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Carbon Sequestration: A Fresh Perspective
Carbon dioxide capture, transport and storage are the three major components of Carbon dioxide Capture and Sequestration (CCS). Technologies in all the aforesaid components play a vital role. Pradnya P Gune, Senior Technology Manager - Marketing, Aker Powergas Pvt Ltd, Mumbai, writes.

The deployment and management of energy lies at the core of a nation╩s continued growth and development. For this growth to be sustainable it is important that the challenges it poses are addressed effectively. Burning of fossil fuel to cater to a nation╩s energy needs leads to large point emissions of carbon dioxide (CO2). Carbon dioxide being a greenhouse gas is a significant contributor to climate change.

Figure 1, next page, shows forecasted CO2 concentration if we continue with our business practices as usual. Large efforts are required to maintain CO2 concentrations in stable range of 450 to 500 PPM. Thus mitigating the emission of large point anthropogenic (caused by humans) carbon dioxide is one of the major tools to combat the global climate change challenge. In Indian context, coal has been the main stay of electricity generation. It will continue to remain so in the near foreseeable future, though there is every effort being made to deploy renewable sources of energy.

The technological options for reducing Carbon dioxide emissions as recognised by IPCC (Intergovernmental Panel on Climate Change) include:
1. Reducing energy demand by increasing the efficiency of source and/ or user device.
2. Decarbonising energy supplies (either by switching to less carbon intensive fuels e.g. moving from coal to natural gas, and/or by increasing renewable energy sources and/ or nuclear energy).
3. Sequestering Carbon dioxide.
4. Carbon dioxide utilization e.g. dry ice, fire extinguishers, chemicals, polycarbonates etc.
The magnitude of CO2 emission reduction required to stabilise the atmospheric Carbon dioxide concentration calls for the use of a combination of various technologies. Thus, Carbon dioxide Capture and Sequestration (CCS) assumes greater significance as it is a scalable technology and can be used for CO2 capture from large point sources of emission like power plants, cement plants, refineries and iron and steel plants. In India, CCS family of technologies are extremely important due to our reliance on abundant domestic coal sources.

The major components of CCS include Carbon dioxide capture, Carbon dioxide transport, Carbon dioxide storage.

Carbon Dioxide Capture: Carbon dioxide capture technologies are usually distinguished based on following:
Pre-combustion Capture: In this case, all types of fossil fuels can be gasified with less than stoichiometric amount of oxygen and some steam at higher pressures to generate mixture of CO + H2 usually called Synthesis Gas. This is subject to ┬water gas shift reaction╩ in presence of steam to shift CO to CO2 & H2. Carbon dioxide from this CO2 laden gas is captured to leave Hydrogen rich fuel gas. IGCC is an example of pre-combustion carbon capture.

Oxy Fuel Combustion: The fuel is burnt in a mixture of oxygen and recycled flue gases. This generates flue gases that are mainly CO2 and H2O with no nitrogen present in the flue gases. The condensable water vapour can be easily removed from flue gases leaving behind mainly CO2 that can be sequestered.

Post-combustion Capture: Carbon dioxide is captured from flue gases post combustion before the gases are vented to atmosphere. All the above three methods are highly cost intensive and have penalties on thermal efficiencies. The relative economics of each method depends on fuel source, maturity of technology and needs to be studied in detail on a case to case basis. There are some technological challenges faced in implementing these technologies, e.g. high energy requirement for amine regeneration, corrosion and amine degradation issues in case of post combustion, cost effective gasifier designs and gas turbine designs for pre combustion, competitive oxygen plant designs are some such examples. There is also a need to demonstrate the processes at large enough scale and less energy intensive.

Carbon Dioxide Transport
The Carbon dioxide captured is dried, compressed to near supercritical pressure and transported to a safe storage site. It can be transported by using road, rail or pipelines depending on the quantities involved. CO2 transport in pipelines is implemented elsewhere outside India and is relatively proven. Currently, such infrastructure is not available in India.

Carbon Dioxide Storage
Potential options for Carbon dioxide storage include Depleted oil & gas fields, Deep coal seams that are not mine able, Basalt formation, shales etc, Saline aquifers, Enhanced Oil Recovery (EOR), Enhanced Coal Bed Methane (ECBM).

Saline aquifers represent the largest sink for CO2 though it is hard to estimate their storage potential without the detailed geological data. Data shows that storage resource in India is unevenly distributed. Highly suitable storage resources are mainly offshore or in coastal region.

Oil & gas fields in India are relatively smaller in CO2 storage capacity in India. Only a few fields like Bombay High have relatively large storage capacity. However these reservoirs will not be available for CO2 storage till they are depleted. There are opportunities for EOR using CO2 and it is a fairly mature technology.

We are beginning to see some development in EOR in the case of depleting Indian fields. Unmineable coal mines have limited storage opportunity (between 600 m to 1000 m below surface) and are mainly located in Eastern part of West Bengal, Jharkhand, Orissa, and Uttar Pradesh & Chhattisgarh.

In India, major basalt formations are located in west-central part and might have good storage potential. But the storage of carbon dioxide by reaction of aluminosilicate minerals has currently not matured well enough for large scale implementation. For many large point sources in India, the geological storage sites do not appear to be located close by thus adding large cost of CO2 compression and transportation from source to sink. There are many critical factors to take into account in geological storages like deeper understanding of geological characterisation, ground water contamination, leaks to atmosphere and risks to human health.. Smart monitoring techniques need to be employed to get early warning of leakages or loss of containment. There is a lack of knowledge about long term effect of CO2 storage thus bringing to fore liability issues. The provenness and regulatory framework for storage currently does not exist. Public acceptance of long term geological storage of CO2 is equally crucial.

Aker Solutions has a long term commitment to the capture of Carbon dioxide from various industrial sources. It has developed and applied carbon capture technology solutions since the beginning of the 1990╩s. In its quest to commercialize carbon capture technology, Aker Clean Carbon (ACC), which is an Aker Solutions company, was established by a group of in-house expert engineers. ACC continues to remain focused on continual developments in the carbon capture technology through its R & D program and is committed on reducing emissions from greenhouse gases in the future.

Conclusion
Although currently CCS has been recognised by IPCC as one of the important options for climate change mitigation and is considered to be technically feasible on an industrial scale, there are several challenges faced in Indian scenario. Though there is a growing awareness of CCS and the fact that CCS has been included in the UNFCCC╩s CDM, India is yet to recognize CCS as part of National Action Plan for Climate Change. High costs of capture, high penalties on thermal efficiencies in implementing CCS, health and safety issues, high costs of compression and transport, lack of transportation and storage infrastructure, lack of policy framework for storage are some of the hurdles in CCS implementation on commercial scale in India in near future. At the same time there is lot of interest from the industry regarding developments in CCS technologies and new emerging solutions are being closely watched, as industry recognises the need for sustainable growth.