CFB fuel flexibility in practice, at Zabrze and Naantali28 October 2018
Fortum has inaugurated its new 75 MWe/145 MWt multi-fuel CHP plant in Zabrze, Poland, which employs circulating fluidised bed boiler technology supplied by Sumitomo SHI FW under a full turnkey plant construction contract (see MPS, August 2017, pp 28-30).
The new power plant complies with BAT (Best Available Technology) environmental standards and, as well as power (about 730 GWh/y), will provide district heating (approx. 550 GWh/y) to some 70 000 households in Zabrze and in Bytom, southeastern Poland.
The new plant replaces two elderly coal-only CHP units at Zabrze, which has been a power plant site since 1897.
Initially the new plant will run on refuse-derived fuel (RDF) and coal, but it can also use a mixture of these fuels and biomass. The amount of RDF can be up to a half of the total fuel usage. By taking advantage of locally sourced RDF, the facility will contribute to improved waste management in the region.
The Zabrze CHP facility is Fortum’s biggest investment in Poland to date. As well as the power plant, the 200 million euro project s has included a 10 km long heating connection between Zabrze and Bytom and new boilers in both towns.
Good boiler availability at NA4
Meanwhile, the new multifuel boiler at the NA4 power plant of Turun Seudun Energiantuotanto (TSE), Naantali, Finland, provides another demonstration of the fuel flexibility benefits of CFB technology, in particular demonstrating the possibility of using whatever fuel is currently available at the lowest cost.
The new 146 MWe/250 MWt plant, which entered commercial operation in December 2017, has reported 100% boiler availability – although due to turbine issues the boiler has been running at lower load factor (60- 70%) than planned. “The boiler runs very well either with 100% coal or 70% biofuels, and there have been no combustion issues,” says Tapani Bastman, managing director of TSE.
During the first months of commercial operation, the NA4plant has generally been run with a fuel diet consisting of 70% biofuels, 4% peat and 26% coal, with the biofuel being a mixture of various types of forest wood chips, bark and sawdust.
The NA4 plant has shown its multifuel capabilities and flexibility with different fuels and fuel qualities, able to operate with a wide range of fuel and at the same time achieve guaranteed steam parameters.
“The main driver for the investment was that the existing 50 year old pulverised coal fired plant with its three blocks was reaching the end of its life cycle. We had to figure out how to ensure district heat deliveries in the future in an economical, efficient and sustainable way,” says Bastman.
TSE produces district heat and electricity for Turku and Naantali and the surrounding area, as well as process steam for nearby companies. It is owned by Turku Energy, Fortum Power and Heat, and Turku’s neighbouring municipalities of Raisio, Kaarina and Naantali. The company’s 15 km district heating duct is one of the longest in Europe.
Another driver was the desire to combat climate change and replace coal as a fuel with renewable energy sources – mainly local biofuels – in the long run. Additionally, the Finnish government has decided that the use of coal in energy production will be prohibited by law as of 2029.
“For all these reasons, we wanted a multifuel power plant that would feature high fuel flexibility and enable us to burn all kinds of biofuels together with peat or other fuels when it is no longer possible to use coal”, Bastman says. “In Finland, there are changes in energy taxation every time there is a change in government. Market prices swing back and forth based on political decisions, so we cannot rely on one fuel – we need a solution that enables us to change the fuel mix flexibly.”
TSE opted for CFB technology to fulfill its fuel flexibility requirement and chose Valmet to supply the boiler island, as they considered its tender to “have the best ratio of quality to price.”
Valmet delivered a CYMIC boiler with 100% coal firing capacity and a design fuel mixture that also includes wood-based biomass (0–75%), agro-based biomass (0– 15%), peat (0–95%) and solid recycled fuel (0–5%).
“Our target is to run the plant 70% on biomass. In order to run purely on biomass, we would have to make an additional investment. In the meanwhile, we have other interesting fuel alternatives to consider, too,” Bastman notes.
The new 390 MWt CFB boiler is designed to achieve both high efficiency and high reliability, regardless of the demanding fuel mix. High reliability is ensured through low-maintenance boiler components and redundant design in the auxiliary equipment.
The power plant includes a reheat steam system, a two-stage district heat system, and bypass lines to produce heat-only when necessary. The steam parameters were selected to be quite demanding when fuel quality is taken into account: main steam 555°C/160 bar, reheat steam 555°C/40 bar.
Some of the design fuels, such as agro-biomass and SRF, contain quite high amounts of chlorine (0.2–0.4%), representing a risk of high-temperature corrosion in the finishing superheater and finishing reheater, especially as the steam temperature in both superheated and reheated steam is quite high, 555°C, as already mentioned.
In order to mitigate the risk of high-temperature corrosion, both the finishing superheater and reheater surfaces are located inside the bed material in cyclone loop seals. The bed material protects the heat surfaces against corrosive gas components.
The old coal-fired units, NA1 and NA2, are no longer used, while the future of the NA3 coal-fired unit is uncertain. One possibility might be to add a flue gas condenser to NA4 to increase its district heat production, enabling NA3 to be shut down.