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Coal, as a source of energy, is firmly out of favour now due to climate change considerations. Most countries, including India, have plans to phase out coal over the next few decades. India, in fact, has officially announced that it would not set up any new coal-fired power plants after 2022.
However, we still have to live with coal for some more time at least. In spite of the rapid increase in power generation from renewable sources like wind and solar, over 60% of India’s electricity is still generated in thermal power plants. And it is estimated that even in the best-case scenario, coal would continue to be the mainstay of India’s energy mix for at least three more decades.
Efforts are on to ensure that pollution emanating from coal is at least reduced a bit in these intervening years. A variety of “clean coal technologies” is being deployed or experimented with to realise the objective. The modern “super-critical” power plants also emit lesser pollutants.
Most thermal power plants burn coal to generate heat, which is used to convert water into steam. The pressure of the steam is then used to move turbines that produce electricity. The quality of coal is an important factor in deciding the efficiency of the plant — the amount of electricity generated per unit of coal burnt — as well as the waste that is released. Typically, coal power plants release a lot of carbon dioxide (CO2) that is a dangerous greenhouse gas.
The varieties of coal found in India have an additional problem. They have high ash content. Burning coal in the conventional pulverised mode results in the release of a lot of fly ash, a major contributor to air pollution and a health hazard. Several techniques have been put in place to capture this fly ash after it is produced, but they are not very efficient. Alternatively, coal is passed through an extensive “pre-processing” process called “washing” to remove some of the ash content before it is burnt, which is also not very effective.
A group of researchers at IIT Madras has now come up with a more effective way of managing this problem. Apart from ensuring that the ash is removed as chunks from the reactor bed itself, their procedure reduces the formation of CO2, and instead generates synthetic gas (syngas), which is a mixture of clean fuel gases like carbon monoxide and hydrogen, as by-products which can then be put to a variety of uses.
The group utilised a well-known coal gasification technique in which coal is only partially burnt with a very limited supply of oxygen in the ‘bubbling fluidized bed gasification reactor’. At about 100 degree celsius, all moisture from the coal is drained out. At higher temperatures, between 300 and 400 degree celsius, gaseous fuels trapped inside coal, like nitrogen, methane and a mixture of many other hydrocarbons, are released. When temperatures reach between 800-900 degree celsius, the carbon in the coal starts reacting with oxygen in the air, as well as steam supplied along with air, to form carbon monoxide (CO), hydrogen and carbon dioxide (CO2). By controlling the amount of air and steam, it can be ensured that significant amounts of carbon monoxide (CO) and hydrogen (H2) are formed. Production of CO2, which is a greenhouse gas, can be minimized. Careful systematic studies have been conducted to arrive at the regime of operation, air to coal and steam to coal ratios. It has been found that the addition of steam becomes favourable in the case of high-ash Indian coals. Therefore, optimized performance can be established in the case of Indian coal by following this operational procedure.
In fact, this technique can be extended to produce syngas of high calorific value by enhancing the oxygen content in the oxidizer, and the H2 to CO ratio can be improved by adding steam in appropriate quantities.
The researchers also showed that adding biomass, like rice husk along with Indian coal imparts catalytic effect and improves the gasification performance significantly.
Vasudevan Raghavan, one of the researchers associated with the experiment, said that the process would improve the attractiveness of Indian coal for use in power plants. Coal is cheaply available in India in very large quantity, but it is not preferred due to the high ash and low energy content. Raghavan added that existing power plants would need to replace their traditional reactors with gasification reactors, and operate them as illustrated by his team to take advantage of this procedure. In Indian coal mine mouths, such gasification reactors can be established to take care of rural power needs.