The elements of low carbon emission coal fired generation.
There are renewed calls to construct new, base-load, coal fired power plant in north Queensland. The value proposition being that new, high efficiency low emission plant (HELE) will have a lower carbon footprint than the older coal plant being displaced.
In the same way that replacing your old V8 with a hybrid reduces emissions, replacing an old, sub-critical power plant with a new HELE power plant also reduces net emissions. The hybrid still burns petrol, but not as much as the old V8; likewise the HELE power plant burns coal, but not as much as the old sub-critical plant.
The second value proposition is that lower cost power in the north of the State will help improve the cost of metals production in Queensland hopefully trigging additional investment in value add processing to our minerals exports.
It always strikes me as inefficient to move Queensland zinc concentrate to eastern Asia so that it can be smelted with power produced by Queensland coal. In my view it is a more economically and environmentally efficient outcome to have the smelting undertaken here in Queensland where the minerals and the energy are both produced, but that’s a topic for a future blog.
I don’t intend to put forward a political view on what type of power we should have as that is a matter for the policy makers. This blog deals with engineering and is for the purposes of stimulating informed discussion.
That said, if we are going to put in a new base load coal plant in Queensland, let’s make sure that we design it with the highest efficiency and lowest emissions profile that we can.
Power plant efficiency is, very simply, the ratio of the energy that is exported as electricity to the energy that goes into the plant as fuel. In the case of a coal plant, it is the ratio of the electrical energy produced divided by the energy contained in the fuel.
Efficiency matters, because more efficiency means less fuel and therefore less carbon emissions.
The principal factor affecting efficiency is the difference in temperature and pressure between the steam going into the turbine and the steam coming out of the turbine. The higher the temperature and pressure of the steam going in, the more efficient. The lower the temperature and pressure of the steam exiting the turbine, the more efficient.
The essence of a HELE plant is to increase the steam temperature and pressure as far as possible; this is limited by the properties of the materials used in the superheater tubes (the hottest part of the steam cycle just before the steam leaves the boiler), the main steam pipe and the high pressure turbine inlet.
Siemens currently offer steam turbines that can accept steam at 30 MPa and 600 C; Shanghai Electric sell a turbine that accepts steam at 25 MPa and 600 C. This compares with Queensland’s oldest power plant, Gladstone, which operates at 17 MPa and 540 C. Shanghai Electric claim efficiencies of up to 45.4 percent.
HELE plants are also know as ultra-super-critical. Like cooking in a pressure cooker, as you increase the pressure, the boiling temperature raises. Boilers have always operated at higher pressures. Over the passage of time, as material properties have improved, the pressure has increased so that higher efficiencies can be achieved. Sub-critical boilers operate below the ‘critical point’ of 22.1 MPa, where there is a distinct change in phase between water and steam. Super-critical boilers operate above the critical point of 22.1 MPa.
Above the critical point, the steam does not pass through a distinct boiling phase, it simply gets thinner. We have four super-critical plants in Queensland, these being the most efficient and newest of Queensland’s coal fired power plants inlclude: Callide Power Plant (Callide C), Tarong North, Millmerran and Kogan Creek. Millmerran operates at 24.1 MPA and 566 C.
An ultra-super-critical plant does not fundamentally differ from a super-critical plant, it simply operates at temperatures above 600 C.
The availability of a cooling medium dictates the lowest temperature at which steam can exit the turbine. Power plants operate with a vacuum in the condenser that sucks steam through the turbine. The lower the vacuum, the higher the efficiency. The vacuum pressure is dictated by the temperature at which the steam condenses, much like cooking at low temperatures in a vacuum suos-vide.
Availability of water for evaporative cooling or access to sea water cooling has a significant impact on power plant efficiency. Our inland power plants that rely on dry cooling (Millmerran and Kogan Creek) are disadvantaged by their higher condenser temperatures and pressures, when compared with power plants that have access to water, such as Callide, Tarong or Gladstone.
The third major element of cycle efficiency is the reheat cycle. The reheat cycle draws steam from the exit of the high pressure turbine and returns it to the boiler to heat up again prior to it going through the intermediate pressure turbine. This process allows additional heat from the boiler to pass through the turbine without the steam having to be condensed and evaporated again.
The recently decommissioned Hazelwood Power Station in Victoria didn’t have a reheat cycle. This was one of the contributions it its high carbon emissions. We are now seeing the emergence of dual reheat cycles. In this process the steam is sent back to the boiler for a third time between the intermediate pressure turbine and the low pressure turbine.
Feedwater heating also plays a role in improving overall cycle efficiency. The feed water heater draws steam from intermediate stages in the steam turbine and uses this steam to heat the feed water before it enters the boiler’s economiser. This process allows some power to be generated from the superheat in the bled steam prior to it being drawn off, which enables the latent heat released as the bled steam coolsto heat water, rather than being exhausted to the atmosphere in the condensers.
Finally, air heaters are used to transfer heat from the flue gas exiting the boilers into the incoming combustion air. This means less energy from the coal is required to heat the air up to furnace temperature and more of this energy is transferred into the steam cycle rather than being lost up the stack.
Coal quality also plays a vital role. The most important element being the ratio of LHV (lower heating value, or net calorific value) to carbon mass. LHV is the amount of energy released from the fuel after latent heat (the energy lost up the stack as water vapour) is removed. Dry coal provides the most net energy per kilogram of carbon dioxide formed.
Ash content is not significant to carbon dioxide production, as it simply passes through the system, so a dry high ash coal not suitable for export can be a better choice of fuel than a low ash coal with higher moisture content. The use of high ash coal for domestic power production also allows our mines to make better use of these products that are less competitive on the sea-borne market. This is when the cost of transporting ash becomes significant.
I will talk about carbon capture and storage in another blog. A serious carbon capture and storage program has the potential to substantially reduce carbon emissions.
Arche Energy is one of the few consultancies operating in Queensland with significant thermal generation experience spanning development, construction and operations. Arche Energy stands ready to assist in the development of new base load plant in North Queensland.
