Coal continues to play a significant role in the energy market and indications are that for economic reasons, coal fired generation is set to increase worldwide. With the focus now turning towards the reduction of carbon dioxide emissions, increased efficiency is at the top of the agenda for equipment manufacturers.
Peter Kawa, Alstom, CT, USA
King coal is back with a vengeance and is now predicted to reclaim its traditional place in the energy market, making up some 30 per cent of generation sector output. The main reason for this resurgence is economic, with gas prices much higher than they were several years ago and coal the natural alternative. Coal is also abundant in areas of the world where demand for energy is exploding, but key to this resurgence is the concept of clean coal combustion.
The ability of today’s technologies to achieve near-zero emissions for certain products of coal combustion – namely nitrogen oxide, sulphur dioxide, particulates and mercury – has made coal competetive and increasingly the preferred choice for clean power generation. However, one of the main questions facing the industry now is that of carbon dioxide and its relationship with global warming and climate change.
Wai Gao Qiao Power Station in Shanghai, People’s Republic of China features 2 x 900 MW supercritical units
The power generation industry is currently engaged in research efforts to find ways of effectively capturing CO2 and other industries, particularly the oil sector, is advancing storage technology with the aim of delivering cost-effective techniques as part of a long-term solution to the global warming problem. Several technologies are promising and demonstration projects are under way both in Europe and the United States.
Efficiency: the key technology
Concurrent with these developments, the generation industry is concentrating its research efforts on improving the efficiency of power plants as the single best way to reduce emissions intensity per kWh of energy produced. Seemingly modest improvements in efficiency are significant, since each one per cent increase in absolute efficiency in a coal fired plant can result in as much as a three per cent reduction in CO2 emissions. While coal fired plants are not in the same league as the 59 per cent efficiences achieved by gas fired plants, considerable strides are being made to increase efficiency towards 50 per cent net on a higher heating value (HHV) basis. And, in the last 15 years, efficiencies of coal fired power plants have increased from around 35 per cent to more than 40 per cent on an HHV basis. For example, a sub-critical boiler operating at a pressure of 165 bar and steam temperatures of 538à‚°C will generate electricity at an efficiency of approximately 37 per cent (based on HHV). For such a unit, each MWh of power could generate as much as 0.850 tonnes of CO2. By way of comparison, an ultra-supercritical boiler operating at a pressure of 379 bar and temperatures of 732à‚°C will generate power at an efficiency approaching 48 per cent (based on HHV). For this unit, each MWh of power could generate as little as 0.650 tonnes of CO2, a reduction of more than 23 per cent. Figure 1 shows the evolution of steam parameters and the net effect of technological advances in terms of efficiency over time.
Figure 1 : Evolution of steam parameters
Development of supercritical steam cycles
Since the 1950s the technology of coal fired power plants has advanced considerably. Initially, the major focus of development was on increasing capacity but by the 1970s, owing to environmental concerns, the reduction of primary pollutants such as oxides of nitrogen and sulphur (NOx and SOx) and particulate matter became the priority. In the 1980s and 1990s, fuel resources and global warming were starting to become important issues and today, clean coal technologies must address both the environmental and the economic needs of power generators.
The first generation of supercritical boilers was designed for constant-pressure operation, with supercritical pressures maintained in both furnace wall and superheater over the normal operating range. In the 1970s, sliding pressure designs were introduced to meet the needs for generation units to cycle and load-follow to meet changing grid demands. Modern state-of-the-art sliding pressure designs allow reduction of pressure with load, providing a number of advantages to plant operation and performance, including improved lifespan of all cycle components, from the feedwater system to the high-pressure turbine, and improved steam temperature control and plant heat rate over the load range.
The steam parameters of temperature and pressure to create the optimum performance in terms of efficiency and performance are limited by the available materials used to make key components. Therefore, there has been and will continue to be a strong emphasis on tubing and header material developments for further efficiency increases.
New project inquiries are increasingly specifying supercritical technology with elevated steam temperatures of 600à‚°C/620à‚°C for superheaters and reheaters, and it is anticipated that the proportion of units operating at supercritical cycles will continue to increase. Supercritical steam cycles have been applied in Europe and Asia with increasing frequency in the last decade and the United States has just entered a new cycle of orders for coal capacity. Power generators are also selecting supercritical cycles for a significant proportion of these projects.
Typical of this new generation of plants is Wai Gao Qiao, 2 x 900 MW units in Shanghai, in the People’s Republic of China. The Phase II and the Phase III units currently being built use advanced sliding-pressure tower-type supercritical boiler technology for high cycle efficiency, resulting in both lower fuel costs and lower emissions per MWh. The plant efficiency of Phase II is 42.4 per cent while the steam parameters are 27.56 Mpa and 605à‚°C/613à‚°C.
Equipment suppliers worldwide have been very cautious in increasing the steam conditions above 600à‚°C /620à‚°C for new product offerings, a commercial posture that is hardly surprising given the highly risk-averse attitude of the market. The technical basis for this caution has been largely focused on several key materials issues that have been cited repeatedly as obstacles.
In the broadest terms the issues of greatest concern are primarily the risk of accelerated fireside corrosion in the superheater/reheater tubing, and to a lesser extent the furnace wall tubing, due to the higher temperatures of operation. Accelerated steam-side oxidation at the higher metal temperatures is an additional concern.
Each of these concerns reflects valid questions regarding material behaviour at the temperatures envisioned for the operation of the advanced ultra supercritical boilers at temperatures of some 700à‚°C. In large part, the US and European ultra supercritical programmes have been designed to develop the information necessary to insure that each of the concerns is addressed. Currently in Europe a large-scale component demonstration is being carried out. At the Scholven F plant in Germany, for example, the critical boiler components for a 700à‚°C application have been in operation since August 2005.
From a technical point of view, the main processes involved in the construction and operation of coal fired power plants do not change irrespective of steam conditions and therefore ultra supercritical technology can benefit from experience gained over decades in terms of reliability and availability. The risks related to the introduction of a new technology are consequently minimized to a number of critical components that need to be tested in advance.
Figure 2 : Heat rate improvement against steam conditions
Areas under development include a new chromium steel alloy ‘T23’ which has higher creep strength values for use at high steam temperatures found in the waterwall section of the boiler heat exchanger. Chromium alloys are also being considered for superheater and reheater tubes. For temperatures above 620à‚°C, a range of ‘austenitic’ materials are under consideration, which are more resistant to oxidation and corrosion than traditional alternatives. Meanwhile, development for thick-walled components, such as high-pressure outlet headers and steam piping, have centred around new tungsten alloyed chromium steels with the addition of tungsten improving the creep rupture properties of the materials making them suitable for the high steam parameters found in modern boilers. Figure 2, below, shows the improvement in heat rates as related to improving steam conditions.
Technology is being developed to remove CO2 for subsequent sequestration. Two major approaches involve oxy-firing and flue gas treatment to remove CO2. In the former approach, coal is burned in the presence of an oxygen/ CO2 mixture, rather than air. The subsequent flue gas is very rich in CO2 and can be compressed for subsequent sequestration. In the latter approach, coal is burned in the presence of air and the flue gas is treated to remove CO2, a technology which is already commercial. Sorbents are being further developed to improve the economics of the process. Demonstrations of both oxyfiring and flue gas treatment are planned over the next few years with specific examples including a 5 MW chilled ammonia process to remove CO2 at Pleasant Prairie in the USA and a 30 MW oxy-firing demonstration plant at Schwarze Pump in Germany.