Advanced Ultra Supercritical (A-USC) thermal power plants are the latest in thermal technology. This futuristic technology has been spurring the growth of next-generation turbines, says Pratosh Saxena.
Coal is the cheapest available fuel and generates more than 60 per cent of the electricity in India and people in the know believe that coal will remain an important part of the nation´s fuel mix to avoid potentially devastating economics consequences. But there are also other people who love to hate coal for its damaging effect on the environment since coal burning emits carbon dioxide & greenhouse gas into the atmosphere at a time when climate change is a significant issue. Thus environmental agencies in India and around the world are enforcing stringent regulations and thermal power plants are fighting for survival like never before.
Advanced Ultra Supercritical (A-USC) development programs serve an important mission to improve the economics of electric power generation while reducing adverse effects on the environment. In India already some progress has been made under national mission under NAPPC for development of 700¦c A-USC and MOU signed on technology collaboration between IGCA/BHEL/NTPC to develop A-USC technology was signed in August 2010 and planning commission approval also received in October 2013. Recently, the Principal Scientific Adviser to government of India Dr. R. Chidambaram chaired a meeting to push the commissioning of India´s first 800 MW A-USC thermal power plant in Kalpakkam by 2017.
Plant Efficiency Improvement The adoption of A-USC steam cycle for thermal plants on a wide scale has the ability to improve overall system efficiency, reduce solid waste, reduce water consumption as well as provide benefits of lower emissions both on land and in air.
The steam cycle of A-USC operates at very high pressure and temperatures and thus takes full advantage of the advanced parameters like higher expansion in turbines, more stages of feed heating and higher input levels in steam generators contributing to higher system efficiencies. Additionally, developers of the future theme for plants are considering large size machines to take full advantage of economies of scale, thus reducing carbon, land, coal & water footprints per MW generated.
Pushing the technology envelope to simultaneously minimise pollutants and fuel consumption through improved plant efficiency is the goal of every utility owner. Efficient approach to achieving these goals is by selecting coal-fired steam generator operating at AUSC steam conditions. At these extremely high pressures and temperatures, a coal-fired power plant can operate with a net plant thermal efficiency over 44 per cent based on the higher heating value of coal. Future development efforts target net plant efficiencies at or above 48 per cent within the next decade. The A-USC steam generator in the DOE/OCDO study is under design for 792 MW gross output. The steam generator conditions are 36.2 MPa, 735.5C/760C with inlet feedwater temperature around 332.8C. Estimated net turbine heat rate is 6,633 kJ/kWh. A net plant efficiency of 44.6 per cent to 45.6 per cent is possible with steam generator fuel efficiency of 89 per cent to 90 per cent and auxiliary power between 6.5 per cent and 7.5 per cent of gross generation. Over the years supercritical technology has evolved and the table describes the same:
Technology Comparison of Supercritical, Ultra-Supercritical and Advanced Ultra-Supercritical Steam Generators A comparison of key features of Super-critical, USC and A-USC is given in Table-2. Most of the A-USC features are the same as for USC with particular exceptions related to the following: Final superheater and reheater tube banks will use materials like 740H and 230 nickel. Steam piping is 740H nickel or better. Minimum circulation flow load is more likely 5 per cent to 10 per cent higher than USC which limits temperature control range.
Smooth or ribbed tubes may be used in a spiral furnace enclosure. Fewer and slightly larger tubes are used, increasing the mass flux and routing the tubes around the furnace periphery to pass through the varying heat flux zones providing more even fluid outlet temperatures. Near the elevation of the furnace arch, transition headers and piping are used for the conversion from spiral wound tubes to vertical smooth tubes in the upper furnace and arch. The superheater, reheater and economiser heating surface arrangement are a combination of radiant and convective heat transfer surface. Pendant radiant platens are supported from the roof. Double reheat cycles would have portions of the second (low pressure) reheater in both down passes and some final outlet pendant surface.
The A-USC steam generator should have the same gas-side temperature profile as a conventional supercritical unit while the steam is at higher temperature. The energy absorption from the steam generator inlet to outlet is lower and the amount of steam flow to be heated is lower for the same power output as compared to a lower efficiency steam cycle. The higher efficiency reduces the fuel input and the combustion products´ gas weight flow, which also means the size of equipment from pulverisers, furnace, flues, air heaters, and environmental systems are smaller. The furnace size should be smaller for the same relative capacity due to lower firing rate.
The convection surface tubing weight will increase due to heat transfer at a lower mean temperature difference between the hot gas and the steam.
Challenges Challenge of selecting the right material and its cost The development of steam generators that are capable of operating at A-USC steam conditions is a real challenge. Major components, such as in-furnace tubing for the waterwalls, superheater/reheater sections, headers, external piping, and other accessories require development and advancements in materials technology to allow outlet steam temperatures to reach 760C.
The US Department of Energy (DOE) and the Ohio Coal Development Office (OCDO) Materials Development Program for A-USC technology includes task categories for conceptual design and economics, material properties testing, steam-side oxidation, fireside corrosion, welding and fabrication techniques, coating development, and testing. Industry experience gained in the materials development program with new and better analysis methods; in finding the proper handling techniques throughout the procurement, fabrication, delivery and operating process; and achieving lengthy materials service exposure time has been essential to the new product introduction. Testing conducted in the program for the thermal coefficients of expansion, hardness, toughness, and other mechanical properties is important to the design and fabrication of materials. In addition, welds and weldments for both thick sections and tubes were tested..
To achieve 760C steam temperatures, longer creep rupture strength testing at higher temperatures is very important. Some of the creep rupture tests have now achieved 30,000 hours..
The steam turbine rotor is one of the largest components of the steam turbine system. The material and design for rotors are unique to each manufacturer. The rotor for A-USC design involves welding nickel-based alloy and ferrite steel to minimise use of expensive nickel-based alloy and due to difficulties in producing a large ingot for mono-block nickel-based alloy rotors. The middle of the rotor is nickel-based alloy and the ends are ferrite steel..
Actual size weld trials are under process between nickel-based alloy TOSIX-II to ferrite steel and welding TOSIX-II to TOSIX-II. Fabrication processes were also tested to acquire knowledge on handling the new alloys in processes such as bending, machining, swaging, and welding. Shop welding practices, particularly with dissimilar metal welds (DMW), were tested in many combinations of product forms and materials. Field welding procedures were evaluated when installing and repairing test sections to determine procedural limitations..
Economic considerations It is important to give the proper and due consideration towards the arrangement/location of the steam generator relative to the steam turbine because of the need to minimise the use of high cost nickel alloy steam piping. One can also explore the possibility of moving the steam generator steam outlet close to the steam turbine inlets by physical repositioning or by using nonconventional arrangements like: New steam generator arrangements have been proposed that primarily change the steam lead terminal point on the steam generator. There has been some consideration for placing the steam generator partially in the ground and /or raising the steam turbine pedestal. Some have also suggested dividing the turbine, for example, the high pressure (HP) and intermediate (IP) sections could be located at the higher elevation and the low pressure (LP) section could be located at the conventional pedestal condenser location..
Designers have also proposed a steam generator design that lays the steam generator height down with a horizontal gas flow arrangement and thus the steam turbine immediately at the side..
Challenge of designing the steam generator for Indian coal Indian coal faces serious techno-economic design considerations, as it is known for its high ash content along with high silica/quartz , resulting in erosive nature of coal, requiring much lower gas velocities passing through the convection tube banks. Indian coal requires special erosion protection provision required on the pressure parts and pulverisers. The impact to the design arrangement and cost is significant. The size of the gas flow area increases about 50 per cent and amount of the heating surface increases due to lower heat transfer rates.
Compared to a steam generator using US eastern bituminous coal, the furnace of a steam generator using Indian coal is about 78 per cent larger in volume and about 50 per cent taller. The furnace width is also about 38 per cent more and thus impacts the length of the nickel alloy super-heater/re-heater outlet headers.
Conclusion Though there exists an ongoing debate both in favour and against these clean technologies these are characterised by higher capital investments and longer pay-back periods, but the most important step is to come up with the design of an A-USC plant that should be high in efficiency/performance and cost affordability/acceptability.
The need for energy, together with the economics of producing and supplying energy to the end user should remain central considerations in power plant investment decision and operating strategies. Inevitably, there will be a point at which technology for higher efficiency and lower emissions come at a cost which can´t be justified. But economic and regulatory bodies should step in and encourage the balance in favour of higher efficiency and lower emissions technology like A-USC.
The author is Head-Proposal Engineering, Tata Projects Ltd
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