|Pilot-Scale Oxy-Fuel Combustion of Victorian Brown Coal|
|By Dr Lian Zhang, ARC Future Fellow, Department of Chemical Engineering, Monash University|
|Oxy-fuel combustion is a process of burning coal in the mixture of high purity oxygen and recirculated flue gas (RFG) to deliver a CO2-rich gas stream that is ready for direct storage/sequestration. It has the potential to be more cost-effective than existing subcritical brown coal-fired boilers coupled with CO2 capture by amine solvents. |
|In Australia, commissioning of the first stage of a pilot-scale 30MW black coal oxy-fuel boiler is under way at the Callide A power station in central Queensland. In the USA, the FutureGen 2.0 project is committed to commencing a 200MW full-scale oxy-fuel plant by 2014-18.|
Monash University has been a leader of research in the oxy-fuel combustion of Victorian brown coal since 2007. Our current project with BCIA involves collaboration with a Chinese boiler manufacturer (Shanghai Boiler Works Co Ltd), three local power generators (TRUenergy Australia and IPHR-GDF SUEZ Australia (Hazelwood and Loy Yang B)), and two international research institutes (Chubu University, Japan and Shanghai Jiao Tong University, China).
|One of the challenges of Victorian brown coal is its high moisture content, typically 60-65% by weight. As a result, brown coal-fired boilers are typically 2-3 times larger than black coal boilers of the same capacity, and are thus more expensive. They also produce more greenhouse gas emissions, for example, ~1300kg-CO2/MWh-electricity sent out, compared to ~900kg-CO2/MWh for Australian black coal-fired power plants. |
Oxy-fuel combustions has great potential to improve the efficiency and cost-effectiveness of power generation from Victorian brown coal. For example, Figure 1 shows that using a high oxygen fraction, such as 30%, with a coal containing 65% moisture, can reduce the flue gas volume by 50%. Such a dramatic reduction helps reduce the boiler size and capital cost.
|Figure 1. Coal and flue gas amounts for a 100 Mwe oxy-fuel boiler as a function of coal moisture content|
|With oxy-fuel combustion, the high moisture content of brown coal can even be an advantage, since the steam produced can function as a diluting agent, and hence, the recycle ratio for flue gas can be as low as 20% to achieve an oxygen content of 30% in inlet gas, see Figure 2. |
|Figure 2. Flue gas recycle ratio as a function of coal moisture content and O2% fraction|
|More importantly, the oxygen-enriched gas atmosphere in an oxy-fuel boiler ensures a more efficient combustion of brown|
|coal, creating efficient, high-temperature supercritical/ ultra-supercritical conditions, which helps to offset the energy penalty of oxygen production.|
We have found that Victorian brown coal burns very efficiently under oxy-fuel conditions. Figure 3 illustrates typical flame propagation results, showing that oxygen concentrations of around 30% result in stable combustion.
|Figure 3. Flame pattern for wet coal (30 wt% moisture) burning in oxy-fuel mode|
|Working with our project partners, we plan to demonstrate the feasibility of oxy-fuel combustion of Victorian brown coal at pilot scale. We have already shipped bulk samples of brown coal from three mines in the Latrobe Valley to our collaborators in China and Japan. Pilot-scale boiler trials will be conducted in a 3MWh boiler at Shanghai Boiler Works (shown in Figure 4) in August/September this year. |
|Figure 4. Photos of the 3MW pilot-scale facility. From (a) to (g) are front view of furnace, bottom four coal-fired burners with sampling /measurement of gases/particle above them, natural gas burners, flame TV supervision device, convective heat exchanger, flue gas recycling pipelines and mixer of O2 and RFG|
|These tests will help us to better understand the combustion characteristics of both dry and wet brown coal in oxy-fuel mode. We will be looking at potential pollutant emissions and their control requirements, as well as ash deposition, fouling and slagging propensity during operation. By working closely with local power generators and a major oxy-fuel boiler manufacturer, our project will support the commercialisation of this low emissions technology in Victoria.|