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BIOGAS AS AN ALTERNATIVE FUEL FOR AUTOMOBILES
Updated: Nov 6, 2023
FUELS USED IN INTERNAL COMBUSTION ENGINES AND THEIR LIMITATIONS
There are so many fuels used in I.C. Engines, but they have certain physical and chemical properties. In other words, fuels used in I.C. Engine are re-designed to satisfy performance requirements of engine system, in which they are used. The limitations of fuels that are used presently are as follows.
Gasoline has low octane number. It contains many impurities. All petroleum fuels oxidize slowly in presence of air. The oxidation of unsaturated hydrocarbons results in formation of resinous materials called gum and reduces its lubricating quality and tends to form sludge and warmish on piston and rings. It has less efficiency compared to other fuels. It has high cost. It has less knock resistance as well as energy per unit mass.
Electricity is generated in power stations that use fossil fuels or nuclear power. Another problem is with batteries in these vehicles. The life of batteries is also low and they are quite heavy. Cost of replacing these batteries is high.
BIOGAS AS AN ALTERNATIVE FUEL FOR AUTOMOBILES
Biogas is an environmentally friendly, renewable energy source. It is produced when organic matter, food or animal waste is broken down by microorganisms in the absence of oxygen. Biogas is composed mostly of methane (CH4), carbon dioxide (CO2), hydrogen (H2), nitrogen (N2), water vapor (H2O) and hydrogen sulphides (H2S).
After removing the carbon dioxide, the gas becomes a homogeneous fuel containing up to 80% of methane with the calorific capacity of over 25 MJ/m³. The most important component of biogas, from the calorific point of view, is methane. The other components are not involved in combustion process, and rather absorb energy from combustion of methane as they leave the process at higher temperature than the one, they had before the process.
Properties of Biogas used for engine performance are:
• The high calorific value of biogas is one of the major reasons why it is used as a fuel.
• It is a renewable source of energy and environment friendly source.
TECHNICAL PARAMETERS OF BIOGAS
Technical parameters of Biogas are very important as it has a huge effect on the combustion process in the engine.
• Ignitability of CH₄ in mixture with air: CH₄: 5...15 Vol. %, Air: 95...85 Vol. %
• Mixtures with less than 5 Vol. % and mixtures with more than 15 Vol. % of CH₄ are not properly ignitable with spark ignition. -Combustion velocity in a mixture with air at p = 1 bar: cc = 0.20 m/s at 7% CH₄, cc = 0.38 m/s at 10% CH₄
• The combustion velocity is a function of the volume percentage of the burnable component, here methane. The highest value of cc is near stoichiometric air/fuel ratio, mostly at an excess air ratio of 0.8 to 0.9. It increases drastically at higher temperatures and pressures.
• Methane in a mixture with air ignites at a temperature which is about Ti = 918K to 1023 K.
• Methane number, which is a standard value to specify fuel's tendency to knocking (uneven combustion and pressure development between TDC and BDC). Methane and biogas can be used in engines of higher compression ratios than petrol engines as they are very stable against knocking.
• Stoichiometric air/fuel ratio on a mass basis is the ideal ratio of air to fuel that burns all fuel with no excess air.
PROBLEMS OF USING BIOGAS IN IC ENGINES
• High CO₂ content reduces the power output, making it uneconomical as a transport fuel. It is possible to remove the CO₂ by washing the gas with water. The solution produced from washing out CO₂ is acidic and needs careful disposal. Chemical agents like NaOH, Ca(OH)₂, and KOH can be used for CO₂ scrubbing from biogas.
• H₂S is acidic and if not removed can cause corrosion of engine parts within a matter of hours. It is easy to remove H₂S, by passing the gas through iron oxide or ZnO.
• There is high level of moisture/ water vapor in biogas.
•The gas can vary in pressure and quality.
PURIFICATION AND UPGRADATION OF BIOGAS
Before using biogas as a vehicle fuel, it should be subjected to following processes so that the impurities are removed and the quality of the fuel is increased.
Biomass feedstock is subjected to pre-treatment and then to anaerobic digestion which degrades the microorganisms under anaerobic conditions. Then biogas is produced.
For using it as a vehicle fuel, biogas should be purified and made free of contaminants such as hydrogen sulphide and water vapor. Hydrogen sulphide is removed by anaerobic digestion or upgradation and water vapor is removed by refrigeration or adsorption by a drying agent.
Upgradation of biogas includes the removal of carbon dioxide gas which in turn increases the concentration of methane thus increasing the energy density of biogas.
CO2 has no combustion value and is highly corrosive when wet so its removal is must to improve the biogas quality. The processes to remove CO2 are as follows -
a) Caustic solution, NAOH - 40% NAOH + CO2 → NAHCO3
b) Refined process, K2CO3 - 30 % K2CO3 + CO2 → 2KCO3
CO2 removal from biogas can be done by using chemical solvents like aqueous solution of alkaline salts, i.e., sodium, calcium hydroxide and potassium or mono-ethanolamine (MEA), di-ethanolamine and tri-ethanolamine. Biogas passed through 10% aqueous solution of MEA can reduce the CO2 content from 40 to 0.5-1.0% by volume. The rate of absorption is affected by the concentration of solution and NaOH solution has a rapid CO2 absorption of 2.5-3.0%.
DESIGN OF A BIOGAS VEHICLE ENGINE
The biogas produced from biogas production plants has to be enriched with 95% methane and then can be used in vehicles engines which run on natural gas. Biogas vehicle engine is same as natural gas vehicle engine, although due to the low energy density of biogas, consumers may want to install additional fuel cylinders to extend their driving range. Biogas vehicle engine can be classified broadly into the following three types.
• Dedicated Gas Engine – The gas engines which can run only on biogas or natural gas are dedicated gas engines. They do not have any secondary fuel supply system. A 3-cylinder 4 stroke 800 cc vehicle showed that it is possible for a motor vehicle to run on biogas.
Gas engines are spark ignited engines where biogas/ natural gas is stored at 200 to 250 bars in pressure vessels made of steel or aluminum composite materials conveniently gas cylinders are placed without using luggage space.
1. Battery: The battery provides electricity to start the engine and power vehicle electronics/accessories.
2. Electronic control module (ECM): The ECM controls the fuel mixture, ignition timing, and emissions system; monitors the operation of the vehicle; safeguards the engine from abuse; and detects and troubleshoots problems.
3. Exhaust system: The exhaust system channels the exhaust gases from the engine out through the tailpipe. A three-way catalyst is designed to reduce engine-out emissions within the exhaust system.
4. Fuel filler: A nozzle from a fuel dispenser attaches to the receptacle on the vehicle to fill the tank.
5. Fuel injection system: This system introduces fuel into the engine's combustion chambers for ignition.
6. Fuel line: A metal tube or flexible hose (or a combination of these) transfers fuel from the tank to the engine's fuel injection system.
7. Fuel tank (compressed natural gas): Stores compressed natural gas on board the vehicle until it's needed by the engine.
8. High pressure regulator: Reduces and regulates the pressure of the fuel exiting the tank, lowering it to an acceptable level required by the engine 's fuel injection system.
9. Internal combustion engine (spark-ignited): In this configuration, fuel is injected into either the intake manifold or the combustion chamber, where it is combined with air, and the air/fuel mixture is ignited by the spark from a spark plug.
10. Manual shut off: Allows the vehicle operator or mechanic to manually shut off the fuel supply.
11. Natural gas fuel filter: Traps contaminants and other byproducts to prevent them from clogging critical fuel system components, such as fuel injectors.
12. Transmission: The transmission transfers mechanical power from the engine and/or electric traction motor to drive the wheels.
• Dual Fuel Engine – In the dual fuel engine, there are two separate fuel systems and the engine automatically switches to the backup diesel/ petrol when the primary source of biogas/ natural gas runs out. Also, a switch is installed on the dashboard which allows the driver to easily switch between the natural gas or biogas back over gasoline or diesel at any time including while driving.
The dual fuel engine configuration has original diesel injection system and the gas is ignited by the injection of small amount of diesel oil. This means that the engine starts on 100% diesel and the fuel injection technology increases the biogas ratio as high as the drive cycle will allow (max to 90%). This technology has several advantages over the spark ignition as it maintains the overall efficiency found with the compression ratio.
A gasoline car can also be converted to bi-fuel gas operation by adding a second fuel supply system and storage for methane. It has a fuel storage cylinder usually underneath the vehicle or the trunk. It also includes stainless steel fuel lines, a regulator and a special fuel air mixer.
Most of the gas driven personal cars are converted vehicles that have been retro-fitted with a gas tank in the luggage compartment and a gas supply in addition to the normal liquid fuel system. 2.4 liter bi-fuel five-cylinder engine powered by biogas uses petrol as a backup.
1. Battery: The battery provides electricity to start the engine and power vehicle electronics/accessories.
2. Electronic control module (ECM) (gasoline): The ECM controls the gasoline mixture, ignition timing, and emissions system; monitors the operation of the vehicle; safeguards the engine from abuse; and detects and troubleshoots problems.
3. Electronic control module (ECM) (natural gas or biogas): In a bi-fuel natural gas configuration, the natural gas or biogas ECM communicates with the gasoline ECM and controls the natural gas or biogas mixture, ignition timing, and emissions system; monitors the operation of the vehicle; safeguards the engine from abuse; and detects and troubleshoots problems.
4. Pressure sensor: The pressure sensor senses the pressure of the fuel exiting through the fuel tank.
5. Pressure regulator: Reduces and regulates the pressure of the fuel exiting the tank, lowering it to an acceptable level required by the engine’s fuel injection system.
6. Gas Injector: It consists of a nozzle which introduces the gas to the combustion chamber for ignition.
7. Gas Distributer: It distributes the biogas or the natural gas from the fuel tank to the gas injector.
8. Bi-fuel switch: This switch enables the driver to change the supply of the fuel from gas to petrol/diesel or vice-versa.
9. Gas tank: Stores compressed natural gas or biogas on board the vehicle until it's needed by the engine.
10. Petrol/Diesel tank: This tank stores gasoline or diesel on board the vehicle until it's needed by the engine.
11. Internal combustion engine (spark-ignited): In this configuration, fuel is injected into either the intake manifold or the combustion chamber, where it is combined with air, and the air/fuel mixture is ignited by the spark from a spark plug.
• Biogas/ Diesel blend Engine – It is also possible to run an engine on biogas/ diesel blend (example 90% biogas 10% diesel) using modified diesel engine. The engine runs upon injecting biogas into the engine on the intake stroke (since the methane does not ignite upon compression). The diesel is injected and ignited which then ignites the biogas, effectively acting as a spark plug.
EFFICIENCY OF BIOGAS ENGINE
• Dedicated biogas engines – Since methane has a higher knock resistance as compared to gasoline, therefore dedicated gas engines attain a higher compression ratio. We know that the efficiency of the internal combustion engines increases with higher compression ratio. Thus, these engines have higher efficiency than gasoline operation.
• Dual fuel engines – Engines which use diesel or gasoline along with biogas hold a promise to provide the same efficiency and power output as in diesel or gasoline engines respectively.
COST OF PRODUCTION
Production cost mainly depends on the cost of the feedstock used. Production cost for biogas is typically USD 0.22/ m3 to USD 0.39/ m3 methane for manure-based biogas production and USD 0.11/ m3 to USD 0.50/ m3 methane for industrial waste-based biogas production. Purification and upgradation cost depend partially on trace gases resulting from the feedstock being used and mainly on the size of biogas upgrading unit. Then the distribution and transportation cost also add to it. Thus, the total cost of biomethane as a vehicle fuel varies with the cost ranges of USD 0.40-1.63/ m3 from manure and USD 0.28-1.63/ m3 from industrial waste.
APPLICATIONS OF BIOGAS ENGINE
Though the technology of biogas engine is yet to be developed but in the recent times, biogas engines are being used in some vehicles stated as follows.
• In India, a trial was performed on a 3-cylinder 4 stroke 800 cc vehicle which ran on biogas. It had a fuel consumption of about 9km/ kg of biogas and covered 60km on approximately 1 hour.
• In Britain, the Volvo S80 Bi-Fuel has a five-cylinder 2.4 liter bi-fuel engine powered by biogas gives a range of 250-300 km and the reserve petrol tank provides an additional range of 350 km.
• Biogas produced from farms is compressed and then used for fueling farm vehicles. The diesel/biogas dual fuel model has been demonstrated at farm-based biogas systems with systems running on 95%/5% biogas/ diesel blend. AGCO’s Valtra tractor is a biogas/diesel dual fuel tractor.
• A number of European countries are exchanging their buses with biogas driven engines. The leading biogas fuel countries are Sweden and Switzerland. Biogas Max EC project is being developed to document the experience.
• The city of Guelph biogas bi-fuel 2007 Dodge Ram 5.7 Hemi truck has a range of roughly 200 km when both 70 liter biogas tanks are full and an additional 500 km on gasoline. The raw biogas contains 60% and 40% CO2. Guelph boosts
the gas quality to 86% of methane by addition of natural gas.
• A Kolkata based company recently designed the cheapest bus of India fueled by cow dung Biogas. It charges only Rs. 1 for 17.5 km of route.
• Dedicated biogas engines are used in light duty, commercial vehicles and in city buses. Several bus manufacturers offer gas buses, from conventional two-axle articulated buses.
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