Production of Biogas from Multiple Feedstocks and its Application as a Fuel for Spark Ignition Engines - A Review

The passenger cars run by petrol are mostly used for transportation in India. The increases in fuel demand as the vehicles increases, high fuel cost due to price fixation daily and pollute the environment with their combustion products, engineers concern have to look an alternate fuel for petrol in Spark Ignition (SI) engine. Alternative fuels such as LPG (Liquefied Petroleum Gas), CNG (Compressed Natural Gas) are used in SI engines are current development. The use of biogas is also a better exchange for LPG and CNG fuel. Because biogas is easily producible, has a high octane number, lower emission level, wider flammability range, lower cost per energy unit of biogas, high self-ignition temperature and it also resists knocking, which is desirable in SI engines. Limited authors have attempted to use biogas as a substitute fuel for petrol in SI engine and dual fuel in Compression Ignition (CI) engine on their experimental studies. This paper reviews those studies and gives an observation for researcher’s to make use of biogas as an alternative fuel for spark-ignition engines efficiently.


INTRODUCTION
The enormous growth in industries and an increase in population is the main reason for the heavy depletion of fossil fuel. Owing to the current energy scenario, the entire world is looking alternate to fossil fuels that should be renewable ones. Gaseous fuels are widely used nowadays because of their wide ignition limits and also have the capability to form a uniform mixture which reduces pollution. Various alternative gaseous fuels are compressed natural gas, liquefied petroleum gas, hydrogen, biogas, producer gas. Among the all renewable energy sources, biogas is particularly significant because of possibility of use in internal combustion engines (Mustafi et al., 2008), which are the main power source for transport sectors and also biogas is become more popular in rural areas for cooking and is produced from cow dung, other animal wastes and also from plant wastes by anaerobic decomposition. It is also known as gobar gas. Biogas is used as an alternative fuel in petrol engine and dual fuel in the diesel engine. Biogas is primarily composed of methane (CH4), carbon dioxide (CO2), hydrogen (H2), nitrogen (N2) and minute impurities like hydrogen sulphide (H2S). Biogas can be subjected to clean by removal of impurities by scrubbing process. It is made up of twothird methane and remaining by carbon dioxide by volume. Biogas is safer in many aspects compared to other fuels. Ignition temperature for biogas is higher compared to gasoline and diesel fuel, which reduces ignition delay and makes it less hazardous. Biogas can be upgraded to synthetic gas by Removal of CO2 is necessary to increase the density and calorific value of the gas to meet quality (Qian et al., 2017). An attempt was made by a researcher to a covert diesel engine to SI engine fueled by biogas and an observed 35% and 40% less power compared to diesel and gasoline fuel respectively (Dobslaw et al., 2019). Inconsequent with another study was done by Neyeloff and Cunkel (1981) in a Cooperative Fuel Research (CFR) engine fuelled with simulated biogas in different compression ratios They have noted storing and transportation were main challenges for biogas. Thring (1985) concluded that biogas would be glowing appearance just where it is close to the production site. This review paper is covered the potential biogas production and its application as a fuel in IC engines. The objective of this review is to make a simple for the researcher to do future development.
are cow dung, sewage, crop residues, vegetable wastes, poultry droppings and pig manure. Biogas is produced by hydrolysis, acidogenesis, acetogenesis, methanogenesis. Various types of biogas plants based on design are It consists of airtight container made of steel or brick masonry is called a digester with inlet and outlet for raw material feeding and getting manure respectively. Another drum is called floating drum which floats over fixed digester employing rope and weight through the pulley. It has an opening at the top as a gas outlet. The ideal temperature for the production of biogas is 30 0 to 40 0 C and the required pH will be 6 to 8. Inhibitors can also be used to produce methane like ammonium sulfate. In case of cow dung as a feedstock for biogas production, cow dung is mixed with water in 3:1 ratio and fed to the digester through an inlet and set up is placed in an open atmosphere and exposed to sunlight. After two or three weeks, cow dung is digested to yield biogas. The biogas main constituent and their percentage are listed in Table 1. Some important thermodynamic properties of biogas are listed in Table 2.

BIOGAS AS A FUEL FOR CI ENGINE
Biogas cannot be used as a substitute fuel in diesel an account of high self-ignition temperature and high Octane number. But it can be used as a primary fuel in dual fuel mode in the modified existing engine and either diesel or biodiesel as pilot secondary fuel. Modification of diesel engine into a dual-fuel engine has the following advantages: Operation on diesel fuel alone is possible when biogas is not available. Any contribution of biogas from 0% to 85% can substitute a corresponding part of diesel fuel while performance remains as in 100% diesel fuel operation (Van Ga et al., 2015). Various research on biogas as a fuel in the dual-fuel engine are listed in Table 3.

BIOGAS AS A FUEL FOR SI ENGINE
The use of biogas as an SI engine fuel offers several advantages. Biogas is a clean fuel causes clean combustion which results in low particulates and nitrogen oxides and reduced contamination of engine oil. The basic modification is required in SI engine is the provision of an air/gas mixer instead of the carburettor as the engine is designed to operate on an air/fuel mixture. From the previous studies conducted by Jeong et al. (2009);Arroyo et al. (2014); Kukoyi et al. (2016); Krishnaiah et al. (2017), it was noted that researchers have used both simulated biogas and direct biogas in their experimental studies. This review study has taken both cases and listed their result in Table 4 and 5.  The brake thermal efficiency is lower by 2.43% and the cylinder peak pressure is higher by 6.55% for dual fuel mode when compared with diesel mode.
NOx emission reduced by 2.6 % and CO emission increased by 3.3% compared to diesel at full load Ambarita et al. (2017) Tiger Diesel Engine R175 AN Simulated biogas (mixing of natural gas and CO2) Output power and efficiency increases with increasing engine speed and SFC decrease.
High CO and HC as the flow rate of biogas increases.
Prajapati et al.   Raw biogas 18% of reduction in brake power, 66% of increase in brake specific fuel consumption and 12% of reduction in break thermal efficiency when the engine is fueled with raw biogas.
The emission components such as CO and NOx are significantly reduced by 40% and 81.5%, respectively, while, the unburnt hydrocarbon (UHC) and CO2 emission were increased by 6.8% and 40%, respectively.

CONCLUSIONS
From the comprehensive review study in various authors findings on biogas as a fuel in the IC engine, the review study concludes the following • The production of biogas from various feedstock is feasible and its composition and properties are pertinent for substitute fuel in IC engines.
• The performance of diesel engines in dual fuel mode of either diesel or biodiesel and biogas is almost equal to diesel fuel operation.
• The emission characteristics of biogas fuelled SI engine are found relatively lower in particulates, carbon dioxide and nitrogen oxides emissions • Biogas is a very efficient, sustainable and environmental friendly fuel which helps to reduce dependence on conventional fossil fuels and the management of waste, Finally, biogas may be recommended as an alternative fuel in CI engine.

Suggestion for Future Development
• For the long-run operation of CI engine running on biogas fuel require the following attentions • Developing the low cost biogas purification and upgrading technologies • During the biogas simulation, it can be enhanced by methane enrichment or the addition of hydrogen to improve its flame quality.