Clean Coal Technologies: The way ahead for lowering carbon emissions

The way ahead for lowering carbon emissions

Given coal’s dominance in the power generation segment in India, clean coal technologies (CCTs) hold a key position in the country’s sustainable energy security strategy. Reduction in environmental pollution caused by emissions and efficiency enhancement are the two most important factors which make a case for cost-effective, efficient and environmentally compliant technologies. CCTs help in reducing air emissions and other pollutants from coal-based power generation plants. Initially, the focus of CCTs was on sulphur dioxide and particulate matter, but of late, it has shifted to carbon dioxide (CO2) (due to its impact on global warming) and other pollutants. The application of CCTs gains more importance in the Indian context as the domestic coal supplied to power generation segment is of low quality. The calorific value of Indian coal is low, with the energy content ranging from 2,500 kCal to 5,000 kCal per kg, implying that larger quantities of coal have to be burnt to achieve the same level of generation, thus releasing more pollutants. Therefore, the wide-scale adoption of CCTs has become all the more important for the country.

Government initiatives

One of the most crucial developments has been the revision of the environmental norms by the government in December 2015. The revised norms aim at reducing suspended particulate matter (SPM) emissions to 0.98 kg per MWh, sulphur dioxide (SO2) to 7.3 kg per MWh and nitrogen dioxide (NO2) to 4.8 kg per MWh. The standards have to be met by the various categories of thermal power plants by December 2017. These regulations are expected to increase the focus on CCT-based methods such as coal washing, wet scrubbers, or flue gas desulphurisation systems, electrostatic precipitators and low-nitrogen oxide (NOx) burners, among other things. The draft National Electricity Policy, (NEP), 2016 also lays emphasis on environment-friendly technologies. As per the NEP, the entire coal-based capacity addition during the years 2017-22 and subsequent plan periods shall be through supercritical units. It states that 44 supercritical units of size 660/800 MW with a total capacity of 30,125 MW have been commissioned and 48,085 MW capacity is under construction. With higher steam parameters of supercritical units, the efficiency of these units would be 2-3 per cent higher than the efficiency of the present 500 MW subcritical units. This would lead to corresponding savings in coal consumption and reduction in greenhouse gas (GHG) emissions.

Further, the government has formulated a policy on the automatic transfer of coal linkage (granted to old plants) to new plants in the case of scrapping of old units and replacing them with new higher efficiency supercritical units. The retirement of old and inefficient units of thermal generating stations and replacing them with new and more efficient supercritical units is one of the major initiatives undertaken. Old and inefficient units of a capacity of about 7,751.94 MW have already been retired till March 2017. Further, development of advanced ultra supercritical technology for power plants has been taken as one of the four sub-missions, as part of the National Mission. Under this initiative, a research and development (R&D) project has been taken up to indigenously develop advanced ultra supercritical (AUSC) technology with steam temperature of around 700 °C.

Key clean technologies

CCTs include a variety of technologies to reduce various emissions from the coal industry such as fly ash, particles and gases such as CO2, carbon oxide (CO) and nitrous oxides (NOx).

Supercritical and ultra supercritical technologies

The Majority of the coal-based generation capacity in India is based on pulverised coal combustion technology. The future capacity addition is expected to utilise more efficient pulverised coal combustion technologies, that is, supercritical and ultra-supercritical technology. Supercritical technology is an advanced and commercialised alternative for subcritical technology, and it offers numerous advantages such as much better efficiency in power generation due to higher performance parameters, reduction in auxiliary power consumption, and suitability to various fuels. However, subcritical plants have an important role, especially in the short term. The subcritical technology will continue to be in use, but the performance and efficiency of these plants need to be improved through various renovation and modernisation measures.

Ultra-supercritical technology is still in the development phases in the country. The Indira Gandhi Centre for Atomic Research (IGCAR), NTPC Limited and Bharat Heavy Electricals Limited (BHEL) have signed an MoU for the development of an 800 MW AUSC indigenous demonstration plant. The ultra-supercritical boiler is targeted to have an efficiency improvement of about 10 per cent over the supercritical unit. Further, a 20 per cent reduction in CO2 emission at source, combined with a 20 per cent saving in coal consumption as compared to a subcritical plant, and 11 per cent compared to a supercritical plant are the primary reasons justifying this project. This project is proposed to be set up at an estimated cost of Rs 15.54 billion with a contribution of Rs 2.7 billion from BHEL, Rs 500 million from NTPC, Rs 2.34 billion from the IGCAR, Rs 1 billion from the Department of Science and Technology and the balance Rs 9 billion will be contributed by the Department of Heavy Industry as a grant.

Fluidised bed combustion

Fluidised bed combustion is a well-established method of burning low-grade coal and it allows a greater flexibility to power plants for burning a wide range of coals and other low-grade fuels. Circulating fluidised bed combustion (CFBC) technology is more adaptable to the high ash content in the Indian coal and is suitable for power generation from coal washery rejects, whereas pressurised fluidised bed combustion (PFBC) technology offers higher efficiency. However, PFBC is presently not as commercially mature as CFBC.

Integrated gasification combined cycle

Integrated gasification combined cycle (IGCC) is a combined cycle of steam-based and natural gas-based electricity generation, using coal and natural gas as fuels respectively. This technology has shown capability of power generation at higher efficiency and lower emission levels with respect to pulverised coal combustion technologies as demonstrated in the US, the Netherlands and Spain. The gasification route offers various advantages over the combustion route such as higher overall efficiency due to combined cycle, cleaner power generation due to very low emissions, suitability for carbon capture (pre-combustion) and possibility of poly-generation (producing various chemical products along with power generation).

BHEL pioneered IGCC research in India by setting up a 6.2 MW IGCC pilot plant at its R&D centre in Tiruchirapalli in 1985. It included pressurised fluidised bed gasifier and a 4 MW Mitsubishi gas turbine using cold gas clean up.

NTPC Limited has also proposed to build a 100 MW IGCC plant suitable for Indian coal. The plant is proposed to be set up at its Dadri plant complex. It will be set up in two phases – Phase 1 would entail setting up the gasifier and Phase 2 would entail setting up the combined cycle gas treatment plant. The estimated cost of the project is Rs 150 million per MW. The technical specifications for Phase 1 of the project have been finalised. However, the project has been put on hold as its current techno-economics are not favourable.

Carbon capture and storage

Carbon capture and storage (CCS) technology or CO2 sequestration is the emerging technology for mitigation of CO2 emissions. It consists of capturing CO2 emissions from power plants and then storing them permanently. It also involves transportation of CO2 from power plant location and storage site. There are various technologies for capturing CO2 such as chemical absorption, membrane separation, physical absorption, or cryogenic separation methods. The chemical absorption process is the most developed process but large volumes of absorbents, large equipments and higher costs have limited the applicability of the technology. Physical processes are based on cryogenic cooling or solid absorbents. CO2 capture is an energy-intensive process as it reduces the net power generation from power plants by almost 15 per cent. Thus, it has a very high impact on power generation economics with the cost of power generation being almost double as compared to power plants without CCS. Hence, the applicability of CCS technology has remained restricted till now due to its high cost and lack of pilot installations.

Challenges and the way forward

Development of clean, high efficiency, higher capacity, coal-fired power generation technology is a strategic task. According to the India Energy Security Scenario 2047, an energy projection tool developed by NITI Aayog, intervention of CCTs could lead to a reduction in emission intensity by 38 per cent in 2032, and further to 52 per cent by 2047 at the base level of 2005 emission intensity. Though it is a proven fact that CCTs offer various benefits, the high costs of technology development, inadequate R&D infrastructure, lack of academic–industry interaction and insufficient funding for these technologies are some of the barriers that are hampering their wide-scale adoption by the industry.

Although recent developments in CCTs have been very encouraging, the segment needs to step up to match the global levels. Considering the limited financial resources, a comprehensive roadmap for technologies and its effective implementation would be essential.