Take the example of energy sector: Fossil fuels are the backbone of economy of any nation. To address the challenge of emissions from coal-based power plants, efforts are being mounted to introduce carbon capture and storage (CCS) technology as means of effective mitigation option for stabilising CO2 concentrations, either through reduction of emissions or capture of excess CO2. The captured CO2 is converted to liquid form, making it easier to transport it to desirable location. For storage CO2 can be injected into depleted oil and gas reservoirs. The other possibilities are rocks, which react slowly to mineralisation, deep saline formationsunderground or under the sea bed.
Pre-combustion appears most favourable to remove or minimise CO2 from the fuel before it is combusted. It is being routinely done for natural gas. Coal needs to be, however, gasified. It would be more appropriate to remove CO2 from the syn gas at higher temperatures of gasification so as to reduce overall energy consumption. The CO2 sequestration studies over a range of different materials; rare earths, composites and adsorbents, which can perform at higher temperatures, have been initiated in R&D laboratories.
In the area of coal combustion the pulverised fuel is the most practical technology adopted in 97% of thermal power plants worldwide. Pulverised fuel under super-critical conditions offers improved efficiency. In India plans are underway to introduce new capacity addition using super-critical boilers. Oxy coal combustion is another innovative technology being proposed in may countries to reduce air pollution and increase efficiency of power generation. Burning of coal in 100% oxygen has benefits as well as technical challenges. Rapid progress has been made in the UK, Germany, Canada and Australia in the last two years as it could facilitate low-cost CO2 capture. A 30-mw oxy coal pilot facility has been demonstrated at Vattenfall (Germany) in 2008.
The post-combustion capture technology deals with physical and chemical ways of CO2 separation from flue gas. The challenges include the development of new pressure and volume swing absorption cycles and use of polymeric membranes for achieving low cost regenerative material options. Appropriate materials development, which can withstand required temperature or pressure as the case may be for CO2 emanating from coal gas or natural gas respectively, is the challenge.
Biological alternative of using micro-algae pond to capture CO2 with high productivity from the power plant flue gas appears most cost-effective post-combustion solution. Approx 1.8 kg of CO2 can be fixed for every 1 kg of algae biomass. To isolate algae and genetically improve algal strain for both higher oil content and overall productivity has been the greatest challenge. In this context marine algae could also form a possible solution for Indias thermal power plants located along its long coast line.
Micro-mediated CO2 sequestration using carbonic anhydrase is another option. A proper understanding of enzymes and heterotrophic microbial systems would help in stabilising atmospheric levels through photo-autotrophic and non-photosynthetic CO2 fixation processes. Challenge also lies in biological amplification of soil carbon fixation, requiring enormous data on carbon stock, rate of sequestration and soil emission rate in different forest types for increasing forest cover as means of mitigating climate change.
High CO2 fluxes in oceans and options for storing CO2 on the surface or in deep sea basins exist. Lowest estimate of CO2 that can be stored in the sea floor of ocean basins in super cooled liquid state permanently is no less than 5,000 gigatones. How the marine ecosystem and living resources will be affected in the long run is not known.
Deep under the sea bed is another feasible sequestration option. In Norway North Sea about 13.5 million tonne of CO2 has been injected so far in saline aquifers below the sea bed and is being monitored. Research in these areas as well as in ocean iron fertilisation to increase phytoplankton productivity, which is controversial due to lack of data particularly for the Southern regions, has been initiated in the country.
While post-combustion CO2 sequestration is costly and offers many research challenges, CO2 injection for value addition through enhanced oil recovery or coal bed methane recovery is still in infancy. Very little knowledge base exists in these areas and it is preliminary to talk about a possible business model through injection of CO2 or safe underground storage.
These and many other research priorities exist in CCS. CO2 injection in deep underground formations basalts rocks are expected to provide solid cap rocks and thus high level of integrity for CO2 storage. Basalts react with CO2 and can convert it into mineral carbonates and intertrappean zones between basalt flows are considered to be stable and large reservoirs.
To sum up, it can be said that CCS can become an effective mitigation option of climate change by reducing atmospheric CO2 concentrations, either through reduction of emissions using advanced clean coal technology or capture of excess CO2 for sustainable energy future. The carbon capture technology is yet to be developed and proven as well as made cost-effective and can be deployed only after that. Unlike renewable energy, the CCS technology has not reached the market place, hence research is vital. An exposure to the work being done on coal bed methane recovery at the Central Institute for Mining and Fuel Research, and Geological Survey of India could throw some light into what technological challenges need to be addressed. The CCS research is an area where India can be in the forefront in a global scenario.
The author is former advisor, department of science & technology, and CSIR emeritus scientist. These are her personal views