- Looking for alternative fuels in transportation has multiple incentives for India as below.
- Reducing import dependence for crude
- Reducing carbon emissions
- Reducing air pollution
- In this section we will be looking at alternative fuels from fossil sources. In the next section we will talk about biofuels.
- From solid to liquid to gaseous fuels, the fuels become cleaner in the increasing order.
- However, energy density decreases in that order.
- Combustion efficiency is increase. (Proportion of usable heat is more)
- Cleaner combustion in the absence of residues and pollutants
- Easy to transport. The challenge is to compress it in fuel pumps and tanks.
- However, the problem with gaseous fuels is higher NOx emission due to high temperature of combustion.
- Natural gas is primarily a mixture of lightweight alkanes (aliphatic: straight-chain hydrocarbons). see table
- Typically, C3, C4, and C5 hydrocarbons are removed before the gas is transported.
- In addition, natural gas coming out of the reservoir contains about 9% which is reduced to 2% before being shipped onshore.
- Here again carbon capture technology mentioned above is employed to remove CO2.
- Thus, natural gas that is commercially available is a mixture of methane and ethane.
- The propane and butanes removed from natural gas are usually liquefied under pressure and packed as liquefied petroleum gases (LPG).
|Propane, Butane, Pentane etc.||<5|
Source of Natural Gas
- Natural Gas occurs in nature in the following forms
- Bombay high, Assam’s Digboi, KG basin, Rajasthan (Jaiselmer), Tripura.
- Largest gas fields are located in Iran, Central Asia and Russia. This is why the interest in TAPI (Turkmenistan, Afghanistan, Pakistan, India)
Diluted in Oil: Petroleum
- Petroleum wells are those in which liquid petroleum comes out and the top part is gas.
Diluted in Water: Gas hydrates
- Gas hydrates are natural gas is diluted in water.
- Hydrates are water and gas molecules present in crystalline form.
- Normally they are in the form of ice in permafrost or sea floor which when heated releases methane.
- Gas hydrates are present all over the coast.
Diluted in rocks: Coal-bed methane
- Found in coal seams where hard coal deposits have methane absorbed in it under high pressure.
- Mining involves reducing the pressure of coal bed and releasing the trapped gases.
Natural Gas: Storage, Transport and Use
- Since natural gas contains some amount of propane and butane, we remove them.
- This is because, being heavier and denser, propane and butane liquifies at low pressure of 5 bar and can be used as cooking fuel.
- The pressure in your LPG cylinders is around 12 bar to keep it liquid inside the cylinder. This is done to get high energy per volume.
- On the other hand, its major component methane is lighter and hence needs to be compressed or liquified in order to be used in vehicles.
- Accordingly, we make Compressed Natural Gas or Liquified Natural Gas. (more on this in a bit)
India’s push for gas-run economy
- India has been taking a number of steps to leapfrog into gas-run economy in the near future.
- Currently the share of natural gas in India’s energy basket is 6.7% compared to 23% worldwide.
- This India wants to increase to 15% by 2030.
- Accordingly, India has taken the following steps:
- Expansion of National Gas Grid to about 35,000 Km from current 17,000 Km.
- Expansion of city gas distribution network to cover 96% of India’s population and 86% of its geographic area.
- Setting up of LNG Terminals.
- Methane, being lighter, is compressed at high pressure (around 225 bar) in the vehicle tanks to get a workable energy-density. This limits the size of the tank making it suitable for light-commercial vehicles like cars, auto-rickshaw etc.
- Thus, CNG vehicles have low range and are suitable only for intra-city travel.
Natural Gas alternative in transportation
- While CNG continues to grow rapidly in India, the next logical step is to step up LNG production to replace diesel in heavy-load trucks.
CNG in transport fleet
- In 2022, annual sales of CNG vehicles in the passenger segment more than doubled.
- 49% of the light commercial vehicles sold in 2022 were CNG-powered.
- India plans to increase the number of CNG stations to 10000 by 2030 from current 4,500.
World’s 1st CNG terminal
- India is setting up the world’s 1st CNG in Bhavnagar, Gujarat.
Advantage of CNG as alternative
- No sulphur emissions.
- No unregulated emissions like benzene.
- Low carbon emissions. (1 unit of natural gas produces half the emission as compared to 1 unit of coal)
- No PM pollution.
- NOX emissions will be higher due to higher temperature of combustion.
- Liquid fuels can be alcohols or hydrocarbons. While hydrocarbon liquids are diesel and petrol, alcohol-based fuels are methanol and ethanol.
- Besides alternative liquid hydrocarbons include LNG, LPG, Dimethyl ether, liquid hydrogen (suitable only in rockets).
Gas-to-liquids: Hydrocarbon-based alternatives
- As you have already seen, hydrocarbons in the gaseous form are small chain molecules like C1, C2, C3 and C4 (methane, ethane, propane and butane)
- While gas is voluminous, its density is too small.
- Therefore, to transport and use gas either we need to pump it at high pressure, or we need to liquify it.
- Gas-to-liquid alternatives involve this conversion of gas to liquid fuels. (like LNG)
- When we are liquifying gas what we are really doing is only fusing small chain gas molecules into long-chain ones. This is fancily called polymerizing.
- This will increase the boiling point and eventually become liquid.
- Basically, by polymerizing hydrocarbon gas molecules to liquid, we are only making diesel.
- Higher energy density
- Ease of transport and storage
- Zero sulphur emissions
- Zero aromatics (no closed-rings remember?!)
- Low NOx as compared to gaseous fuels.
LNG (Gas-to-liquid alternative)
- Alternately, we can liquify methane in order to achieve higher energy density.
- However, at normal atmospheric pressure methane liquifies at very low temperature of about -1700C because it has a low Boiling Point about -1600C.
- Thus, in order to transport we need cryogenics which is not feasible for 2-wheelers and 4-wheelers.
- However, LNG, with its higher energy density, occupies three times less space than CNG.
- Thus. it is suitable for long-haul transport like trucks provided they are fitted with cryogenic tanks.
LNG in transport fleet
- LNG is the best bet to replace heavy-duty diesel-based trucks.
- India has more than 10 million trucks, of which the government expects at least a million to run on LNG by 2035.
- India has planned to build 1,000 LNG stations in next 3 years along major highways, industrial corridors and mining areas.
- As we have seen gaseous components of petroleum separation include methane, ethane, propane and butane. While methane and ethane are used in the form of gas (natural gas), the heavier gaseous hydrocarbons are liquified to form what is called liquified petroleum gas.
- This is because heavier gases like propane and butane liquify when compressed become liquid at lower pressures of around 5 bar.
- This is why your LPG cooking cylinders are maintained at low pressure of 12 bar to hold enough fuels.
- LPG is a better alternative than LNG in transportation as it can be liquified at lower pressure and thus suitable even for 2, 3 and 4 wheeler segments.
- Only constraint has been lack of supply and thus LPG use is restricted as cooking fuel.
- Another approach to make liquid fuel is compress the syn gas obtained from gasification process. (remember syn gas is produced by burning any hydrocarbon, be it fossil or biofuel, with steam)
- The mixture of CO and H2 is adjusted in its pressure and temperature to form methanol. (CH3OH)
- CO(g) + 2 H2(g) —-> CH3OH(l)
- Methanol is also called wood alcohol.
- Methanol can directly be used in IC engines or even converted to petrol using a zeolite catalyst.
- Note that methanol can be produced from any hydrocarbon not just coal including natural gas, biomass, and even captured carbon dioxide. Only thing you need to adjust the amount of hydrogen.
Significance of Methanol
- Methanol cars are becoming common especially in China where coal is in abundance.
- Methanol can also be used in fuel-cells and thus could drive the EV revolution.
- India by adopting Methanol can reduce its import dependence on oil and at the same time have a cheaper fuel (at least 30% cheaper than any available fuel)
- Niti-Aayog is set to come out with a roadmap for transition to Methanol Economy.
DME: Di-Methyl Ether
- The most significant thing about DME is that it is liquid at room temperature.
- To make DME all you need to do is to take 2 molecules of methanol and remove water from it.
- CH3OH + CH3OH à CH3-O-CH3 + H2O
- Liquid at room temperature
- Liquifies at 5 bar and thus easily stored.
- Better for NOx reduction
- Low CO2 emissions owning to high H-C ratio.