Carbon Dioxide: Capture, Sequestration, Compressor and Power Cycle

Abstract

With an aim to reduce the effect of climate change, it is necessary to reduce the greenhouse gas (GHG) emissions significantly. Energy industry is one of the largest contributors for the same. Even though the usage of energy from renewable sources is increasing rapidly, the dependency for energy on conventional fossil fuels, such as coal or crude oil, will to remain relatively high for following few decades. One of the ways to curb the carbon footprint is implementation of carbon capture and storage (CCS) technology, where carbon dioxide (CO2) is captured from the atmosphere and stored for long-term in an empty gas or oil fields. CCS is an important component of the low-carbon based technologies which may help us meet the reduced CO2 emission targets.

Introduction

I. INTRODUCTION

Global warming and Climate change is caused by increase in the concentrations of greenhouse gases (GHG) produced due to various human activities. One of the important greenhouse gas is carbon dioxide (CO2), of which the concentrations are being increased at a rapid pace due to usage of carbon-based fuels. A large contribution of the increasing CO2 in the atmosphere is caused by the power industries based on the fossil fuel. For the year 2010, electricity and heat generation contributed for  41% of the CO2 emissions around the world. One of the options to reduce CO2 release is through the carbon capture and storage (CCS). CCS consists of capturing and separation of CO2 from different industries and energy-related sources, transporting it to a storage location for prolonged isolation from the atmospheric air. The International Energy Agency (IEA) has estimated that 17% of the reduction as per the requirements can be targeted with implementation of CCS. Hence CCS is an important aspect of the newly developing low-carbon technologies to meet the overall CO2 emission reduction. Adding on to it, the IEA has estimated that without CCS, the total cost for the reduction of the emissions by 50% would increase by 70% by the year 2050.

On an average, the global CO2 emissions have gone up by 1.0% per year since 1990 to 1995 and 1.4% between 1995 and 2001, a rate just below that of the consumption of energy in both the periods. In the individual sectors, there was no increase in CO2 emissions from the industries between 1990 and 1995 (0.9% since 1995 to 2001).  There was an overall increase of 1.7% each year (2.0%) for transport sector, 2.3% each year (2.0%) for construction sector, and decrease by 2.8% per year (1.0%) in the agricultural sector as per IEA, 2003. Total emissions from burning of carbon based fuel consumption and flaring of natural gas was approximately 24 GtCO2 per year i.e. 6.6 GtC each year in 2001 of which the developed countries were contributing for 47% of energy-related emissions . The developing nations accounted for 13% of 2001 CO2 emissions. Developing nations in the Asia-Pacific region have released 25% of the global CO2 while the rest of the developing countries contributed for 13% of the total value.

II. CARBON CAPTURE TECHNOLOGIES

Huge amount of CO2 is emitted during the combustion of natural gas. This amount of Carbon dioxide is either released into the atmosphere or used in plants to manufacture secondary items for example in food processing industry. However, even though small quantity of the carbon dioxide emissions are reused by manufacturing industry, majority of the CO2 eventually ends up being released in the atmosphere. The general goal for the carbon capture and storage technologies is to produce carbon dioxide such that it can be stored or transported. For this, the carbon dioxide should be compressed to the liquid state for it to be transported easily through pipelines and eventually pumped into a geological formation. The carbon compression stage can thus be defined as part of the CCS technology. The technologies utilized for CCS are categorized as pre – combustion, oxy - combustion and post – combustion. These technologies are called so due to the timing when the carbon dioxide is eliminated during fossil fuel combustion .The oxyfuel or oxy – combustion process, is still under the developing stages and will need some more time before it becomes accepted commercially . This technology is used by power plants which is similar in a way to that used by various industrial activities which devoid of burning.

A. Post Combustion

The post combustion process (PCC) absorbs the CO2 generated by the flue miasma following the combustion of fossil commodities or carbon based materials. The highest quantity of the electrical energy used by the world is obtained from power plants through the combustion process. As of today, the main process in coal fired power plants is the combustion of coal fused with air in the boiler or the furnace. It results in an exothermic reaction and the steam released from the process is used to drive a turbine generator. The high temperature flue gas that flow out of the furnace consists of nitrogen from the air and water vapor in small concentrations. The carbon dioxide is also produced from the hydrogen and carbon of the fuel used. Sulfide dioxide, nitrogen oxide and fly ash are also released due to the burning of impurities from the coal. Such toxic gases and few others like mercury should be eliminated as they are considered as pollutants owing to the emission standards. According to the scientists, chemical reaction is described as the best option for capture of CO2 from the flue gas of a pulverized coal plant. A solvent known as mono ethanolamine (MEA) is required to facilitate this chemical reaction process. MEA is a part of the different amine compounds. The flue gas is primarily scrubbed in an absorber. The absorber is used in the capture of approximately 85% to 90% of the CO2 produced. The dissolved CO2 in MEA is injected into a vessel known as the regenerator or the stripper. In this vessel, steam is used for the release of CO2. The Carbon dioxide produced following this process is highly concentrated. The gas is then compressed and conveyed to a location where it can be stored. The solvent used for this process is then forced back and recycled in the absorber. The Post Combustion process is suitable for capturing the CO2 from pulverized coal power plant or a natural gas fired boiler. The coal fired power plants have flue gas with carbon dioxide levels being denser when  compared to the natural gas combined cycle, but with the use amine based solvents and capture systems, it is still possible to obtain higher efficiency .The natural gas is very pure hence the flue gas stream obtained from it is very clean. As a result, there will be no requirement for the cleanup for capturing the CO2 from flue gas.

B. Pre Combustion

The Pre-Combustion technology is based on the separation of carbon dioxide from fossil fuel commodities before the combustion process being started. It can be explained also as a reaction of fuel and O2 gas to produce carbon monoxide, hydrogen, and fuel gas. After the removing of carbon dioxide from this, we can obtain pure hydrogen. Carbon dioxide can be obtained through integrated gasification. The Pre-Combustion Process can also use for power plants that use natural gas. The primary step of the elimination of carbon from fuel is to it to a form which is easier to capture. The reaction between coal, steam and oxygen gas is the phenomenon for power plants powered by coal which occurs at high pressure and temperature. Output of this chemical reaction is a fuel made up of Carbon Mono-oxide and a mixture of hydrogen also known as syngas. Further, this gas goes through a combustion in the power plant to produce power. The power generated by this gas is generally referred as Integrated Gasification Combined Cycle (IGCC) power. In secondary step of this system, the carbon monoxide generated in first step is converted into carbon dioxide through a reaction with steam. This results in the formation of CO2 and hydrogen. Selexol, a glycol-based solvent, is used to capture the carbon dioxide through the chemical process. This leads to production of purified hydrogen stream which can be used in other power plants for producing the power. Comparatively simpler and cheaper separation of CO2 as to the operating pressure is higher and very high concentrations of CO2 with the use of IGCC plants, leads to being most preferred option though it is an expensive process when compared to the normal coal-based combustion plants. The operation for pre- combustion technology is based on physical absorption and then releasing the carbon dioxide as there is drop in sorbent pressure, compared to the use of a chemical to catch the carbon dioxide such as amines as used in post combustion capture process. The use of IGCC includes a few drawbacks such as there is a loss in energy during the CO2 capture due to the shift reactor and other steps during the process.

C. Oxy Combustion

A carbon capture process is the oxy–combustion method which has been a recently developed technology. This process makes use of pure oxygen for the combustion and thus eliminating the quantities of nitrogen. Fly ash is also removed from flue gases, thus resulting to output gas consisting of only CO2, few water droplets and some impurities such as sulfur dioxide. For the removal of water vapor from this, the flue gases are compressed, and the temperature is reduced. As a result, through this process, we are left with pure carbon dioxide which can be stored directly. The key advantage of oxy – combustion over other methods is the avoidance of use of an expensive Carbon dioxide capture system. Replacing the Carbon dioxide capture systems in post combustion process, the oxy combustion incorporates an air separation unit (ASU) which produces extremely clean oxygen with purity of approximately 95% to 99% for oxyfuel process when compared to IGCC plant of the same volume. Significant cost is affected by the Air separation unit. Generally, extra gas transformation is needed to limit the air pollutant concentration so as to meet the appropriate environmental guideline. This results in a reduction of unwanted materials in the flue gas recycle. As the temperature required for burning of pure oxygen is higher than air, oxy combustion process makes use of large portion of the stream as the flue gas is used back in the boiler to maintaining optimal operating temperature. System’s Sealing is one of the important aspects in the design to maintain the required oxygen and nitrogen as found in the gas. Sealing of the system prevents the leakage of air into the flue gas. Hence, during maintenance, it is considered as one of the critical issues as the leakages at the joints and flanges are difficult to eliminate specifically along the flue gas duct. It is necessary to eliminate pollutants from the Oxy-Combustion systems as this may reduces the efficiency of the process to 90%.

III. SEQUESTRATION

Among the several methods of CO2 transport, transportation with the help of pipeline is the most economical considering the huge amounts of CO2 and long distances. Various studies for understanding the pressure drop behavior of supercritical CO2 and the dense carbon dioxide phase through the pipelines have been conducted. From results, we can interpret that the pressure through the pipeline keeps on dropping till the CO2 evaporates and leading to blockage of the pipeline. As a result, we can conclude that there is a maximum safe transport distance. If it is required to transport CO2 beyond the safe limit of transportation, there will be requirement of boosting pump stations along the pipeline. Carbon dioxide can be transported for long distances in two states i.e., either as a supercritical fluid or as a sub-cooled liquid. Due to the lower density and high pressure drops, gas-phase transport is a disadvantage.

Commonly, transportation of carbon dioxide in the sub-cooled liquid state has a few advantages compared to the supercritical state transport, of which the most important is that due of the lower compressibility and comparatively higher density of the liquid within the similar pressure range, there are lower pressure losses and enables the use of pipe of smaller size.

Considering the case of the transportation of fluid through pipeline, it generally occurs under isothermal and adiabatic conditions. For a given  length of a CO2 pipeline segment that is buried and has no insulation, it can be treated as isothermal. In this case, the temperature of CO2 is of the soil that is surrounding the pipeline. Therefore, the transport of CO2, which was started in a supercritical state, goes to a gaseous state process during some point through the pipeline due of the pressure drop. Considering a constant pipe diameter, velocity of carbon dioxide increases along the pipeline. In a few, this may eventually lead in a very high pressure drop or even “choking” conditions.

Among the viable options or method for CO2 storage, the preferred option have been geologic formation especially the depleted oil and gas reservoirs considering environmental risks and uncertainties related to the geologic