Wood fuel for a carbon-neutral Copenhagen

On 1 September 2016 the official ground-breaking ceremony took place for Denmark’s latest combined heat and power station. Fuelled on wood chips, with a rated boiler capacity of 500 MWt, it is set to be one of the world’s largest biomass fuelled circulating fluidised bed boiler based plants, second only to the proposed MGT Teesside facility in the UK (see p 30).

The new Danish CHP unit, called BIO4, is located at the Amagerværket site close to the centre of Copenhagen. Supplying 25% of the greater Copenhagen area’s district heating demand once commissioned in 2019, the new plant is under construction on an existing CHP generation site.

Carbon-neutral Copenhagen

BIO4 emerges from an ambitious plan to make Copenhagen the world’s first carbon- neutral capital, by 2025. One of the core measures of this plan is a 20% reduction in heat consumption by 2025 relative to 2010, with all heat supplies being carbon neutral.

The project is being developed by greater Copenhagen’s municipal utility HOFOR. With more than 1 million customers, HOFOR is a non-profit municipally-owned entity with a turnover of around DKK5 billion a year, employing some 1000 people.

As well as providing heat and power, HOFOR supplies water and waste water services, town gas to 150 000 customers and district cooling.

It is also expanding its portfolio of green energy. For example, a renewable energy section was established in 2010 with the objective of building about 360 MW of on- and offshore wind capacity by 2025. Around 65 MW is currently installed, including 14 offshore turbines – three of which are in place at Prøvestenen in Copenhagen and can be seen from the site of the new biomass power plant.

The city council and HOFOR will give citizens the possibility of directly investing in the wind turbines, with around a third of existing projects already available for public investment.

In addition, the company operates a heat pump with a view to exploiting historically low electricity prices in Denmark, where more than 100% of national demand is occasionally available from wind capacity alone. Under the 2025 plan this heat pump development programme is set to be expanded while some peak load and reserve plants are to be converted to CO2 neutral fuel. 

HOFOR also has both geothermal and solar thermal capacity, installed in small plants, and the plan implies investigations to establish a new geothermal facility.

Under the terms of the plan, the utility is also required to achieve energy savings of more than 3% of the annual district heating demand annually over the period 2015–2020.

The BIO4 project

BIO4 is to be located on the site of the existing Amagerværket power station in the harbour area. Acquired from Vattenfall in January 2014, there are currently three units on the site.

Unit 1, a coal-fired plant, was converted to biomass in 2009-10 and now runs on wood pellets. Unit 2 was a coal and oil fired plant which was decommissioned in 2007. Unit 3 is a 600 MWt (250 MWe) coal fired facility which was built in 1989 and is scheduled for decommissioning once the new bio-fuelled installation is operational.

The plants supply baseload district heating to Copenhagen. The Amagerværket site, initially established in 1970, is located a mere 1500 m from the city centre, opposite the royal apartments.

Although official ground breaking only recently took place, HOFOR has already purchased two new harbour cranes to cope with the significantly greater volumes of biomass coming ashore when compared with the fuel volumes associated with higher calorific value coal, and is currently expanding the harbour to take two ships simultaneously.

The new plant is expected to use 1.2 million tonnes of wood chips annually, around one ship load per day. It is an absolute necessity for HOFOR that the biomass is sustainable, with suppliers meeting certain criteria determined in co-operation with other major energy companies in Europe in the “The Sustainable Biomass Partnership”.

Delivering such biomass volumes represents a significant logistical challenge. HOFOR had originally planned to replace the existing 600 MWt coal-fired plant with a plant of the same installed capacity. However, as Carsten Schneider, project director for HOFOR and leader of the more than EUR600 million CHP project, tells MPS: “We saw that it would be a challenge to get that much biomass into the harbour. So we decreased the electrical capacity a little bit and increased the heat production instead, as the demand is there for the heat”.

Currently installing piling, with some 6000 concrete piles (900 cm2 x 20 m long), due to be installed, major construction of the process equipment is scheduled to begin in July 2017.

Says Schneider: “We’ll continue with the concrete and piling so that it is ready for process erection. In May next year we’ll start with the boiler.”

“We expect to have the first supplies of biomass at the end of 2018 with commissioning of the in 2019,” he adds.

A multi-part contract

Arranged as a multi-contract supply, rather than as a turnkey project, Schneider explains: “We want to have full insight into what [the suppliers] are doing and not have just one single point of contact as in a traditional turnkey supply. We understand that others prefer that, but we want to have access to all parts, to get the best suppliers working on all systems and ensure good competition amongst the suppliers. We are also aware that we take on the risk of co- ordinating many suppliers, but we believe we have the team to do that.”

For example, in a deal valued at more than EUR 150 million in total, the fuel storage, conveying system, combustion system, cyclones and boiler are all being supplied by Valmet in two different contracts.

Valmet’s scope of delivery includes a 500 MWt CYMIC-brand CFB boiler.

The district heat capacity of the plant will be 415 MJ/s while the power plant’s net electricity production capacity will be 150 MWe, which implies a total thermal output from the power plant that is significantly greater than the boiler thermal rating.

This is because the boiler thermal rating assumes steam at high temperature, suitable for electricity generation, while the district heating capacity is provided by low temperature heat.

Also, the plant employs a flue gas condenser to extract low temperature heat from condensation of water vapour in the flue gas, for use in the district heating system. This provides a significant amount of low temperature heat because of the high water content of the wood chips, something like 45%.

As Schneider explains: “Instead of letting the vapour go out through the stack, we can condense it (and recover the latent heat of vaporisation) using the district heating return flow to cool down the flue gas, producing about 115 MJ/s that would normally go up the chimney, so it’s a lot. In addition we also use combustion air in a humidifying system to cool down the flue gas even further and can also make use of this in the district heating system.”

“It means even though we have a combustion temperature of 800-830°C, all of the energy is utilised to such an extent that the final temperature of the flue gas as it goes out of the chimney is down to 30-35°C”.

The overall result is a plant with a “very high level of efficiency, around 115%”, says Schneider. (The efficiency can exceed 100% because it is calculated on a “lower heating value” basis, where the calorific value of the incoming fuel is estimated on the assumption that the latent heat of vaporisation of water in the combustion products is not recovered, as it will be at BIO4.)

Schneider adds: “We also have to contend with very demanding environmental requirements, considering the location of the plant, the proximity to neighbours and to the city itself, so we have a very advanced system for flue gas treatment.”

The plant will use a bag house filter for dust together with a wet scrubber system supplied by Lyon-based LAB, part of France’s CNIM.

The steam turbine, supplied by Doosan Skoda, employs a one cylinder, 26 stage, configuration, with two district heat extractions and full bypass to allow electrical production to be curtailed to zero with a corresponding hike in heat production.

Once cranes have unloaded the ships, the wood chip will initially head to a screening system that removes oversized material and contaminants such as metal. Material is then conveyed to two giant storage facilities. Each A-frame storage shed is 175 metres long and around 25 metres tall. In total it will hold 105 000 cubic metres, something like seven days of production.

There are two conveyors which transport screened material to silos ready for immediate use. The conveying system will feed the boiler with 900 cubic meters of wood chip an hour, equivalent to a lorry-load every six minutes.

“We have chosen a high-redundancy approach. There are two conveyors even though we have a conveying system that offers 99.5% availability. We want to have high reliability throughout, so we have double redundancy on the power plant,” says Schneider.

With the final investment decision from the board given in May 2016 – the day after contracts were signed with the primary suppliers – and environmental and building permits in place by 1 September, 2016, the BIO4 project is currently exactly on schedule.

Architectural concept

Given the proximity to the city, one of the more striking elements of this project is the architectural design concept, which emerged from a competition launched last year.

Schneider explains: “As we have people living in this area we have to be aware of noise, environmental issues, but also produce something for the eye through the architecture of the plant. We take something from the city by putting up large buildings – we believe we have an obligation to give something back”.

The competition was won by the Danish firm Gottlieb Paludan Architects, working in collaboration with landscape architects Møller & Grønborg and lighting specialists Speirs + Major.

Says Schneider: “The power plant as it is today has high fences with barbed wire on top. Everything is saying ‘It’s dangerous, you’re not welcome here’ and we wanted to change that. At the same time we have a plant which has to be operated. This was the challenge when releasing the architectural competition.”

Clad with thousands of debarked tree trunks and equipped with a staircase leading to a viewing platform, the vision is to invite visitors to see a CHP plant in operation and also to enjoy a spectacular view of the city.

“The winning concept incorporates tree trunks at the perimeter, representing, of course, our fuel, and a sustainable approach to power and heat production. At various points the façade employs glass allowing you to look into the power plant, into the boiler house, into the turbine hall and the flue gas treatment system. You have to climb 45 m to get to the viewing platform. From there you can see Sweden, or you can see the whole city of Copenhagen.”

The top of the plant is about 65 metres high.

Commenting on the design Jesper Gottlieb, of Gottlieb Paludan, said: “In the end we are telling a story, a story you can move around in, the story of the forest, and the story of sustainability.”

Schneider is perhaps more pragmatic: “Wood is the cheapest fuel, this is the baseload, this is the best we can do in Copenhagen.”