Results of Basic Study On Oxy-Fuel Combustion System for CO2 Capture

1. Introduction

One of the main topics at Hokkaido Toyako summit held in Japan, July, 2008, was “Environment and Climate Change”. As the long-tern Goal, the G8 leaders agreed to seek to share and adopt it with all Parties to the United Nations Framework Convention on Climate Change, with respect to the goal of achieving at least 50% reduction of global emissions by 2050. Concerning the global warming, Kyoto Protocol entered into force in February 2005, and the intension of Japanese government towards a low-carbon society, that means CO2 emission reduction of 60 – 80 % as compared with the present amount of CO2 emission by 2050, was expressed in the Fukuda vision. From a global viewpoint, however, thermal power plants are still releasing CO2 in large quantity at present, which indicates the necessity for a power generation system with Carbon Dioxide Capture and Storage (CCS) in more effective and economical manner.

Among all the fossil fuels used at thermal power plants, coal produces the greatest amount of CO2 per unit calorific value. Thus, emissions from power plants using pulverized coal seem to be one of the significant sources in capturing CO2 effectively.

In the process of capturing CO2 through oxy-fuel combustion, O2 is separated from combustion air and used for burning the coal. In this process, it is theoretically possible to improve the CO2 concentration in the flue gas up to 90 % or over, by simply capturing CO2 from the flue gas. When the oxy-fuel combustion technology is applied to power plants, flue gas, which mostly consist of CO2, are re-circulated and mixed with O2 before combustion for the purpose of controlling the flame temperature. With this system, it has been confirmed that the process characteristics help to reduce NOx emission. But there are some development issues, including the necessity of saving motive power for oxygen production and the necessary integration among the units for oxygen production, power generation and CO2 capture process. In addition to the above, some studies towards the realization of oxyfuel combustion system would be necessary, including the mixing performance of O2 with recycled flue gas, a behavior of the fly ash in the recycled flue gas and etc.

In this paper, study results regarding the oxyfuel combustion system are introduced.

2. Results of basic study towards the realization of oxyfuel combustion technology

For the purpose of the realization of the oxyfuel combustion system, the various activities have been studied and reviewed. In this section, study results on this field are outlined.

2.1 Behaviour of fly ash in the high oxygen concentration atmosphere

In the oxyfuel combustion system, oxygen is mixed with recycled flue gas before burning the coal. It is concerned that recycled flue gas includes the fine fly ash at the simple system without desulphurization facilities in some areas and the fly ash exist in the flue gas with high temperature and high O2 concentration locally in the process of mixing. Therefore, the behaviour of fly ash in the atmosphere with high temperature and high O2 concentration depending on the carbon content in fly ash is tried to confirm.

In the test, O2 concentration in the supply gas and carbon content in the sample by mixing the powdery activated carbon with fly ash are adjusted. The fly ash sample is charged into the small cell which is approximately 1cm3, and set in the glass tube installed in the electric furnace. Gas temperatures around the sample cell are monitored and gas temperature is increased controlling the rate of 3 degree C per a minute up to the setting temperature. The supply gas is entered from the bottom of the glass tube and heated at the lower section of the tube filled with the ceramic balls by electric furnace. A sample cell is setting at the upper section of the tube and the test gas heated at the lower section is flow around the cell up to the top of the glass tube.

Test results show the characteristics of ignition and weight decrease depending on the O2 concentration and carbon content in the sample and are classified into 3 types of results. First is the area of ignition. Temperature of the sample is rapidly increased at a certain temperature. The sample is rapidly oxidized with red heat. Second is the area of no ignition with weight decrease. The sample is slowly oxidized without apparent temperature increase, however the sample weight decrease is detected. Third is the un-reacted area. Apparent temperature increase and weight decrease of the sample is not detected because oxidation rate of the sample is very slow.

2.2 Mixing of O2 with recycled flue gas

In the oxyfuel combustion system, oxygen is mixed with recycled flue gas before the combustion in the furnace because of keeping the flame temperature. In the normal oxyfuel process, oxygen is mixed with recycled flue gas at the outlet of air-heater which is upstream of burner wind-box with considering of avoidance the oxygen leakage at the air-heater. This means that mixing area is only the duct to the burner after the oxygen injection point. If there is the higher distribution gradient of O2 concentration at the burner wind-box, flame temperature and combustion characteristics are different among the burners. The higher distribution gradient like this should be avoided in order to maintain the safety condition and the stable operation of the boiler. In addition, considering the lifetime and the safety for the duct, the O2 concentration with higher temperature region should be as low as possible on the duct wall, as described in the above section. Therefore, mixing simulation study was performed to confirm the distribution gradient of O2 concentration at the burner inlet.

Temperature of recycled flue gas and O2 are approximately 300 degree C and normal temperature for each. Flow rate of O2 is a quarter of flow rate of recycled flue gas and thermo-fluid analysis is performed using STAR-CD.

From the mixing simulation results, the appropriate O2 injection nozzle was finally obtained. In this paper, results of 2 types of the O2 injection nozzle are introduced. One is the normal (unconsiderable structure and direction) injection nozzle type and another is the final improved injection nozzle type obtained by evaluating the results of any types of nozzles.

3. Conclusion

These results obtained in this study will be reflected with the design of the demonstration plant for Callide Oxyfuel Project and the mixing simulation like this will also be put into practical use at the commercialization stage of oxyfuel combustion system.

Oxy-fuel combustion technology has been attracting and increase attention worldwide because it has a potentiality for providing a breakthrough solution that helps reduce CO2 emissions. In relation to this technology, many countries are conducting research and development towards the realization of this technology which is one candidate of the CO2 capture from the power plant.

We would like to have much knowledge and information through the studies including above and the demonstration operation at Callide-A, and contribute to reduce CO2 from the pulverized-coal fired power plant using oxyfuel technology.