“The world’s largest boiler for waste incineration”
The new Block 6 boiler in Västerås, Sweden is a multi-fuel unit that can use both waste and biomass. The SEK 2.85 billion (≈ EUR 310 million) investment by the municipality-owned energy utility Mälarenergi seeks to provide inexpensive and climate-friendly heat and electricity to its clients. The odour problems that plagued local residents last summer are now well on their way to being solved.
– It started in 2008 when I began working for Mälarenergi as project manager, said Magnus Eriksson, now in charge of plant development at the company. Back then we worked to determine the status of the existing facilities. For a long time the company had seen that there was a need to renew the facility. Mälarenergi has four goals; renovate and modernise the facilities, maintain low heating cost rates to clients, spread the fuel risk and reduce carbon dioxide (CO2) emissions. With the new Block 6 facilities, Mälarenergi has halved its CO2 emissions.
– We have gone from 600 000 tonnes of CO2 down to about 250 000 tonnes per year, said Magnus Eriksson.
Co-combustion better than gasification
In 2008, Mälarenergi had a fuel mix with a relatively large amount of fossil carbon. Revenues from electricity certificates for Boiler 4 and Boiler 5 was due to end in 2013. Electricity certificates are part of a national Swedish premium scheme for generators of renewable electricity. According to Eriksson there were two factors that spurred interest in making an investment into a new baseload boiler with waste as fuel.
– We first looked at the possibility of building a gasification plant for waste-to-energy gas production, feeding the gas to the existing Boiler 4. On paper it looked a better option compared to building a brand new circulating fluidised bed (CFB) or grate boiler. It was even more profitable than building a biomass boiler as baseload. But gradually we realised that Boiler 4 was not quite up to standards, said Magnus.
It turned out Boiler 4 was more worn than expected. It would require high maintenance costs and it looked like it would be hard to cope with the steam data needed.
– We got a quote on a gasification plant, but we found no commercial terms in the offer. At the time there was a similar facility in Lahti, Finland that first had to be commissioned and test run. The combination of Boiler 4 being in poor condition and the lack of commercial conditions for a gasification solution meant that we did a reboot and concluded that co-incineration was the best option. This is the solution that has been completed, said Magnus.
CFB for steam data
Contracts for each of the major process components were signed in December 2011.
– We opted for a CFB boiler to handle the fact that we wanted a single boiler. If we had chosen a grate boiler we would have had to build two. The Valmet CFB boiler has a fuel output of 167 MW and is the world’s largest boiler for waste incineration. Grate boilers are not built at that size, explained Simon Jansson, Project Manager for the Block 6 renovation project.
The boiler can receive as much as 60 tonnes of waste per hour. One advantage of CFB technology over grate is that it enables a higher electric output because the plant can deliver higher steam data. Block 6 can supply steam at 470 °C and 75 bar pressure.
– Grate boilers are typically designed for up to 425 °C. The difference is due to the end reheaters sitting in the sand lock on the CFB boiler, which means that they are protected from corrosion. The superheater located in flue gas comes up to about 380 °C. That compares with the grate boiler corrosion situation where the superheater in the flue operates at a temperature of around 425 °C, said Jansson.
This means that less advanced materials in the superheater are needed, resulting in lower investment and maintenance costs, while also getting better steam data enabling higher power output compared to grate boilers.
Adapted for service
The boiler was supplied by the Swedish arm of Valmet, with a large part of the engineering work conducted by Valmet in Tampere, Finland. It is a traditional CFB boiler with a lot of masonry.
– It is suited for service in that we have an overhead crane in place so that we can lift out the superheater elements when they need replacing in the future. We expect that the hottest superheaters will hold at least two years, but hopefully four to five years. They should be able to be changed very quickly, explained Simon Jansson.
Two important elements of a waste-fired boiler are the fuel feeding and ash removal, as large volumes and difficult fractions of ash need to move through the system.
– The boiler also has a secondary input for biomass. It’s a bit unusual to have separate entries for waste and biomass. There are four lines of waste input and two lines for biomass feed, which provides increased reliability. We are very happy with the fuel infeed. One challenge is getting improved ash removal. But in general we are very happy, said Simon Jansson.
Unique fuel preparation
The waste fuel used is sorted household and industrial waste. The proportion delivered by truck from the local area will increase gradually.
– Today, 80 percent is imported via boat. There will be about 100 boats per year. But the goal is that 50 percent will come locally by truck. In total we have permission to receive 540 000 tonnes. But in practice, we estimate that there will be about 400 000 tonnes per year, says Magnus Eriksson.
The waste fuel arrives at the CHP packed in bales that can be stored temporarily in a large cooled warehouse. Mälarenergi bought an existing industrial building and turned it into a fuel stock. Next to the fuel store is a unique fuel processing plant supplied by Finnish company BMH. Here the waste is put into a receiving bunker, then crushed and passed through three separators before it is ready to be fed into the boiler. There are three parallel processing lines that work independently of each other.
– We crush the fuel and then capture contaminants in three steps. First, a magnetic separation followed by a non-magnetic separation of non-ferrous materials such as aluminium and stainless steel, and finally we have a wind sieve that separates the heavy fractions. The CFB boiler requires a better separation and decomposition of fuel compared to a grate boiler, but it also means that we can recycle more metal fractions from waste, explains Magnus.
Summer with odour
The first summer with Block 6 was characterised by the smell.
– It was a tough summer with a lot of pressure from local residents. For a while I sat down, called each of the neighbours to try to explain the situation, says Magnus Eriksson. We have now come a long way to sorting out the problems and by next summer at the latest there should not be any more odour problems from the new facility.
Magnus Eriksson gives some examples of problems and the actions taken or planned to resolve the problems.
– We use activated carbon filters to take care of the smell, but it turned out that they didn’t last as long as we had anticipated. We are reviewing the operation of the carbon filters to make them work better. We keep a better eye on the moisture content of the filters and are working to preheat the filters. The fuel preparation building has 13 filter compartments with five tonnes of activated carbon in each filter. The carbon was changed twice and now we are about to change it again.
– We have an odours panel around Västerås, which consists of a number of odour-trained citizens who report to us if they detect any smells. We also work with suppliers of waste bales to ensure that they arrive whole. We stopped using suppliers who couldn’t meet that demand and we are working to get better routines in place. For example, management of ports, where we are in the plant and how we move between different zones as well as how we handle clothing if we were in zones with strong odours.
– The processing plant is divided into zones and staff cannot move from the dirtiest zone to other parts without changing clothes. We rebuilt the changing rooms so staff enter from one side and leave from the other side, to keep smells under control. In the beginning sometimes staff went from the processing plant into the office or out to town for lunch without changing clothes. Now we have rules to avoid this, says Magnus Eriksson.
– We are looking at taking exhaust air from fuel preparation and using it as combustion air in the boiler to minimise the risk that the smell spreads. We are also reviewing the charcoal filters on the indoor buffer storage.
– There will simply not be any smells in the summer of 2015, says Magnus Eriksson.
Next boiler project
The new Block 6 boiler runs as base load. When the outdoor temperature starts to get colder Boiler 5 is run on biomass fuel. Boiler 4 was previously fuelled on peat and coal but is currently not used. Instead some of the older boilers are used as additional peak load, when it is the coldest. They use pine pitch oil and fossil oil as fuel. The next step for Mälarenergi is to build an additional cogeneration unit, called Block 7. This project is now in the study phase. Unit 7’s capacity is planned to be 150 MW and is scheduled for completion in 2020. This will enable Mälarenergi to reduce their greenhouse gas emissions further. The fuel is expected to be recovered wood.
– A strategy decision may be taken during the first quarter of 2015. After that, we can get started with various permit applications and begin preliminary planning phase, revealed Magnus.
According to Eriksson the driving force is the same as for Block 6, to renew the facility, reduce carbon dioxide emissions, keep good district heating prices and high fuel flexibility.
–We are proud of Block 6, which we believe is one of the world’s largest single boilers for waste incineration, with a unique fuel management system. Mälarenergi is in the midst of incredible change to cut a large proportion of its CO2 emissions from fossil sources by 2020, concluded Magnus Eriksson.