Coal-based power generation is the mainstay of power generation in the country, accounting for over 75 per cent of the generation in 2015-16 (provisional). Amidst growing environmental concerns, it is essential to put in place checks and balances to limit the harmful emissions from thermal power plants (TPP), including SO2, NOx, mercury emissions and particulate matter. Clean coal technologies (CCTs) play a vital role in curbing emissions from TPPs. As per the International Energy Agency, CCTs include coal upgrading, efficiency improvements at existing power plants, improved combustion technologies and other near-zero emission technologies. Besides, CCTs also lead to more efficient combustion of coal.
Types of CCTs
Some of the popular CCTs are supercritical boilers, fluidised-bed combustion (FBC), oxy-fuel combustion and integrated gasification combined cycle (IGCC) technologies. Besides this, emissions from TPPs can also be reduced through timely renovation and modernisation and the use of add-on devices/processes.
Supercritical technology
Supercritical boilers operate in high temperature and high pressure conditions, resulting in reduced fuel consumption as well as lower greenhouse gas emissions. Based on the steam conditions in the boiler, the technology is categorised into supercritical, advanced supercritical and ultra-supercritical technologies. In addition to the steam conditions, these boilers have other clean air technologies including impro-ved designs of burners, boiler furnaces and steam super-heaters and gas cleaning systems. Besides, supercritical and ultra-supercritical boilers have a higher capacity than subcritical boilers and, therefore, also lower the number of boiler units required for a given capacity.
The use of supercritical boilers is rapidly gaining traction in India. Adani Power Limited commissioned the first supercritical plant of the country at the 4,620 MW Mundra TPP in Gujarat. The boilers for the plant were imported from China. Currently, there is a lot of focus on increasing the localisation of supercritical technology in the country. In 2015-16, Bharat Heavy Electricals Limited (BHEL) supplied supercritical boilers to various TPPs in the country, including the 1,980 MW Lalitpur super thermal power project and the 1,000 MW Bellary TPP in Karnataka. Further, BHEL has formed a national consortium with NTPC Limited and the Indira Gandhi Centre for Atomic Research for indigenous development of 800 MW advanced ultra-supercritical power plants in the country under the National Mission on Clean Coal (Carbon) Technologies.
FBC
In FBC, coal combustion takes place in a combustion bed created using pressurised air. With strong airflow, the coal bed becomes highly turbulent and rapid mixing of particles occurs. This results in fluidisation of the coal and the formation of bubbles in a boiling liquid. FBC can be used to burn wide varieties of coal, including low-quality coal and waste coal, as well as biomass and other feedstock. Further, in this technology there is in-combustion sulphur removal by mixing crushed limestone/dolomite along with coal as well as lower NOx production due to low combustion temperatures. Another advantage of FBC is that it can be effectively used for combustion of large pieces of coal, thereby resulting in the saving of coal crushing costs.
There are three popular variants of FBC systems – bubbling fluidised-bed combustion (BFBC), circulating fluidised-bed combustion (CFBC) and pressurised fluidised-bed combustion (PFBC). In BFCP, the airflow into the bed is strong enough for fluidisation but not large enough for a continuous outflow of fine particles. In CFBC, the air is blown into the bed with enough pressure to elutriate fine particles out of the bed. One of the key features of CFBC is that it allows power plants to burn a wide range of coal varieties and other fuels (including biomass waste). Meanwhile, in the PFBC system, combustion takes place under high pressure with the underlying combustion process based on either BFBC or CFBC.
The use of CFBC is beginning to increase in the country. In July 2015, Neyveli Lignite Corporation commissioned the 500 MW Thermal Power Station II Expansion Project at Neyveli, the country’s first project of this size deploying CFBC boiler technology.
Other technologies
Another CCT is IGCC wherein coal and steam are used to produce hydrogen and carbon monoxide from coal and these are then burned in a gas turbine with a secondary steam turbine (in a combined cycle power plant) to produce electricity. IGCC entails higher capital costs and is yet to gain prominence in the country. Another CCT is oxy-fuel combustion, which entails burning of coal in pure or enriched oxygen to create flue gas composed primarily of carbon dioxide and water. In the absence of nitrogen in the combustion atmosphere, the only pollutant produced is carbon dioxide, which can be captured by amine scrubbing.
Carbon capture and storage (CCS) is another technology that aims to manage carbon dioxide emissions from plants. CCS entails separating the carbon dioxide from flue gas, transporting it to a storage location and injecting it into suitable underground geological formations (such as depleted oil and gas fields, unmineable coal seams and saline water-bearing reservoir rocks). All forms of CCS require careful preparation and monitoring to avoid environmental damage. The technology also requires a large initial investment.
Add-on technologies
There are some add-on technologies for pollution control that can be deployed along with CCTs. There are three broad categories of add-ons – pre-combustion, in-combustion and post-combustion. One of the popular pre-combustion emission clean-up technologies is coal washing. Use of washed coal improves plant efficiency and reduces emissions. Taking cognisance of the importance of washed coal for emission reduction, the Ministry of Power has mandated Coal India Limited to supply only crushed and washed coal from all it subsidiaries after 2017.
Some of the in-combustion pollution control technologies include use of improved burners, use of limestone for sulphur removal in FBC and gasification, and the use of scrubber technology filters in smoke stacks. Further, post-combustion clean-up technologies include electrostatic precipitators, flue gas desulphurisers, and selective catalytic reducers.
To conclude, the success of a particular technology depends on various site-specific factors such as ambient conditions, temperature and availability of the cooling water as well as technical factors such as greenfield/brownfield plant, size of the plant, fuel properties and load factors. Thus, while it is critical to adopt CCTs to mitigate environmental issues and improve the efficiency of combustion, it is essential to make a judicious choice of technology for optimum gains.