Carbon Dioxide (CO2) is considered the most significant of the green house gases with emissions estimated at 6Gtc/Year from the combustion of fossil fuels alone (Freund and Ormerod 1997). Atmospheric concentration of this gas has increased from 280 to 380 ppmv (parts per million by volume) which represents a 35% change since pre-industrial times (IPPC 2007). The global warming phenomenon caused by the concentration of these green house gases has led to an increase in atmospheric temperature, rise in oceans levels, melting of ice caps and a more frequent occurrence of natural disasters. With the publication of several assessment reports on climate change, it is now understood that carbon emissions from fossil fuel combustion, industrial processes and land use change have lead to an increase in atmospheric carbon dioxide concentrations ( Intergovernmental Panel on Climate Change (IPCC) 2007). During the last 150 years land use and land use changes were believed to be responsible for about one third of all human emissions of CO2. These emissions are also believed to be responsible for the acidification of oceans. Other reports have also made projections of future economic costs of adaptation and mitigation which have shown the cost effectiveness of acting early to reduce carbon dioxide emissions and subsequently atmospheric carbon dioxide (Stern 2006).
There have been global calls for governments and regulatory bodies to put up regulations that make CO2 reduction targets legally binding. This is based on the recognition of the fact that mitigation measures have to be put up to combat climate change. Efforts to mitigate the green house gas problem have traditionally focused on avoiding the production of carbon dioxide by reducing the use of fossil fuels (referred to as 'CO2 abatement') (Nordhaus 1992). The climate change committee of the United Kingdom for instance has called for an 80% reduction in the concentration of green house gases in the atmosphere by 2050. For these targets to be achieved there will be need for the development of an alternative energy sources or the use of low carbon fuels will have to be explored. This is a global problem which needs urgent attention as recently published research results show that the growth rate of CO2 emissions between 2005 an 2006 exceeds the worst case 2001 IPCC scenario (Raupach et all 2007).
As a result of this, power supply technologies that have adopted carbon capture and storage as part of their processes do in fact contribute to the reduction of carbon dioxide emissions. The combination of this with other methods of carbon capture and sequestration will lead to reduced emissions.
Carbon capture and storage (CCS) has been endorsed by professional bodies and governments alike as a major mitigation option for the reduction of CO2 emissions from stationery sources such as fossil fuelled power stations (IPCC 2005).
Carbon sequestration has been identified as a long term method of mitigating the effects of fossil fuel consumption. Before sequestration, carbon is collected in form of gaseous CO2. After collection, sequestration then begins. This refers to the geo engineering technique for the long term storage of co2 or other forms of carbon for the mitigation of climate change usually done through biological, chemical or physical processes.
It has also been defined as the uptake and storage of carbon dioxide by one of several mechanisms to reduce the concentration of the gas in the atmosphere and its contributions to global warming. Carbon may be stored in living (vegetation and forests) or non living (soils, geologic formations, oceans and wood) reservoirs.
There are several approaches to the process of carbon dioxide sequestration. All of these approaches fall into one of either the terrestrial, geologic or ocean storage methods. It must be noted however that carbon sequestration involves the capture, storage or even usage of the gas. Sometimes the process also involves transportation of the gas for onward storage. Capturing the gas from the atmosphere is assumed to be a more expensive venture because of its low concentration compared to other gases like Nitrogen and oxygen.
As result of this, most proposals seek to combine CO2 capture with power generation. Some of these approaches include:
Industrial use of co2 in plastics and chemical industries Sequestration by wood burial Biological conversion to fuel Geological sequestration in coal beds Injection into active oil wells Injection into exhausted gas or oil wells Ocean disposal Injection into aquifers This paper aims to describe and evaluate three major techniques of carbon dioxide sequestration suitable for power plants. The techniques to be examined include sequestration of carbon dioxide in wood, gas and coal beds.
CARBON SEQUESTRATION VIA WOOD BURIAL
This method of carbon dioxide sequestration is as a result of the observation that natural forests are typically littered with dead trees. Studies have shown that organic substances like wood in landfills decompose at very slow rates (Micales and Skog 1997). With this, it has become clear that wood harvesting and burial could be a viable method of carbon sequestration. Land vegetation helps to sequester large amounts of CO2 the world over. The stored carbon continuously returns to the atmosphere when vegetation dies and decomposes. This method basically involves collection of dead trees on the forest floor and the selective logging of live trees. The trunks of the dead trees are then buried in dug trenches on the forest floor or suitable landfills. Logs can also be piled up above the ground sheltered away from rain. In this way, the buried wood has much longer residence time and it transfers carbon from a fast pool of decomposition of about 10 years, to a much slower pool of about 100-1000 years. In the case of dead trees, they reduce heterotrophic respiration, and are thus immediately effective as a carbon sink. In the case of selective logging of live trees, the following re-growth in the spaces left by cut trees can act as carbon sinks which depends on the rate of re-growth. In practice, these two methods do not defer much because fallen trees leave spaces or gaps for smaller trees to grow. This is quite similar to selective logging of live trees. It must be noted here that fine wood such as branches and twigs decompose faster, occupy more burial space and are more costly to clean up. As a result of this only coarse wood is considered suitable for burial.
In quantifying the potential size of this carbon sink a model based on observed climatology with seasonal precipitation, temperature sunshine, wind speed and vapour pressure can be used. A simulation can be run until there is convergence where tree growth is balanced by mortality. This model is referred to as the global dynamic vegetation and terrestrial carbon model VEGAS (Zeng 2003). This model simulated the full terrestrial carbon cycle.
This translates to the fact the higher the death rate of coarse wood the larger the carbon sink it can provide. The tropical forests like the Amazon and Congo Basins provide the highest rate followed by the temperate and boreal forests. The tropical forests have a carbon sequestration potential of 4.2 GtC y-1, the temperate forest has 3.7 GtC y-1 while the boreal region has 2.1 GtC y-1 potential. The model only considers potential vegetation and no agriculture.
On the implementation and cost of this method, the major requirements will be a good network of roads and paths that would allow machine access to these forests and trenches evenly distributed around the forest. The trench sizes will need to balance several factors such as cost of trench digging and the transportation of dead wood, minimizing forest disturbance and selecting sites that prevent decomposition. Transportation may be needed in area with shallow soils while on site burial is encouraged in order to eliminate transport costs. The technology for selectively cutting trees is low tech and has been around for years. The cost of this method will even be lower where forest management is already in place i.e. where machinery and roads are in place but the price will increase as the total area of forests increases. In making use of machinery however, CO2 is further emitted. These factors need further evaluation if this method is to assume global status.
CARBON SEQUESTRATION USING COAL BEDS
CO2 Long term storage in coal beds may help to reduce the accumulation of the gas in the atmosphere. Injecting CO2 into gassy coal beds leads to the production of coal bed methane (CMB) thereby reducing cost of sequestration (White et all 2003). Crucial to this method is the safety of CO2 storage in coal bed basins which means that there is low risk of leakage or seepage back into the atmosphere. Basically, there is need to have an understanding of coal-gas interactions and their effects on CO2 in terms of structural deformation and transportation in coal seams. In predicting long term effects of sequestration in coal beds, a number of processes such as gas adsorption on coal surfaces, replacement of coal bed methane with CO2, factors that might induce gas release and factors that might affect the storage capacity of this carbon sink are of utmost importance. Reliable estimates of the retention capacity of coal beds are required for economic assessments of the viability of this method (Bromhal et all 2004).
Recent proposals that the injection of CO2 into coal beds provide a viable option in mitigating the increase of global emissions of this gas have lead to interests in having a better understanding of coal-CO2 interactions and the adsorption capacity of coal seams for CO2 sequestration (EPA 2005). Particularly, the need to determine gas in place concentrations and their impacts on safety issues is very important.
In order to ensure optimum relationship between cost of sequestration and efficiency of storage for a coal reservoir, simulations of gas-coal interactions need to be done. This should take into consideration geological properties such as coal rank, pressure, permeability and porosity of geology, physical and chemical processes, fluid flow and structural interaction as well as changes in phases (Leneveu 2008). The original balance of the sink will change with CO2 injection and this reduces the partial pressure of water down by driving it out. This leads to a decrease in water pressure and also to methane (CH4) desorption form coal surfaces. The structural deformation and variable saturation models have been developed for this. These models have been useful in providing insight s on how to avoid carbon leakage and seepage by effective stress monitoring and coal bed basin displacement during injection. The candidate coal bed may be used for depositing CO2 if the deformation can be kept within safe limits for a long term perspective. In future the validation of this structural deformation model will be needed and it will also include changes in transport phases.
CARBON DIOXIDE SEQUESTRATION USING GAS WELLS
Depleted natural gas fields are attractive targets for carbon dioxide sequestration by direct injection. This is due to the fact that they have a proven record of gas recovery, integrity against leakage, existing wells and pipelines and history of land use in gas production and transportation. The international Energy Agency (IEA) has estimated that as much as 140 GtC could be sequestered in these depleted natural gas fields (IEA 1997). Prior studies have shown that additional methane (CH4) can be recovered from depleted gas fields by CO2 injection (Van Der Bugt et all 1992).
The general process involves the injection of CO2 into producing wells at some distance while taking advantage of reservoir repressurization to produce more methane. The production of methane can be used to augment the cost of CO2 injection. This process has been termed Carbon Sequestration with Enhanced Gas Recovery (CSEGR). It basically involves injecting CO2 into the wells at known temperature and pressure levels (Vargaftif et all 1996). CO2 will be superficial once injected into the formation due to relatively high pipeline pressure. Strong cooling occurs due to flashing of supercritical liquid like CO2 to gas. Joule Thompson cooling also occurs as CO2 gas expands in the reservoir as a result of low pressure. The formation is dried by the injected gas and this is another heat consuming process. Since the formation residual gas and liquid are at high temperatures, heat is available for the expanding gas. Eventually the temperature around the well may become low leading to hydrate formation and associated decreases in injectivity. Pure carbon dioxide hydrate can form at 0 degrees Celsius and 20 bars pressure (Haneda et all 2000).
Simulations of CO2 injection into depleted gas fields have been carried out but field testing of the process to validate simulation results are yet to be done. This should be the next step in the process. The process consists of many steps which are subject to theoretical and practical limits alike. The more prominent of these limitations appear in terms of reservoir processes as opposed to gas transportation, CO2 availability, land use and economic and policy factors.
The development of economic, technical, environmental feasibility and acceptability of sequestration techniques has crucial implications for meeting the demands for energy, food and fibre while reducing the emissions of green house gases (GHG). As the world energy use still revolves around fossil fuels the practicability of sequestration could provide options for future near zero GHG emissions. Compared to other alternatives, carbon sequestration has the potential to reduce the cost of bringing down concentrations of green house gases in the atmosphere. If successful, it can further support domestic and global economic growth.
Should sequestration prove technically and economically viable, fossil fuels may continue to play vital roles in energy supply. As a result of this, the long term potential of advanced technologies for carbon sequestration are crucial both in emission reduction and reducing cost for achieving those reductions.