PANORAMA SOCIAL Y POLÍTICO EN EE.UU. (1965-1970)

4.2.2.1 Overview

Oxyfuel plant refers a process in which coal combustion takes place in an environment with high O2

concentration. The oxygen is produced from ambient air using cryogenic processes to separate air into O2

and N2. Apart from flue gas recycling, the process is otherwise very similar to that of an advanced PC plant.

Oxyfuel technology has been used in the glass and metal industries to improve combustion efficiency and because it improves product quality (Anheden et al., 2005). No full scale plant has been built for power

generation as the process is not economically viable; the technology is expensive to build and inefficient when the cost of separating air into oxygen and other constituents is taken into account.

Oxyfuel plant is saved as a rational option for coal fired power generation as CO₂ capture is much easier than from standard PC plant: the volume of flue gas is much smaller, and has a high concentration of CO2

– avoiding the need for a separate capture process. The small volume of flue gas is mainly due to flue gas recirculation, which is required in order to maintain flame stability and a low temperature.

4.2.2.2 Plant process

The thermodynamic principle of an oxyfuel plant is similar to a PC plant: coal is combusted to produce heat to drive a Rankine cycle and hence produce electricity. The main difference is that flue gas is recycled and the coal is burnt in N2 free O2 provided by an air separation unit and combustion products from the flue gas recirculation process. A schematic of an oxyfuel power plant is shown in Figure 4-6.

Air separ ation unit

Boiler HP Turbine Gener ator

Condenser

ESP Chemical

cleaning Air

Nit r ogen

Coal HP steam

Flue gas

Flue gas recir culation

Figure 4-6: Schematic of Oxyfuel combustion process

Firstly the intake air is separated into oxygen and nitrogen in a cryogenic air separation plant. The resulting air pumped into the boiler is 95% pure oxygen (Buhre et al., 2005). Therefore combustion products contain no nitrogen- only CO2, water and trace gases as well as particulates. Particulates are removed by an ESP. Around 60-80% of the flue gas is recycled i.e. mixed with the pure oxygen (Jordal et al., 2005). This is done to maintain temperature and heat flux profiles in the boiler (Buhre et al., 2005) i.e.

control flame temperature and stability in the boiler by creating similar burning conditions to air. There are two types of flue gas recirculation: internal and external. External flue gas recirculation is the current process applied to power generation. Internal flue gas recycling originates in the steel industry and results in the so called oxyfuel flameless regime which has better emissions performance (Villermaux et al., 2009). However, this would need full redesign of the furnace to be applicable to oxyfuel power plants (Villermaux et al., 2009).

WATER

The flue gas that is not re-circulated has high CO2and water vapour concentration, along with other minor constituents. After desulphurisation, the main constituents are CO2and water vapour, which are easily separated.

4.2.2.2.1 Plant efficiency

Section 4.2.1.2.1 discussed improvements that could be made to the Rankine cycle to become more efficient. The same applies to oxyfuel. Therefore it is best to concentrate on the components of oxyfuel plant that differ significantly from standard PC plant, namely; the air separation unit (ASU) and flue gas recirculation.

The ASU separates air into oxygen and nitrogen with high purity. Cryogenic ASU is a widely used technology in other industries and for other processes (e.g. gasification). Air is first filtered, compressed and cooled to remove water vapour before being passed through micro filters to remove CO2 and any remaining water vapour. This process is required as otherwise the CO2 and water would freeze and form a deposit on the inside of the distillation tower. After this has been achieved the gas is cooled to -185^oC.

The air is then sent to the distillation column where any nitrogen leaves from the top of the column and oxygen from the bottom of the column. Cryogenic air separation is a mature technology with patents being filed by Linde in 1895 for cryogenic air separation and 1902 for oxygen production (Linde, (2007)).

Oxygen can be produced at between 95-99% purity, depending on the cost requirement.

As stated at the start of the Section, oxyfuel combustion suffers from cost and efficiency penalties because of the need to install and operate the ASU. The ASU has the biggest impact on reducing overall plant efficiency. Jordal et al (Jordal et al., 2005) state that the ASU may account for up to 20% of the gross power output of the plant, equating to a 6.4% reduction in efficiency (Deutch and Moniz, 2007).

Therefore there is definitely a need for a cheaper and less energy intensive process to produce pure oxygen.

The next substantially different process in the oxyfuel plant is flue gas recirculation, which is used to control flame temperature and stability. It is necessary to reduce flame temperature to reduce thermal NOx

formation (Jordal et al., 2005) and protect boiler materials. There are two different classes of flue gas recirculation: external and internal. Internal recirculation involves the use of high momentum oxygen jets to induce recirculation in the boiler, a method primarily used in the glass and steel industry (Buhre et al., 2005). During external flue gas recirculation, the flue gas is piped back into the boiler after undergoing particulate removal in the ESP. The current method proposed for oxyfuel combustion is external flue gas recirculation.

Figure 4-7: Projected Oxyfuel plant efficiency (data source: (Lako, 2004))

As previously stated, the main reduction in plant efficiency is a result of ASU operation. The standard supercritical oxyfuel cycle is estimated to be around 38.5% efficient with a slight increase in efficiency (3%) due to improvements in boiler efficiency and reduced energy usage in the FGD stage compared to a standard plant but the ASU reduces efficiency by 6.4% (Deutch and Moniz, 2007). Therefore the overall efficiency of an oxyfuel power station is expected to be 35.1%, approximately equivalent to a subcritical PC plant.

4.2.2.2.2 Reliability and O&M

As no oxyfuel power plants have been built for commercial power generation, it is difficult to give an accurate estimate of potential reliability and O&M issues. Two issues enable a forecast of reliability and O&M issues to be made. Firstly, oxyfuel combustion has been used in the glass and steel industries for several years. Secondly, laboratory experiments and test rigs suggest that there should be little difference between PC and oxyfuel plant reliability and O&M issues, although it is noted that for a true assessment of availability full scale demonstration plants are required (Buhre et al., 2005).

Buhre et al (Buhre et al., 2005) report on a study carried out at the Energy and Environmental Research Corporation (EERC) on a 3MW pilot reactor which found no operational difficulties with oxyfuel technology. The issue of high concentrations of sulphur dioxide in the combustion chamber as flue gas is re-circulated before FGD takes place was also a concern, although there is evidence to suggest that oxyfuel combustion certainly produces less NOx, and perhaps even less SO2 than standard PC combustion (Buhre et al., 2005).

Jordal et al (Jordal et al., 2005) state that due to the high concentrations of CO2 in the flue gas, high temperature corrosion is more likely due to greater heat flux to the walls and super heaters. This is due to the thermal properties of CO2(it is also the reason why CO2 is considered a greenhouse gas).

4.2.2.3 Technology deployment

The Swedish utility company Vattenfall are building a 30MW pilot power plant in Germany that was commissioned in 2008 (IEAGHG, 2007) which will be useful in providing technology proof of concept.

In addition, a 40MW demonstration plant has also been opened in July 2009 by Doosan Babcock in Renfrew, Scotland (Babcock, 2009)⁶.

4.2.2.4 Future technology developments

Future technology developments in oxyfuel plant design will focus on a more efficient method of separating air into oxygen and nitrogen because at present such a large loss in efficiency is experienced.

Jordal et al (Jordal et al., 2005) state that the EU funded project, ENCAP, is investigating methods of oxygen separation including: membrane separation, ceramic thermal auto recovery and chemical looping combustion.

Jordel et al suggest that future plant will be subject to process integration so that waste heat can be used more effectively e.g. liquid N2 could be used to cool water or for flue gas condensation (Jordal et al., 2005).

4.2.2.5 Conclusion

The current state of the art oxyfuel plant is cryogenic ASU with flue gas recirculation. Although oxyfuel combustion is not competitive with PC plant without carbon capture, it will be shown in Section 4.3.3 that in the case of CO₂ removal, oxyfuel suffers less performance penalties than PC plant. Therefore when considering carbon capture, oxyfuel is a viable option- competitive with PC in terms of cost of electricity and total plant cost. Due to the lack of commercial deployment, no costs for oxyfuel plant exist.

Operational parameters such as the ability to load-follow are also unreported.

The oxyfuel plant has a number of advantages including improved heat transfer in the boiler, low flue gas volume output and the increase in efficiency that comes about as a result of flue gas recirculation. The main disadvantages are that oxyfuel combustion requires a large quantity of energy to separate oxygen-although this is an ongoing research area, and that the technology is relatively immature (30MW test plant).

Apart from improvements in the air separation process, it appears that improvements of oxyfuel plant efficiency are related to the improved performance of PC plant which itself is dependent on the use of new alloys. Hence, the overall performance of an oxyfuel plant would be expected to follow that of a PC plant albeit with possible step changes due to new methods of oxygen separation.

6 http://www.doosanbabcock.com/live/dynamic/News2ShowArticle.asp?article_id={56FB5815-644D-44B8-95A8-FC7BAC057D61}&cmetemplate=dynamic/pressreleaseslist.tmp