Coal Burning And CCUS: Combination To Combat Climate Change

• By burning fossil fuels, we are essentially taking out carbon buried under the earth surface and putting it in the atmosphere. This has increased CO2 content in the atmosphere by more than 415 ppm (parts per million) as of 2020.
• And this is ever-increasing at the rate of 2 ppm.
• Coal, being carbon-heavy (150-240 carbon atoms), should always accompany CCUS technologies to achieve net-zero emissions. (see news headlines in the recent times)  

Greenhouse Effect

• Anything that has a temperature vibrates. In fact at the molecular level temperature is simply how fast the molecules are shaking (kinetic energy).

• When temperature is zero the molecules are simply not shaking. This is absolute zero or 0 Kelvin.
• Further atoms of molecules shake differently. Molecules made of 2 atoms like O2 or N2 shake due to stretching to-and-fro. (see figure)
• On the other hand molecules with 3 or more atoms (like CO2, CH4) shake due to stretching and bending.
• Further the shaking due to bending matches the frequency of infrared radiation. (which is also vibration really)
• Because frequencies of visible light from the sun don’t match the shaking of atmospheric gases (O2 and N2), or the GHGs, light passes through our atmosphere without being absorbed.

• However, since the frequency of infrared radiation from earth surface matches the bending-vibration of the GHGs, they interact with GHGs.
• This interaction shakes the GHG molecules thereby warming these molecules.
•  The vigorously shaking GHG molecules then shakes the surrounding air molecules thereby increasing its temperature.

GHGs Capable Of Trapping The Heat Include

• CO₂: Most significant
• CH₄: Methane: More potent as it traps more heat.
• Other GHGs include nitrous oxide(N₂O), ozone (O), chlorofluorocarbons, water vapour.

CCUS Technologies: Carbon Capture Utilization And Storage

• In January 2018 IPCC said the world should emit not more than 420 gigatonnes of carbon dioxide to have a 67% chance of avoiding a rise of 1.5 degrees.
• Today that figure is down to less than 350 gigatonnes and global emissions are running at around 40 gigatonnes each year.
• This means we need to achieve zero global emissions(net-zero) by 2030–35 to keep total heating below 1.5 degrees, and 2040–50 for a 2-degree target.
• Thus, low-carbon future essentially includes, as its important component, CCUS technologies which essentially reverses the process of taking the underground carbon (fossils) and putting it into atmosphere (in the form of CO₂).
• In simple terms CCUS involves capturing the CO₂ that is released in burning of fossils, compressing, liquefying, and storing underground.
• Alternately CCUS includes converting CO2-forming fuels and converting them into chemicals that form less CO2. Examples include coal-to-products like syngas, producer gas, methanol, di-methyl ether etc.
• CCUS is going to be an important component in hard-to-abate (CO2) sectors like power plants, petroleum refineries, fertilizers, cement, steel industries, etc.

Carbon Capture

• The flue gas that comes out of coal-fired power plants is mixture of gases.
• It typically constitutes carbon dioxide (~10%), nitrogen (70-80%), oxygen (1-10%), water vapor, Sulphur dioxide, NOx including nitrogen dioxide and nitrogen monoxide.
• Essentially flue gas is very dilute CO2.
• While we treat SOX and NOX using the processes described above, to arrest, capturing and storing carbon dioxide is a big challenge.
• Now this is because the principal component of flue gas is nitrogen. The ‘effort’/’cost’(basically energy) used in capturing and storing flue gas goes higher if we dint purify it.
• Thus, carbon capture is all about purifying flue gas to separate CO2 and Nitrogen.
• Note the nitrogen so separated can be allowed to mix in atmosphere as it is stable and thus safe. It is the oxides of nitrogen that is a problem which we have already dealt with. (anyway air is majorly composed of Nitrogen)
• Thus, carbon capture is really a misnomer, it should have been called carbon purification/ flue gas purification.
• There are different strategies for carbon purification. These include,
• Post-combustion
• Pre-combustion
• Oxy-fuel combustion

Post-Combustion Carbon Capture

• As the name suggests the separation of CO₂ and nitrogen is done after the combustion of fuel.

• In other words, under post-combustion carbon capture, we simply treating the flue gas to get purified CO₂.
• The simplest way of doing this is to stream the flue gas, at the exhaust, into a solution containing ammonia salts.
• While the CO₂ in flue gas reacts with the ammonia, the nitrogen floats upwards. You have separated CO2 and nitrogen.
• Now CO₂ dissolved in ammonia solution should be extracted.
• This can be done by passing very hot steam through the solution which heats the solution and drives off pure CO₂. The CO₂ is then compressed, liquefied, and sent to underground storage.

Advantage

• Retrofitting is possible.

Disadvantage

• The very hot steam needed to separate CO2 from dissolved ammonia solution requires energy.
• The energy required to do this is about 25% of the energy produced in a coal plant.
• Thus, post-combustion method reduces the efficiency of coal plants from 34% to 25%.
• Expensive

Pre-Combustion Carbon Capture

• Here the trick is to convert combustible solids to combustible gases (same as gasification we discussed above).
• Thus, these plants are also called integrated gasification combined cycle (IGCC) power plants.
• This can be done either through steam reforming or through controlled oxygen combustion.
• In both these processes coal is converted to carbon monoxide.
• The carbon monoxide so obtained is treated again with hot steam before combustion.
• The water molecules in the steam splits into hydrogen and oxygen.
• The hydrogen from steam and from coal are mixed and the gas is burnt.
• The oxygen reacts with the CO to form CO2, which is then easily separated, compressed, liquefied, and pumped underground.

Advantage

• Reduces efficiency of coal plant by 15% compared to 25% in post-combustion technique.

Disadvantage

• Expensive

Oxy-Fuel Combustion 

• Now we know the input in powerplants are fuel and oxygen and output is energy and flue gas. The concern, from carbon emissions point of view, is the nitrogen in the flue gas. And the source of this nitrogen is the air in the input.

• So, one way is to purify the air before combustion to remove nitrogen.
• We are now left with oxygen which is what we need for combustion.
• Since we are purifying air at source before combustion the technique is called oxy-fuel combustion.
• In order to purify air, it has to be chilled to -200°c at which point it becomes a liquid.
• This liquid air is then gradually warmed until the nitrogen boils off, leaving nearly pure liquid oxygen.

Carbon Storage

• Now that we have dealt with the purification part or carbon capture part, we need to find a place to store the captured CO₂.
• Normally the CO₂ so captured is compressed, liquified and pumped to the place where it is stored.
• Various strategies to store CO2 include

Geosequestration

• Pressurize the CO2 and put it beneath the earth surface. There are different strategies depending on where below the earth surface you pump and store the CO2.

Enhanced Oil Recovery (EOR)

• Under this technique carbon dioxide is reinjected into depleted gas and oil fields. These oil and gas wells have some un-extracted oil and gas left deep below which was not commercially viable to extract.
• The injected-CO2 provides extra pressure, helping to push more of the oil and gas from below.

Advantage:

• Getting oil and gas out of the process is an economic incentive.

Disadvantage:

• Extracting hydrocarbon through carbon capture and storage is paradoxical.

Saline Aquifers

• Brine or salty water sits in some rocks which are porous in nature. Since the water has too much salt here it not useful for drinking or irrigation, you don’t lose anything by storing the unwanted gas.
• CO2, so injected will dissolve in the water, forming carbonic acid which then combines with minerals in the rock to form stable carbonates, locking up the carbon dioxide forever.

Direct Capture By Algae

• Additionally, CO₂ can be directly captured just like what happens in nature. There are 3 strategies to directly capture CO₂.

Algae

• While most plants are very inefficient in photosynthesis (only 0.5% of sunlight is captured), algae have very high photosynthetic efficiency capable of growing very fast capturing the CO2 from atmosphere.
• The flue gases from power plant can be bubbled through water and algae.
• Algae extracts large amounts of the carbon dioxide to feed their growth and very little is left to emit to the open air.
• The so grown algae can be used as input for producing biodiesel (like vegetable oils).

Advantage

• Biodiesel has no sulphur thus produce no SOX emissions.
• Algae-based biodiesel represents net-zero fuel as the CO2 at the exhaust out of biodiesel-run vehicle is nothing but CO2 captured by algae from the atmosphere.

Carbon capture in soils and vegetation

Charcoal Or Biochar

• Charcoal or Biochar is made by partially burning wood or plant matter like coconut husk. It consists of extremely stable and pure carbon.
• Essentially burning wood partially involves restricting the air flow thereby depriving oxygen supply and thus keeping the temperature low.
• The unburnt charcoal can store carbon for centuries.
• Thus, if we make charcoal from wood and then dig it into the soil, we are sequestering carbon from the atmosphere.
• When mixed with soil biochar can significantly improve fertility of the soil as biochar is highly porous which enables it to retain nutrients and encourages growth of beneficial microfungi.

Carbon sinks

Soil and forests act as the most efficient sinks of carbon from atmosphere.

Soil

• Soils are the storehouse of carbon. It holds twice as much carbon as does atmosphere and about 1 trillion tons more than the world’s plants do.
• Carbon cycle broadly involves movement of carbon in various forms from soil to air to plant and vice-versa.
• The amount of carbon flowing in and out of soils in the natural cycle is about 10-20 times the volume put in the atmosphere by the burning of fossil fuels.
• The effect of global warming is that it speeds up rate of chemical reaction leading to carbon emissions from soil at a faster rate. (above 250 C soil carbon losses are rapid)
• In addition, land-use change has led to degradation of soil.
• Industrial meat requires heavily fed cattle which consume 35% of world’s cereal.
• Growing crops for fuels (biofuel) are yet another source of soil degradation.

Following are the farming practices that help soil restore and retain carbon

Zero-till farming

• Basic principle is that ploughing is counterproductive because it reduces the carbon content in soil. So, disturb the soil as little as possible.
• Under this technique we plant the seeds along with fertilizer in a row.
• As soon as the main crop is harvested, a second crop is planted as a cover for the soil preventing erosion.
• This cover crop acts as manure when they are dead
• This acts as a rich source of organic material that earthworms can use to improve the quality of the lower soil.
• The following year, no fertilizers are used. Instead the seeds are planted through this green manure.
• Besides varying the 2nd crop regularly prevents the accumulation of pests and diseases.

Disadvantage

• The only problem is in the absence of ploughing, weed growth cannot be arrested. (basic purpose of ploughing)
• Thus, either we have to use large amounts of herbicide to control weed growth or grow GM crops.

Forest

• Wood is approximately 50% carbon.
• When forest is lost for wood and this wood is burnt carbon is put in the atmosphere
• Deforestation accounts for more than 15% carbon emissions.
• Wood is largely used as cooking fuel. The solution therefore is alternative cooking fuel.
• Alternative cooking techniques include solar cookers and biogas collectors. This is the reason India has stepped up efforts to produce compressed biogas.

Compressed Biogas

• Input (feedstock) can be any organic waste including agri-waste, rotten human and animal waste that essentially gives off methane (Bio-CNG)
• The recent budget has earmarked Rs 10,000 crore under the GOBAR-Dhan scheme (Galvanizing Organic Bio-Agro Resources- Dhan) for setting up 500 new ‘waste to wealth’ plants.
• Around 200 compressed bio-gas plants and 300 community and cluster-based plants are planned to be set up.
• Bio-CNG, in purified form (98% methane) can be used as both cooking and transportation fuel.
• Only thing is it pressurised to around 250-300 bar.
• SATAT (Sustainable Alternatives Towards Affordable Transportation) also envisages setting up if CBG plants to produce and supply CBG to oil and gas marketing companies.