Pre-combustion capture technology is a method in which a carbon based fuel is refined before it enters the combustion chamber in order to produce a gas rich in carbon dioxide (CO2) and hydrogen (H2), from which the CO2 is separated and the H2 is used as fuel. First, the fuel is converted into a gaseous state under heat and pressure in the presence of steam. In the combustion chamber, the oxygen supply is controlled such that only a portion of the fuel burns completely. This process produces the heat required to decompose the fuel and produce synthesis gas (syngas) which consist of H2, carbon monoxide (CO), and trace amounts of other gases (see equation A). The CO then reacts with water (H2O) in a water-gas-shift reactor to form CO2 (see equation B). This increases the concentration of CO2 to 40% and H2 to 55%. The higher concentration of CO2 at this point favors separation and capture technologies. The H2 rich syngas is then used as fuel to generate electricity in a combustion turbine (The Energy Lab, 2009).
CxHy(s) + xH2O(g) → xCO(g) + (x+y/2)H2(g)
< 0 kJ/mol, where x and y are variables
CO + H2O → CH4(g) + H2O(g) -206 kJ/mol
CO(g) + H2O(g) → CO2(g) + H2(g) -41 kJ/mol (Nord, 2009)
Exhaust gases of pre-combustion plants are directed to a heat recovery steam generator, which consists of tubes with water flowing through them. The heat from the exhaust gases dissipates into the water, vaporizing it into steam. The steam flows into a turbine to generate more electricity. As it powers the steam turbine, the steam exits into a condenser, which changes it back to water and the whole process is repeated (Metcalf Energy Center, 2009).
Current pre-combustion capture plants capture 91.6% of the average plant's CO2 emissions (Simbeck, 2009a).
- Both the combustion of H2 and the heat recovered in the flue gas via a device called a Heat Recovery Steam Generator are used to generate electricity (The Energy Lab, 2009).
- H2 rich gas may be used to power fuel cells. The Department of Energy is currently investigating this possibility (Massachusetts Institute of Technology (MIT, 2009).
- The high pressure of CO2 makes capture chemically favorable (MIT, 2009).
- Pre-combustion capture may also reduce emissions of certain air pollutants (MIT, 2009).
- Pre-combustion plants emit approximately 101g of CO2 per kWh, whereas plants not implementing pre-combustion capture emit roughly 824g of CO2 per kWh (Nord, 2009).
- Capital costs for pre-combustion plants are higher than for conventional coal plants (The Energy Lab, 2009).
- Existing coal plants may resist conversion to pre-combustion plants (MIT, 2009).
- If any step in the process fails, the plant may have to shut down. Plants may also experience feed problems as well as corrosion due to syngas (MIT, 2009).
- This technology cannot be added to existing plants (retrofitted).
- With this technology, a new plant's efficiency will decrease from 33.6% to 32.0% (Simbeck, 2009a).
- $1,667 million to build an average 500MWe plant (Simbeck, 2009a)