Abstract
Biogas has emerged as a promising alternative to fossil fuels in internal combustion engines in recent times. It could be used as the primary fuel in Compression Ignition (CI) engines in combination with a small quantity of a high cetane fuel in two modes – dual fuel or Homogeneous Charge Compression Ignition (HCCI). This chapter compares the performance, combustion, and emission parameters of a CI engine operated with biogas in dual fuel and HCCI modes vis-à-vis conventional diesel operation. The effects of biogas composition (quantified in terms of the methane content), location of secondary fuel injection and engine load are investigated. It is observed that the use of biogas has the potential to reduce both NOx and smoke emissions simultaneously, with HCCI mode offering ultra-low emissions. Operating the engine in dual fuel mode can provide high thermal efficiency and significant diesel substitution.
TopBiogas Production
Different waste-to-energy (WTE) techniques have been explored to convert biological waste, such as industrial and municipal solid waste (MSW), into biogas (Demirbas et al., 2011; Gunaseelan, 1997). These techniques can be divided into four broad groups: a) hydrogenation, b) pyrolysis, c) gasification, and d) bio-conversion. A digester is one of the most common means of generating biogas, where biomass is subjected to anaerobic digestion. Anaerobic digestion is a three-stage method of bioconversion involving hydrolysis, acid formation and methane generation. The biogas thus produced is mainly a combination of CH4, CO2 and H2S (Khoiyangbam, Gupta, & Kumar, 2011). Some important properties of biogas are presented in Table 1. The cost of methane generation by anaerobic digestion is similar to those of other sources of biomass energy such as synthesis gases and ethanol (Chynoweth, Owens, & Legrand, 2000). There are different types of anaerobic digesters available, e.g. floating drum and fixed dome designs (Khoiyangbam et al., 2011).
Key Terms in this Chapter
Homogeneous Charge Compression Ignition (HCCI) Engine: A CI engine where one or more fuels are homogeneously mixed with air in the intake, inducted into the cylinder and auto-ignited by compression.
Simulated Biogas: A prepared mixture of methane and carbon dioxide.
Compression Ignition Engine: An IC engine where fuel is ignited by injecting it into hot compressed air, e.g. diesel engine.
Secondary Fuel Energy Ratio (Sfer): Ratio of the energy released by combustion of the secondary fuel (diesel or DEE in the present case) to the overall energy release.
Dual Fuel Engine: A CI engine where the primary fuel is inducted along with air and the secondary (pilot) fuel injected into the cylinder to ignite the compressed primary fuel-air mixture.
Methane Enrichment: Purification of biogas by removing CO 2 in order to improve the combustion properties of biogas.
Biogas: A combustible gas mixture formed by the decay of biomass in an oxygen deficient environment. Its primary constituents are methane and carbon dioxide in variable proportions.
Ignition Delay: The interval between the end of fuel injection and first instance of auto-ignition of the injected fuel. This definition is applicable to CI engines with in-cylinder injection.
Methane Fraction: Fraction (or percentage) by volume of methane in biogas.