UNIT 4 BIOGAS, NATURAL GAS AND LPG AS FUELS
Compressed Natural Gas (CNG)
Environmental degradation is on the rise due to the increased motor vehicle
population. One of the strategies adopted to curb deteriorating environmental
quality is the use of alternative fuels. Natural gas is the world„s most abundantly
available fuel, supplies over 25% of the energy demand in the developed world and
requires little or no treatment prior to use. Thus, it is a low-cost fuel as compared to
diesel and gasoline. Natural gas essentially contains methane and is renewable
environment-friendly fuel. Natural gas is compressed to 200 bars to improve its
energy density and on-board storage capacity and is termed as compressed natural
gas (CNG). The use of CNG as an automotive fuel results in significant reduction in
the level of pollutants such as CO, HC, NOX and PM. Additionally, the use of CNG
results in the reduction of greenhouse gases (C02), owing to the lower carbon-to-
hydrogen ratio of fuel as compared to other hydrocarbon fuels. In India, CNG is a
leading alternative fuel, which is being promoted as an alternative to diesel and
gasoline. The lower cost of CNG as well as its eco-friendly characteristics makes it an
attractive fuel for India.
Natural gas is a very good SI engine fuel, and it was used from the very early
days of engine development. However, relative to liquid petroleum fuels, the ability
to store sufficient amounts of natural gas for on-board vehicles has presented a
significant barrier to its broad use as a transportation fuel. Significant advances
have been made in high-pressure cylinders that can store natural gas at high
pressure (up to 200 bar) that are made of lightweight materials including aluminum
and carbon fiber.
Engine modifications for CNG operation
To enable the conventional diesel engine to operate in CNG mode, some
rework of the engine is needed to match the CR to that of natural gas. This work
usually involves machining the piston to lower the CR. The addition of an ignition
and fuel control system are the other main modifications. The result is an engine
with long life and very low particulate emissions. In order to overcome the decrease
in volumetric efficiency with use of CN G, the engine can be fitted with a
turbocharger.
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Production of CNG
Natural gas is present in the earth and properties of natural gas are dominated by
methane. Methane is widely acknowledged to be produced from four sources:
1. Organic matter that is decomposed in the presence of heat.
2. Organic matter that is converted through the actions of microorganisms.
3. Heavy hydrocarbons like oil that produce methane when heated.
4. Coal that releases methane over time by decomposition.
There is also a theory that methane is present in large quantities deep within the
earth„s crust, from which it migrates upward via cracks and fissures. Very large
reserves of natural gas are believed to lie at depth of 4600-9200 meters. Natural gas
requires very little processing to make it suitable for use as an automotive fuel.
Water vapor, sulfur, and heavy hydrocarbons are removed from natural gas before
compressing it to 200 bar and storing it in cylinders. This processed gas is now
known as compressed natural gas (CNG).
Properties of CNG
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CNG Storage
CNG is stored at 200 bar pressure in stainless steel cylinders. A bank of cylinders is
called as "cascade". Since weight is not a concern for CN G storage cylinders, most of
these cylinders are made almost exclusively from carbon steel. The regulations to
which these cylinders are made include the Indian Gas Cylinders Rules, 2004 or the
American Society of Mechanical Engineers (ASME) pressure vessel code. ASME
cylinders are preferred for refueling system storage because they are larger in
volume and minimize the amount of high-pressure plumbing needed to connect them
with the compressor and dispensing system. CNG storage cylinders are placed on a
concrete slab without an enclosure, though a canopy over is useful in protecting the
cylinder valves from rain.
In India, CNG cylinders are manufactured from a special steel alloy and are
seamless in construction. Their compact size allows them to easily fit even in a small
car. The 50-liter capacity is the most regularly used. CNG cylinders are designed and
built to withstand high pressure. The maximum pressure in a CNG cylinder is
approximately 200 kg/cm2 (about 2840 psi). The cylinders are provided with “safety
burst disc" in order release the high pressure gas.
CNG Dispensing
A CNG-dispensing system includes the basic elements of a compressor, compressed
gas storage and means of dispensing CNG into vehicles. CNG dispensing nozzles
have a finite lifetime and should be replaced at the end of their useful life to prevent
failure. Dispensers for CNG provide a convenient means of filling CNG from the
storage system into the vehicle. They will typically incorporate a metering device to
measure device to measure CNG flow and an on-off switch, which is activated by the
removal of the refueling nozzle.
Material Compatibility for CNG
CNG is compatible with stainless steel, aluminum, copper and other alloys as well as
all elastomers. A serious threat to the materials compatibility of CNG fuel system is
considered water vapor. Water vapor CNG can cause steel and cast iron to rust and
aluminum to corrode. The presence of water greatly accelerates the corrosion
properties of the hydrogen sulphide that might be found in the natural gas. For these
reasons, it has been recommended to remove water vapor from CNG to prevent it
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from condensing in the system under static conditions. Fittings directly mounted on
the cylinders must be electrochemically compatible with the CNG cylinder material.
CNG Fuel Kits
The CNG fuel kit is an assembly, which is fitted on an SI engine, in order to allow
the engine to run on the gaseous fuel. The kit required for CNG operation generally
contains various valves, connectors and gauges. These kits are approved by the
Central Motor Vehicle Rules, 1989.
1st generation kits — venture or carburetor system — simple in construction and in
expensive — non reliable and leakage.
2nd generation kits — gas-air valve or the variable venture system — has drawbacks
similar to 1st generation kits
3rd generation kits — throttle body injection — no possibility for charge
stratification
4th generation kits — manifold injection with charge stratification facility -
improved fuel economy and reduced emissions.
5th generation kits — direct injection technology.
Layout of a typical CNG kit
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CNG kit generation
Electronic control unit (ECU): The ECU is usually microprocessor based. It
communicates with various sensors through a CAN BUS interface. ECU does data
recording and storage, efficient fuel management, deceleration fuel cut off, engine
and vehicle speed limiting, faculty diagnosis, etc.,
Cylinder: The cylinder used to store CNG at a working pressure of 200 bar. It is
fitted with a shutoff valve and a safety burst disc.
Pressure regulators: the function of pressure regulator is to release the pressure in
the CNG from 200 bar to just below atmospheric pressure. This negative pressure is
also a safety feature that will not allow gas to pass through when the engine is not
running.
CNG solenoid valve: The solenoid valve is used to cut off the fuel supply to the
engine as and when required. It is operated through an ECU and acts as safety
device during emergencies.
Selector switch: a selector switch is fitted at the dashboard for bi-fuel vehicles,
enabling the driver to choose either the CN G mode or the petrol mode of operation.
The electronics built into this unit also ensures safety by switching off the gas
solenoid whenever the engine is switched off.
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Gas-air mixer: for the carburetion kits, the gas-air mixer is a unique component,
specially designed to suit each engine model. It precisely meters gas and mixes it
with air and deliver an appropriate air-fuel mixture to the engine.
Performance and Emission Characteristics of CNG and Gasoline in SI
Engine
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From the above graph, the maximum power output with the gaseous fuel was
approximately 15% lower than that of the base gasoline engine. The decline in power
output was attributed to lower charging efficiency resulting from increased flow
resistance of the air. It is seen from the graph that the intake manifold pressure of
the natural gas engine was identical to that of the base gasoline engine, which
means that the volume flow rate of the unburned mixture inducted into the engine
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was the same. On the other hand, the quantity of energy provided by natural gas per
unit volume of unburned mixture is approximately 11.5% lower than that for
gasoline. Consequently, at an ideal level of thermal efficiency, an equivalent
relationship exists between the 8.6% decline in charging efficiency and the 11.5%
decline in power output resulting from the use of natural gas. Even if charging
efficiency equal to that of gasoline were obtained, the quantity of energy provided by
natural gas per unit volume of mixture would still be approximately 3.5% less.
Consequently, power output would also drop by 3.5% at an identical level of thermal
efficiency.
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The natural gas has approximately 22% less carbon weight per unit energy of fuel
than gasoline is thought to be one factor behind the THC reduction. Another factor is
thought to be the absence of wall flow that occurs with gasoline but not with natural
gas because of the latter„s gaseous state. The formation of NOX in an engine is
governed by localized temperatures, gas concentration and gas flow, among other
conditions, owing to the occurrence of non-equilibrium reactions. NOX forms more
slowly with natural gas than with gasoline. One major reason for this is the lower
adiabatic flame temperature of natural gas which is approximately 700C lower than
that of gasoline. The level of CO2 emissions can be easily be estimated from the fuel
properties and quantity of heat released, since the combustion efficiency of fuel is
generally around 98-99%. The C02 formation rate for natural gas is approximately
22% lower than that of gasoline. Naturally, the high octane number of natural gas
allows the use of a higher compression ratio. Consequently, CO2 emissions would be
further reduced as a result of the improved thermal efficiency obtained by increasing
the compression ratio.
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Liquefied Petroleum Gas (LPG)
Liquefied petroleum gas is a by product of natural gas processing and petroleum
refining. LPG includes several light hydrocarbons whose main distinguishing
characteristics is that it can become a liquid when put under modest pressure (10
bar). Propane and butane are the most common LPG constituent and for vehicle use,
LPG essentially consists of 70%-80% propane. In its natural state, LPG is a colorless,
non-toxic gas that comprises at least 90% of propane, 2.5% butane and higher
hydrocarbons and the remaining portion is made up of ethane and propylene. An
odorant is added to the gas so it can be detected for safety reasons. Under moderate
pressure, propane gas turns into a liquid mixture, making it easier to transport and
store in vehicle fuel tanks. Compared with gasoline, propane can lower carbon
dioxide, carbon monoxide, and other toxic emissions.
The energy content of propane is less than gasoline, which means that it achieves
fewer miles per gallon than gasoline. As a result, more propane is needed if the
vehicle is to travel the same distance as a similar gasoline or diesel vehicle. In fact,
propane vehicles have a higher octane rating than gasoline, allowing for a higher
compression ratio in the engine and greater engine efficiency. This also, reduces
“knock” and allows the engine to run more smoothly. Because the fuel is already in
the gaseous state, it readily mixes with air in the combustion chamber to allow for
nearly complete combustion. This reduces certain exhaust emissions, such as carbon
monoxide and minimizes the problems related to starting the vehicles in cold
climates.
Production of LPG
LPG is produced by two processes, namely catalytic cracking and hydro cracking of
crude oil. In petroleum geology and chemistry, cracking is the process whereby
complex organic molecules such as kerosene or heavy hydrocarbons are broken down
into simpler molecules by the breaking of carbon-carbon bonds in the precursors. Oil
refinery cracking processes allow the production of “light" products such as LPG and
gasoline from heavier crude oil distillation fractions such as gas oils and residues.
Fluid Catalytic Cracking (FCC) produces a high yield of gasoline and LPG while
hydro cracking is a major source of jet fuel, diesel, naphtha and LPG.
About half the LPG is produced in association with the production of natural gas,
and the other half is produced in association with crude oil refining. It is undesirable
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to leave LPG as a natural gas due to its tendency to liquefy under modest pressures
existing in natural gas pipelines. The major use of LPG is for heating spaces in
homes and for commercial purposes, though it is also an important feedstock for
photo chemicals. The production of LPG has been constant over the year; however,
consumption is the highest during the winter because of the large amounts used for
space heating.
Propane, the second components of LPG, could also be used as vehicle fuel in the
pure form with appropriate engine modifications.
Properties of LPG
Storage of LPG
LPG is stored in both above ground and underground tanks. The LPG industry uses
many different types and sizes of tank, ranging from 100 to 30,000 gallons. Most
LPG tanks comply with ASME specifications and are built to withstand working
pressure of 250 psi. LPG is compressed into a liquid for ease of storage and delivery.
For storage and transportation, LPG is pressurized and LPG tanks are sealed.
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Sealed tanks eliminate evaporative emissions or spillage. If there is a leak, LPG
vapors sink to the ground and pool, creating a potentially hazards situation. An
odorant is added to make leaks more detectable. Since LPG vaporizes when released
from the tank and is not water soluble, it doesn„t pollute underground water sources.
LPG is extremely volatile and burns twice as hot as gasoline fire. Steel is the most
common material for LPG tanks, though aluminum is also allowed and is popular for
portable LPG tanks. Vehicle fuel tanks are relatively thick wall steel construction. In
the event of a vehicle crash, they are less prone to rupture or cause fires than
gasoline tanks. Vehicular LPG tanks often offer a limited storage capacity, resulting
in an insufficient autonomy (from 300 to 400 km). On some vehicle models, the tank
is placed in the boot or it is located in the spare wheel„s housing. Moreover the
weight of the storage tanks ranges between 0.6 and 0.8 kg per liter of stored LPG.
Material Compatibility for LPG
LPG tanks are made from commonly available and inexpensive steel or from
aluminum. LPG is a good solvent for other hydrocarbons and the plasticizers used in
elastomers. Impurities in LPG can include water particulates and foreign matter,
and residue, which includes dissolved components that are left behind when LPG is
evaporated. The LPG residue may be corrosive to metal fittings and it has been
observed to cause LPG vapor hose to become permeable over time. Hoses made from
butyl rubber are not compatible with LPG and will swell and leak. Hoses made from
neoprene should be used and are compatible with LPG. The valves used in LPG
systems must be made from steel, ductile (nodular) iron, malleable iron or brass. Soft
parts of these valves such as gaskets, valve seat disks, packing, seals and
diaphragms must be made of materials compatible with LPG. For other LPG
equipment such as pumps, pressure regulators, vaporizers, meters, strainers,
compressors, etc., aluminum and zinc are acceptable materials.
Safety Systems for LPG
Due to the hazards of spillage, LPG safety system are very critical. Emergency
shutdown switches should be mounted near the dispenser and at a distance between
6 to 30 meters from the dispenser. Leak detection system could be incorporated in
the vicinity of the dispenser; near ground level since LPG vapors are heavier than
air. Like pipeline natural gas, LPG is odorized so that leaks should be noticed by
refueling personnel. LPG fire suppression systems can be dry chemical or water
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based; the decision about which to use depends on the site conditions of the facility
an on local fire protection codes and regulations. Emergency plans to deal with
inadvertent releases of LPG or LPG fires should be worked out with the local fire,
police and emergency response agencies. LPG fires are not recommended to be
extinguished until the source of the burning LPG is shut off completely.
LPG Engine Development
The vast majority of LPG engines to date have been of the mechanical-control type
that meter LPG in proportion to the amount of air used by the engine (air valve and
venture-type mixers). While these systems work well, their capabilities have been
overshadowed by gasoline fuel injection systems that offer superior vehicle
accelerations, drivability and cold-start performance.
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Performance and emission characteristics of LPG and gasoline in a SI
engine
Engine: 4-8, 4- Cylinder Maruthi Zen engine (MPFI)
Ignition timing: 50 BTDC, LPG tank: Wt: 14.2 kg, pressure = 10 bar
At low engine speeds, the break thermal efficiency is higher for gasoline than LPG,
due to low volumetric efficiency of engine during LPG operation. At 3500 rpm, the
brake thermal efficiency was same when then engine was operated with both LPG
and gasoline. However, at high speeds the better combustion characteristics of LPG
displayed higher engine BTE.
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From the above graph, the HC emission is low at all engine speed for LPG than
gasoline. This is due to low carbon content in LPG compared with gasoline.
From the above graph, the CO emission is low at all engine speed for LPG than
gasoline. This is due to low carbon content in LPG compared with gasoline. Better
combustion characteristics of LPG due to high octane rating may be the reason
behind low CO.
From the above graph, the NOX emission is low engine speed for LPG than gasoline,
but it is higher at high engine speeds. This is due to better thermal efficiency
exhibited by LPG compared with gasoline at higher engine speeds.
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Biogas
Biomass is an energy source, which is renewable in nature. When this biomass is
fermented in an anaerobic manner a gaseous fuel termed as biogas is obtained.
Biogas contains a large amount of methane. Approximately 67 m3 of a gas can be
produced from 1 ton of biomass. Biogas can also be produced from agricultural waste,
which contain cornhusks, leaves, animal droppings and other waste. Biogas is
essentially a mixture of methane and carbo dioxide with traces of hydrogen sulphide.
The methane content in biogas varies from 50 — 60% depending on the source.
Biogas production Proceeds in three stages (hydrolysis, acid formation and methane
fermentation) under the action of certain bacteria. This process of waste
decomposition or biodegradation is natural and inevitable. The production of biogas
depends on many factors: the kind of feedstock and its water content, temperature,
pH, etc. the fuel quality can be improved by removing traces of moisture and carbon
dioxide. The energy content of biogas is equivalent to two-thirds of natural gas and
can be burned in IC engines.
Composition of Biogas
Properties of Biogas
Biogas is a very stable gas, which is a non-toxic, colorless and odorless. However, as
biogas has a small percentage of hydrogen sulfide, it smells like rotten eggs, which is
not often noticeable especially when being burned. When the mixture of biogas and
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air burns blue flames is generated producing large amount of heat energy. Some
important properties of biogas are listed below.
Calorific Value: 18840 — 20943 kI/m3
Octane rating: 130
Ignition temperature: 650°C
Stoichiometric air to fuel ratio: 10: 1
Explosive limit to air by volume: 5 -15
Biogas Production
Biogas is produced by AD process. AD is a biological process, which occurs in the
absence of oxygen and in the presence of anaerobic organisms at ambient pressure
and temperature of 35 — 70°C. The container which this digestion takes place is
called a digester. The digestion reaction is carried out by microorganisms like
bacteria. Anaerobic bacteria ferment the biomass to produce biogas.
The AD process has three phases:
1. Enzymatic hydrolysis, where fats, starches and proteins in biomass are broken
down into simple hydrocarbons.
2. Acid formation, where the simple hydrocarbons are then converted to simple
organic acids by bacterial action.
3. Methane formation, where the organic acids are decomposed to form methane and
carbon dioxide.
For all the three steps to occur, the pH value of the digestion should range from 6.5
— 8. The environment must be maintained for suitable growth of methane forming
bacteria.
The general equation for AD is given as follows.
(C6H10O5)n + nH2O —> 3nCO2 + 3nCH4
The factors affecting biogas generation are listed below:
1. The pH value of biomass waste
2. Digestion temperature
3. Solid content of the feed material
4. Rate of feed into the digester
5. Carbon to nitrogen ratio of waste
6. Diameter to depth ratio of the digester
7. Retention time for digestion
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8. Stirring rate of contents of the digester
9. Pressure inside digester
10. Rate of acid accumulation in the digester.
Biogas Treatment (CO2 and H2S scrubbing)
Energy recovery from biogas is becoming more common, but the process is hampered
due to the presence of impurities such as hydrogen sulfide (H2S) in the gas. The H2S
levels can range in biogas from 5000 ppm to over 30,000 ppm. Hydrogen sulfide has
an offensive “rotten egg" odor at concentrations as low as ppb, and is an acute toxic
hazard at concentrations above 100 ppm. When combined with water vapor, it forms
sulfuric acid which can corrode boilers, piping and other equipment. Reducing the
hydrogen sulfide content in biogas reduces sulfur dioxide emissions, equipment
corrosion, and fouling. H23 reduction also offers cost savings through lower
maintenance requirements, and results in greater energy recovery for increased
power generation. Several biogas treatments that remove impurities like hydrogen
sulphide, carbon dioxide, and moisture are available.
Hydrogen sulphide can be controlled by using technologies such as the following:
1. Addition of iron salts in the anaerobic digester
2. Solid-state scavenging systems
3. Non-regenerable liquid scavenging systems
4. Alkaline scrubbing with bacterial regeneration of a reagent
5. Full-scale, amine Claus process technology
6. Regenerable redox reagent systems
7. Modified Claus absorption technology
Out of these technologies, the regenerable redox technology has been successfully
used in Australia.
The working of the system is as follows: The biogas enters the unit boundary or it is
by-passed to a storage /flare system. The biogas and recycled biogas are fed to a
blower that operates a flow rate at least 10% greater than the maximum actual flow
rate, and a pressure high enough to meet hydrostatic head and pressure losses,
ensuring that untreated gas cannot enter the discharge line. In the biogas contractor
vessel, biogas is intimately contacted with the proprietary absorption solution at a
pH of about 7.9 to 8.1, which absorbs greater than 99% of the hydrogen sulphide and
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almost all carry-over particulates. A pH meter is installed in the biogas contactor
vessel that alerts the system when make-up chemicals should be added. Treated
biogas exits this biogas contactor vessel through a mist eliminator. A portion is
returned to the blower as recycle gas, the reminder being made available for
utilization at essentially the same pressure and flow rate as the supply gas. The
sulphur rich absorption solution discharges from the vessel over a weir and is fed to
the bottom of the sulphur recovery vessel through a water column gas seal. The
regenerated solution discharges from the bottom of the sulphur recovery vessel, and
is pumped into the biogas contactor vessel for recontact with the gas.
Make-up water is added into the biogas contactor vessel based on level control in a
still well in the sulphur recovery vessel, which is equipped with a level transmitter
and controller. Air is added through a blower impeller-shroud system to regenerate
the solution. By contacting the spent absorption solution with air, the solution is
generated producing elemental sulfur. Dilute sulphur slurry is discharged into the
sulfur slurry chamber, from which it is pumped to the solids removal system. The
slurry level is monitored in the slurry chamber and, when it is within the desired
level, it pumps sulfur slurry to solid removal system. The filtrate solution from the
solids removal system is returned back into the sulfur slurry tank where it is used to
break up the sulfur slurry froth. The solids removal system is designed for either
continuous sulfur removal or in the case of filter process, to be typically opened every
24- hours. Proprietary chemical additive solutions are fed into the standpipe of the
biogas contactor vessel using metering type drum pumps based on pH in the vessel.
Residual measurements downstream from the unit are made periodically to evaluate
system performance.
Advantages of Biogas
Cleaner burning fuel with less hydrocarbons, NOX and particulate matter
emissions.
Lower cost fuel
Ability to be generated from waste feed stocks
Traps and prevents greenhouse gas methane from being released into
atmosphere
Displaces fossil fuels
Reduces groundwater contamination
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Prevents odor pollution
Boosts rural economy by creating jobs
Provides fuel for cooking and rural electrification
Solves the waste problem for diary, hog and poultry farmers.
Hazards of Biogas
Biogas if untreated is corrosive in nature due to the presence of hydrogen
sulphide
It is flammable and explosive if accumulated in enclosed spaces
It is toxic if accumulated due to the presence of carbon monoxide.
Engine modifications for biogas operation
In SI engine: Biogas can be used in spark ignition engines by modifying the
carburetor. Usually the engine is run in the bi-fuel mode. The vehicle is started using
petrol and then the engine runs on biogas. The use ofbiogas in SI engines requires
the provision for storage and entry of gas along with the throttling of air and
advancing ignition timing. The gas can be admitted through intake manifold by a
flow control valve. The biogas and air are mixed in the carburetor and fed to the
engine intake manifold. The mass emissions produced by using biogas are lower than
that for petrol.
In CI engines: Biogas is used in the diesel engine in the dual-fuel mode. Here both
biogas and diesel are simultaneously burned in the combustion chamber. The
compression ignition (CI) engine is modified by making provision for the entry of
biogas with air along with advancing the injection timing and metering the diesel
supply. The biogas and air are mixed in the mixing chamber, which is located below
the air cleaner. The capacity of the mixing chamber must be equal to the engine
displacement volume. Pilot injection of diesel is required for smooth and efficient
operation of the engine in dual-fuel mode. The admittance of biogas in the engine in
the initial stage increases the engine„s speed.
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Modifications required for operating a stationary SI engine on biogas
Components of a kit to convert a vehicle to operate on compressed methane
and petrol
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Details of mixing chamber
Performance and emission characteristics of biogas in SI engine
The above figure shows reduced brake thermal efficiency with the raw bio gas as
compared to gasoline due to reduced flame speed and poor energy content. Presence
of C02 can lead to improper combustion. With pure biogas the brake thermal
efficiency was improved which is even higher as compared to gasoline. This is due to
improvement in combustion as a result of increased flame velocity.
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The variation of C0 & HC emissions with brake power is shown in figure given
above. Raw biogas produced more C0 & HC emissions as compared to pure gasoline
and pure biogas. This is mainly due to incomplete combustion of raw biogas. In
addition it also causes the formation of HC emissions. With pure biogas there is a
reduction in HC and C0 emission due to improvement in combustion.
The above graph shows the variation of NOX emissions with brake power. From the
graph it is seen that the amount of NOX present in the raw biogas is small as
compared to gasoline. This is due to poor in combustion and also reduce peak cycle
temperature. N0X was observed to be higher for pure bio gas; this may be due to
superior combustion characteristics & increased peak cycle temperature exhibited by
the gas compared to raw biogas and gasoline.
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