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.
Credit: Indian Express Explained
(http://indianexpress.com/article/explained/new-procedure-to-reduce-pollution-from-coal-fired-power-plants-5147729/)
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| Credit: Greenpeace |
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