contain small amounts of contaminants such as volatile organic compounds, sulfur compounds, siloxanes, halogenated hydrocarbons, ammonia, etc. To account for this variation in composition, previous studies have examined the biogas combustion and emission behavior for some specific compositions. Table 2.4 lists two such representative biogas mixtures based on the two common biomass sources, namely agricultural waste and household waste (Bika et al., 2011). Like natural gas and syngas, biogas can be used as a transportation fuel in IC engines, and for power generation in gas turbines and boilers. It can also be used as compressed natural gas, and in solid oxide fuel cells to generate electricity. Moreover, it can be reformed to produce syngas and then used in the above applications.

There is a large body of literature on methane combustion, including ignition, extinction, flammability limits, flame speeds, cellular instabilities, and emissions. Consequently, detailed thermo-transport and kinetic models have been developed to simulate and analyze methane flames in a variety of configurations. Considerable research has also been reported on fire suppression, which has examined the extinction and blowout of methane-air flames using various diluents, such as CO2, N2, H2O, and chemicals (Gunaseelan, 1997; Quesito, 2011). Most of these studies and the associated models can be readily used for analyzing the combustion and emission characteristics of biogas, whose main constituents are CH4 and CO2 with small traces of H2O, and N2. This section provides a brief overview of the fundamental combustion properties of biogas, and its application in IC engines. For more detailed discussion, the reader is referred to the extensive literature available on methane combustion and emissions.

Biogas has lower energy content compared to natural gas. For example, the volumetric heating values of natural gas (94% CH4) and biogas (60%CH4/40%CO2) are 38.6 and 25 MJ/m3, respec­tively. This has consequences for using biogas in natural gas-fired combustion devices, since the lower heating value implies higher feeding rates and lower flame temperatures. Figure 2.17 compares the predicted adiabatic flame temperatures for methane-air and two biogas-air mixtures, shown in Table 2.4.

As indicated, the biogas flame temperature is about 100-200 K lower than that of methane. Lower temperatures imply lower flame speeds and thermal NO for biogas flames compared to those for methane flames. The comparison of laminar burning rates for freely propagating methane and biogas flames is shown in Figure 2.18, which plots the flame speed as a function of equivalence ratio and pressure. The flames were computed using the PREMIX algorithm in CHEMKIN software along with the GRI-3.0 kinetic mechanism. As expected, results indicate lower flame speeds for biogas-air mixtures compared to those for methane-air mixtures. The effect of pressure on flame speed is qualitatively similar for all three cases shown, with the flame speed first decreasing sharply and then relatively slowly as the pressure is increased.

Since biogas is potentially a cleaner and more sustainable alternative to natural gas, it is relevant to analyze methane and biogas flames over different combustion regimes. Figure 2.19 from Aggarwal (2009) depicts the computed structures of methane-air and biogas-air partially premixed flames in terms of temperature, velocity, and species mole fraction profiles.

The counter flow flames were established at Ф = 1.4, pressure = 1 atm, and strain rate = 200 s-1, using the OPPDIF algorithm and GRI-3.0 mechanism, as stated earlier. For all three cases, the flames exhibit a double flame structure with a rich premixed reaction zone (RPZ) located on the fuel side and a non-premixed reaction zone (NPZ) on the oxidizer side near the stagnation plane, which is located by the zero value of the axial velocity. The fuel is completely consumed in the RPZ, producing CO, H2, and intermediate hydrocarbons, which are transported to and consumed in the NPZ. The two reaction zones can also be located by the two local peaks in axial velocity profiles (Fig. 2.19), and by the peaks of CO and CO2 mole fractions, respectively. For example, the RPZs for the three flames are located at 0.85, 0.9, and 0.91 cm, respectively, from the fuel nozzle, based on the peak CO locations, while the NPZ are located at 0.975, 0.963, and 0.95, respectively, based on the peak CO2 locations. The NO profiles indicate a significantly lower level of NO formation in biogas PPFs compared to that in methane PPFs. This may be attributed to the less thermal NO and prompt NO formed, indicated by lower temperatures and C2H2 peaks, in biogas flames compared to those in methane flames.

Like syngas, there are relatively few studies on the performance and emission behavior of biogas-fed combustion devices. Henham and Makkar (1998) and Yoon and Lee (2011) reported experimental investigation on the combustion and emission characteristics of dual-fuel CI engines using diesel and biogas. These studies were able to point out the viability of dual-fuel engines for using fuels with low energy content like biogas. Bedoya et al. (2011) performed an experimental

study of biogas combustion in an HCCI engine for high efficiency and ultra-lowNOx emissions. Kohn etal. (2011) performed experiments on a SI engine operating onLFG and syngas. The syngas addition was found to improve the engine efficiency and reduce emissions of CO, UHC, and NOX.