How state-of-the-art boiler and automation technology were used in a new power plant that’s right in the centre of a capital city.

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Known by locals as the ‘Wedding Cake’ due to its multi-tiered form on the Stockholm cityscape, the boiler house at Värtaverket KVV8 biomass CHP plant has been shortlisted for two prestigious global architectural awards.

And inside the terracotta-clad building in Sweden’s capital city is one of the world’s largest biomass boilers.

The new plant is fuelled entirely by biomass, which comes in chip form from all over Sweden and further around the Baltic region. The chips are fed into the boiler from a dedicated railhead and harbor via a complex subterranean conveyor system down to 40 metres below sea-level through tunnels cut into solid rock.Värtaverket KVV8 biomass CHP plant

Mats Strömberg, senior project manager, says: “Not only is this one of the largest biofuel projects in the world, but we have built it in a tight space, right in the heart of the city, with a lot of the main moving parts and supply systems situated way underground.”

All of the ash from the plant is transported back through the same tunnels to silos at the harbor through a completely closed-loop conveying system.

What makes the environmental aspects at Värtaverket even more challenging is that the plant is right next to a residential area as well as very close to Ekoparken, the world’s first city national park.

Strömberg says: “The target in this city is to get rid of fossil fuels completely before 2030. We will do this by building plants such as KVV8, but also using waste-to-heat plants for district heating and generating electricity in other areas. In fact, Stockholm Exergi has recently announced that coal plants in the city will be completely closed by 2022.”

GOING UNDERGROUND

The decision by Stockholm Exergi to go ahead with the plan for a biomass plant to supply the city with district heating was made in February 2011.

Stockholm Exergi brought in large boiler suppliers right from the beginning to discuss technical solutions and logistics.

Strömberg explains: In the case of the Värtaverket site, we not only had high demands in the capacity of the boiler, we also had major local city planning restrictions when it came to height and space. We needed suppliers who understood how to work with this.

“To get the capacity we needed, we had to install a very large boiler. That meant height as well as width when it came to capacity.

There was no way around it, says Strömberg: “We had to go underground. To do this, we had to blast through solid rock, build the boiler down to 14 meters below ground and the tunnels down to 40 meters below sea level – we created a very big hole, all within a stone’s throw of local residents. We also excavated the tunnels underground from the harbour to the plant for the biomass conveyors and in March 2014 we were ready.”Mats Strömberg, senior project manager

Austrian company Andritz was awarded the contract for the boiler and all automation at the plant in December 2012. Bernhard Haimel, project manager for Andritz, explains: “This was a unique project in many aspects, particularly the manufacture and installation of one of the world’s biggest boilers, with a biofuel fed circulating fluidized bed (CFB) and an output of 345 MW.

“What was particularly difficult in this case was that we had to fit the boiler into a limited space at the same time as apply the very best technology for efficiency, including the need for fuel feeding via six spouts to ensure even distribution of the biofuels, one of the prerequisites for the lowest possible flue gas emissions.”

Andritz also delivered the flue gas cleaning system. Both SNCR and high dust SCR technology is used to ensure lowest NOx and NH3 emissions. For primary desulfurization of the flue gases, limestone is injected into the furnace. For dedusting, bag house filter technology is used with the possibility to add bicarbonate upstream the filter.

Another prime element of the boiler was the Lexor Bed Material Heat Exchangers, which allow the installed superheaters highly advanced steam parameters of 560°C at 147 bar without the normal corrosion risk.

ADVANCED AUTOMATION

The plant has an extremely high degree of automation. Haimel explains: “The DCS system at Värtaverket is very sophisticated and one of our most challenging tasks was to integrate the various technological areas into the common control system.

“Now the entire plant can be operated or at least monitored from several control rooms located in different areas of the power station, both in the newly-built part as well as in the existing buildings. Safety, availability, and efficiency were the key requirements throughout the design of the electrical and automation system and for the selection of suppliers for this project. Consequent observation of these requirements has resulted in one of the most advanced power boilers in Europe.”

Much like catching and using the steam generated from a boiling kettle, the technology at the Värtaverket site has been designed for maximum output, and to use and reuse any steam, heat or power to achieve extra efficiency wherever possible.

Strömberg explains: “The boiler has been designed for maximum output, which is largely based on a well-designed combustion process with high efficiency. But there are also added extras, which allow us to maximize efficiency at the plant by reusing heat and steam. For instance, a so-called heat shift system is implemented in the boiler back pass that cools the flue gas temperature further down after the boiler. The system allows flexibility, switching over heat to incoming combustion air and/or condensate water after the turbine condensers.”

Haimel adds: “The heat shifting system also allows for a lot of flexibility in reusing waste heat and ensures lowest boiler outlet temperatures. This increases the efficiency of the boiler itself by far. Furthermore, the boiler has varying needs going from winter to spring and autumn to winter. The Andritz PowerFluid technology for power boilers allows for the boiler to go quickly and smoothly from a maximum load of 345 MW up to overload operation of 375MW and down to 80 MW, allowing for a wide operation range, which is unique for a biomass application of this size.”

Stromberg continues: “The recovery of district heat from flue gas is maximized by combustion air humidification upstream of the boiler and a flue gas condensation downstream of the boiler. While spraying water into the combustion air, the concentration of vapor water in the flue gas is maximized through the boiler.

“The efficiency of the flue gas condensation after the boiler becomes higher while increasing the condensation effect so that more heat can be recovered. The outgoing flue gas to the stack has a temperature around 40° C. This makes the overall efficiency of the plant very high – and certainly more than 100 per cent.”

Andritz is exhibiting at POWER-GEN Asia in Indonesia in September, where there is also several conference sessions examining CFB boiler technology. For details click here.