A final superheater that can raise steam temperatures to 480 °C in grate furnaces. This was the objective of the SteamBoost development project – which is now reality after five years of successful testing and technical advances that provide effective natural gas savings.
“The result is an innovative incineration boiler design that sets new boundaries for steam temperatures and pressures and thus for the waste incineration plant’s electrical generation efficiency”, says Torbjörn Jonsson, Associate Professor at the Department of Energy & Materials at Chalmers University of Technology.
How SteamBoost works
SteamBoost is an extra final superheater installed downstream of the conventional superheaters. Steam is passed through the final superheater, which is located in the rear of the furnace where flue gas temperatures reach 800-900 °C. Conventional superheaters, located in the boiler’s convection section, are able to raise steam temperature to around 440 °C, but by using the new SteamBoost superheater, steam temperature can be raised a further 10 per cent to at least 480 °C. An important part of the development work comprised a pincer strategy to prevent corrosion. This involves devices to guide the highly corrosive flue gases in the furnace to prevent them reaching the back of the furnace where the SteamBoost superheater will be located. Work was also carried out with materials that can cope with the aggressive environment in the furnace.
”SteamBoost has raised our energy efficiency.”
– Lars Mikkelsen, R&D Engineer and Project Manager for SteamBoost at B&W Vølund.
SteamBoost has raised our energy efficiency
Babcock & Wilcox Vølund has been carrying out tests at the AffaldPlus commercial plant in Næstved, Denmark since June 2012. The tests provided successful results that showed how combustion in the furnace can be controlled and how the most corrosive flue gases can be separated to raise the temperature and thus increase energy recovery. The initial tests showed how many watts per square metre of boiler tube surface in the furnace can be transferred to raise steam temperature, and succeeded in simulating a final superheater that can raise the temperature in a waste-incinerating grate furnace.
During Phase II in 2014, the temperature was raised to 480 °C by adding more boiler tube surface – in the form of longer tubes – in the simulation of the extra superheater.
This is where the collaboration with High Temperature Corrosion Centre HTC was initiated and the project began. The research collaboration entails investigating boiler tube materials that can cope with extra high temperatures.
“We had data from earlier tests at B&W Vølund, but based on a different type of calculation. We joined the project partly to understand and work with the material that B&W Vølund had already produced, and also to link it to extensive studies where we looked at the type of deposits formed and how corrosive they are on various materials”, says Torbjörn Jonsson, Associate Professor at the Department of Energy & Materials at Chalmers University of Technology.
New innovative design
The overarching goal is to increase steam temperature and thus efficiency. This will be implemented through two strategies – a new place in the boiler for superheating, and testing a class of materials that is new in this context.
“Previous experience has shown that flue gases and deposits from waste created an extremely corrosive environment, especially above the grate where the temperature is high. We’ve investigated the possibility of combining a favourable location with a new class of materials in order to reduce corrosion and increase efficiency in this type of boiler.”
The result of the collaboration is an entirely new innovative furnace design aimed at increasing steam temperature in which a socalled SteamBoost superheater is installed in the lower part of the grate furnace where there are fewer corrosive flue gases.
“Together with B&W Vølund, we are carrying out full-scale tests in a commercial installation in Denmark in which we have deployed probes in various materials. We’ve performed two rounds of tests with various types of materials and corrosion probes, and looked at how flexible the materials are in terms of temperature.”
HTC is creating a framework with research into the application where findings can be generalised and help companies to find solutions. B&W Vølund has varied the settings on the commercial boiler and HTC’s scientists have studied how corrosion appears at different temperatures.
“The next step for B&W Vølund is to produce a commercial installation and perform full-scale tests over an extended period.”
In terms of economy, environment and the public good, what would you like to emphasise?
“Here in the Nordics, we’re good at biomass combustion and waste incineration thanks to our earlier transition, and we have extensive experience and research in the field, something we’d like to help pass on. And we could also spread the technology globally by demonstrating a different way to generate electricity. Renewable energy derived from waste not only means we get rid of the latter, but also that we generate electricity and raise efficiency. From a wider social perspective, many different energy solutions are necessary to create a sustainable energy system, and it’s here this type of installation is extremely important.”
How will things develop moving forward?
“From a research perspective, there are considerable opportunities for raising the temperature to improve efficiency if we can find a solution where flue gases contain a lot of energy and are less corrosive. Just how cost-effective this will be is another question, but there is enormous potential”, says Torbjörn Jonsson.
SteamBoost – small but efficient
SteamBoost is an entirely new, innovative boiler design. After just over four years’ development and testing of B&W Vølund’s SteamBoost, the superheater is now ready for full-scale tests at the AffaldPlus combined heat and power plant in Næstved, Denmark.
Field tests have already been carried out on the commercial installation, with different materials being tested during SteamBoost trials, where one to two per cent of the steam was passed through the superheater. The SteamBoost superheater absorbs up to 10 times as much heat while being only one-tenth the size of a conventional superheater, which will help ensure good commercial benefit.
“A small superheater is advantageous as it takes up so little space and because we can use more expensive materials in the steam tubes”, says Lars Mikkelsen, R&D Engineer and Project Manager for SteamBoost at B&W Vølund.
B&W Vølund thinks waste-to-energy is one of the best technologies for converting non-renewable waste to energy and into new raw materials. B&W Vølund aims to build the best installation for this purpose, with the highest possible steam parameters, thereby achieving high electrical generation efficiency. In 2017, the company went all the way and passed all of the steam through SteamBoost.
“We’ve made a detailed boiler design. What’s more, we’ve purchased the necessary materials and equipment for the new design for delivery during the autumn.”
The installation at AffaldPlus will take place during the spring of 2018.
Following installation, full-scale testing will continue for three years.
High steam pressure and superheater temperature are of the greatest importance from an efficiency perspective. To achieve an energy efficiency approaching other types of boiler fuels such as fossil fuels and pure biomass, the superheater temperature must be increased from the 425-440 °C range to 480 °C. This is not possible with existing boilers and materials technology due to excessive high temperature corrosion.
“The new SteamBoost concept has shown that steam temperature can be raised while keeping the rate of corrosion low. The new concept can be integrated fully in the boiler, and externally the power plant has the same shape as a conventional energy installation.”
Moving forward, how do you see SteamBoost in terms of the economy, environment and the public good?
“SteamBoost technology has considerable commercial potential in countries that don’t use district heating. Electrical generating efficiency is an important competition parameter in such countries. The commercial aspects in countries that use district heating depend on electricity and heating prices”, says Lars Mikkelsen.
Babcock & Wilcox Vølund AB builds and operates renewable energy installations.