1. Background

Coal will remain a major fuel for electricity generation in both the developed and developing world for at least several decades. However, to address the issue of climate change, the power generation industry will be required to reduce its CO₂ emissions by, for example, the improvement of cycle efficiency and by increased use of carbon neutral biomass fuels. Whilst these options will achieve limited reductions in carbon emissions in the relatively short term, the longer term need to move to near “zero emission” coal-fired power plants will require the adoption of carbon capture and storage technologies. These can be categorised into three main approaches:

• Pre-combustion capture by fuel decarbonisation (e.g. via gasification using the water shift reaction to produce H₂ and CO₂)
• Combustion with a nitrogen-free oxidant such as occurs with oxy-pulverised coal firing with flue gas recirculation. The combustion products then consist mainly of CO₂ and water vapour and this simplifies the subsequent capture of the CO₂.
• Post-combustion capture with CO₂ separation from conventional [GLOSS]flue gases[/GLOSS] (e.g. by using amine scrubbing).

All three potential technologies will involve lower overall plant efficiency and increased operating costs. [GLOSS]Oxy-coal[/GLOSS] technology is considered to offer the fastest and least risky path towards commercial CCS [1].

2. Oxy-coal combustion

Oxy-coal combustion involves firing with oxygen instead of air by removing the nitrogen from the air upstream of the boiler using an air separation unit. The flue gases then largely contain CO₂ and water vapour so that CO₂ separation and removal is readily achieved. Firing with oxygen, however, results in very high flame temperatures which can in turn lead to overheating problems with existing materials as well as unacceptable changes in the slagging and corrosion characteristics of the boiler. Consequently, it is necessary to recycle or recirculate a proportion of the relatively cool flue gases to reduce the flame temperatures and hence overcome or reduce the problems associated with oxygen firing. The resultant behaviour of the system depends on the degree of recirculation, the composition of the recycled flue gases ([GLOSS]RFG[/GLOSS]) and also on how they are mixed with the fuel and oxygen.

3. Issues in switching to oxy-coal

Scaling Criteria for Oxy/Coal/RFG Burners

At the time of writing, there are no established and validated rules for burner scaling from air/coal to oxy/coal operation, especially when both the burner operation per se, and its impact on boiler performance must be addressed quantitatively. Data obtained in small scale experiments cannot yet be applied with confidence to medium or full scale systems.

Optimal RFG Rates

Switching from air operation to oxy/RFG operation affects the thermal performance of boilers and related operational factors. Optimal RFG configurations will ensure reliable, safe and efficient boiler operation with respect to factors such as the allowable heat transfer and the constraints imposed by the corrosion resistance of existing and likely future materials. It may be necessary to use very high recycling rates of up to 80% to obtain similar combustion and heat transfer characteristics as for conventional air combustion when retrofitting oxy-coal on to existing boilers.

Flame Instability

Under the high recycle rates needed to optimise thermal performance when using oxy/coal/RFG, flames on burners designed to fire with air can become unstable.

Gas-Side Corrosion

The almost complete absence of nitrogen in the combustion gases is compensated by much higher concentrations of CO₂ and water vapour, whilst recycling the flue gases also increases the concentrations of SO₂/SO₃ and HCl in the combustion atmosphere by a factor of 3 to 5. Corrosion of the radiative and convective sections of the boiler is therefore more likely than with conventional pulverised firing systems.

Ash Deposition and Slagging

Oxy/fuel combustion atmospheres can affect the mineral transformations in the coal and hence the composition and properties such as the melting point and viscosity of ash deposits. These can affect the stability of the deposits and hence their “stickability” and “cleanability”.

Exhaust Gas Cleaning

The changes in composition of the boiler exhaust under RFG conditions can substantially affect the performance and costs of the downstream gas cleaning equipment.

Safety

Switching from air to oxygen/RFG mixtures will impact on the risk of explosions in parts of the plant that contain mixtures of coal/oxygen/RFG.

Sources

[1] Lars Stromberg, “Vattenfall Oxy-Fuel Activity”. Keynote Paper 1,1st Oxy-Fuel Combustion Conference, Cottbus, Germany, September, 2009

Acknowledgements

The preparation of this Combustion File was supported by RELCOM; Reliable and Efficient Combustion of Oxygen/Coal/Recycled Flue Gas Mixtures – a project undertaken with the financial support of the European Commission under FP7 Grant Agreement Number 268191.