Oxy-Fuel Combustion

Oxygen fired pulverised coal combustion (Oxy-Fuel), offers a low risk step development of existing pf power generation technology to enable CO2 capture and storage.

Oxy-firing of PF in boilers involves the combustion of pulverised coal in a mixture of oxygen and recirculated flue gas in order to reduce the net volume of flue gases from the process and to substantially increase the concentration of carbon dioxide (CO2) in the flue gases – compared to the normal pulverised coal combustion in air. Oxygen combustion combined with flue gas recycle increases the CO2 concentration of the off-gases from around 15% for pf up to a theoretical 95%. Oxy-combustion is likely to give increased fuel flexibility (same as current pf).

Figure 1 Oxygen Combustion with Recycled Flue Gas (RFG) Red lines show changes to a conventional PF Plant.

Oxy-fuel technology is important to power and coal companies at an international level for the following reasons:

1. The potential for a medium- to long-term, lower cost and lower technology risk, option for achieving near zero emissions from coal-based electricity generation;
2. The potential to retrofit this technology to standard PF technology (sub-critical as well as super/ultra-super critical PF technology).
3. The prospect of applying the technology to new coal-fired plant with significant reductions in the capital and operating cost of flue gas cleaning equipment such as deNOx plant.

There are a number of variants for the proposed oxy-firing of PF boilers, but in simple terms the technology involves modification to familiar PF technology to include oxygen separation; flue gas recycling; and CO2 compression, transport, and storage (Figure 1). Relatively pure oxygen is mixed with a proportion of either wet or dry flue gas taken down stream of the particulate cleaning plant (typically 70% of the total gas flow) and blown into the wind box of the boiler. Primary air to sweep the pulverising mills is substituted with dry flue gas.

The net result of this combustion process is a concentrated stream of CO2, that enables the CO2 to be captured in a more cost effective manner compared to post combustion capture of CO2 from an air-fired boiler.

Oxy-fuel Drivers

The specific reasons for considering oxy-fuel as an option for clean coal technology development are as follows:

1. The existing capacity of PF plant worldwide (old and new) is very substantial, and there are plans for a significant number of new PF plants to be installed around the world.
2. The CO2 capture cost from oxy-fuel is potentially competitive with other emergent technologies.
3. The technical risks associated with oxy-fuel are potentially less than other clean coal technologies because the technology is less complex.
4. In particular countries, the potential for lower capital and operating costs of gas cleaning in oxy-fired PF boilers (deNOx and deSOx) could lead to commercial applications of the technology.

Oxy-fuel Technology Status

The full-scale application of oxy-fuel technology is still under development. However, laboratory and theoretical work has provided an initial understanding of design parameters and operational considerations. In addition there have been a number of investigations using pilot-scale facilities in the US, Europe, Japan, and Canada. Studies have also assessed the feasibility and economics of retrofits and new power plant. Some of the conclusions that can be drawn from the findings to date are as follows:

1. Pilot-scale studies have demonstrated that there are no significant technical barriers to O2/RFG firing of PF boilers
2. Typically, the optimum O2 concentration from the ASU for oxy-fuel applications is around 97 - 98%; and the optimum recirculation rate is generally around 70% which yields about 25 – 30% O2 (vol. %, wet) in the windbox of the boiler, and about 3 - 3.5% O2 (vol. %, wet) at the furnace exit/AH inlet. At these conditions, flame condition and heat transfer characteristics reasonably approximate those for air-fired PF boilers.
3. O2/RFG combustion yields significant reductions in NOx - typically 25 - 50% lower than for the air-fired case.
4. Preliminary cost evaluations indicate CO2 capture costs ($/tCO2 avoided) and electricity costs ($/MWh) comparable with other technologies and lower than conventional PF with amine-based post-combustion capture of CO2.
5. Technical challenges include investigation of flame stability, heat transfer, level of flue gas clean up necessary and acceptable level of nitrogen and other contaminants for CO2 compression, and corrosion due to elevated concentrations of SO2/SO3 and H2O in the flue gas .

CCSD FEASIBILITY STUDY

An oxy-fuel working group was established under the Australian Coal Association COAL21 program and includes the following organisations: CS Energy, Stanwell Corporation, Tarong Energy, Ishikawajima-Harima Heavy Industries (IHI) , IHI Engineering Australia (IEA), Cooperative Research Centre for Coal in Sustainable Development (CCSD), Center for Coal Utilization, Japan (CCUJ), Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Xstrata Coal, Australian Coal Association Research Program (ACARP), & University of Newcastle. The group was formally ratified through the signing of an MOU in September 2004.

The main purpose of the Japan-Australia working group is to carry out a detailed 2 year engineering feasibility study on oxy-fuel technology with three specific deliverables:

1. A detailed assessment of oxy-fuel technology more generally and the specific requirements and costs for a retrofit of a 30 MWe PF boiler at Callide A;
2. A reference design for the Callide A oxy-fuel demonstration project; and
3. A broader evaluation of the potential applications of oxy-fuel technology on a retrofit and purpose designed basis, in Australia, SE Asia and Japan.

The overarching goals of the study are two fold: first, to provide a detailed engineering assessment of the potential application of oxy-firing for electricity generation, and to develop an engineering capability in Australia and overseas to build oxy-fuel plants in the future.

CS Energy’s No. 4 (30 MWe) Unit at Callide A Power Station located near Biloela in Central Queensland (Australia) has been selected as the basis of the study.

The Callide A Oxy-fuel Feasibility Study has been broken into 5 tasks:

• Fundamental Testwork
• Oxygen Production and CO2 Capture
• Boiler retrofit
• CO2 Storage
• Oxy-fuel Applications
  

For more information contact:

Dr Chris Spero
Manager Engineering Technology
CS Energy