Waste to warm the town: new CHP plant for expanding Swedish city

With the recent commissioning of the Torsvik waste-to-energy plant, Jàƒ¶nkàƒ¶ping Energi has modernized its district heating production and almost doubled its power production capacity. The plant represents the latest example of how municipalities in Sweden are looking towards CHP as a way of resolving waste management, environmental and energy issues, writes Alan Sherrard.

by Alan Sherrard

With a population of about 110,000 living in the city and immediate surroundings Jàƒ¶nkàƒ¶ping is Sweden’s ninth largest urban area. Jàƒ¶nkàƒ¶ping is also the administrative seat for the county of Jàƒ¶nkàƒ¶ping. It is estimated that as a direct result of continuous DH and CHP development and expansion, the county has reduced CO2 emissions by 54,000 tonnes per year when comparing 2003 with 1994.

Although emissions of CO2 from DH and CHP production itself has increased over the period, the net gain is attributed to the expansion of DH replacing other heating forms with a dramatic reduction of emissions as a result.

The Torsvik facility is the latest addition to the number of waste-to-energy plants now operating in Sweden. Waste is an important fuel source; in 2005 Sweddish WTE plants treated 3.8 million tonnes of household and industrial waste generating 10.1 TWh heat and 0.9 TWh electricity (Alan Sherrard)
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The local heat and power provider and distributor Jàƒ¶nkàƒ¶ping Energi AB is, as the case in many Swedish municipalities, wholly owned by the municipality of Jàƒ¶nkàƒ¶ping. The company, which has developed and supplied DH to the city since the early 1980s, already has two CHP plants operating in the city.

The smaller of the two is a landfill-gas-fired plant located by the regional hospital. The second is a larger plant with several boiler units located at the Munksjàƒ¶ paper mill. The main boiler uses wood powder and the auxiliary boilers use fuel oil. Together with other smaller DH plants, hydro power stations and wind turbines, Jàƒ¶nkàƒ¶ping Energi supplies district heating to about 30,000 households and electricity to about 50,000 users. In energy terms about 650 GWh heat including process steam, 3.8 GWh district cooling and 80-100 GWh electricity is generated and supplied per annum.

Local waste management and DH dilemma

A number of the production facilities were however beginning to feel their age. The largest of them, the Munksjàƒ¶ CHP plant, has a total installed boiler capacity of 118 MWth and electrical output of 9 MWe. It was originally designed to burn oil for generating process steam for the paper mill back in the late 1960s. The main boiler was first converted to burn coal, then to use peat, and most recently, in 1989, converted yet again to use wood powder and oil. The city itself had seen significant growth over the same period with little space in and around the Munksjàƒ¶ site, adding additional planning constraints in terms of space, noise, traffic and emissions.

The city council found itself at a strategic crossroads. The first problem was escalating fuel prices, in particular oil, which in 2003 accounted for around 27% of Jàƒ¶nkàƒ¶ping Energi’s fuel mix. Then there were insufficient efficiencies due to outdated technologies, with limited development possibilities at existing DH and CHP sites within the city. Combined with new landfill legislation prohibiting the disposal of unsorted household and organic waste, these factors prompted the city council to investigate a new alternative.

Jàƒ¶nkàƒ¶ping was already committed to biological treatment (digestion) of organic waste for biogas production and municipal solid waste (MSW) was being transported long distances to other waste-to-energy (WTE) facilities at considerable cost. Furthermore there were ambitions to further expand the DH network to incorporate new commercial and housing areas. Finally, in late 2003, once all due national authorities including the Swedish Environmental Protection Agency had given the project proposal the all-clear, the city council decided to go ahead and invest 1.1 billion SEK (approximately €119 million) into building phase one of its own WTE unit. Phase two will involve further investment by adding another boiler line at Torsvik using MSW, biomass or natural gas. This is, however, still at a feasibility stage and a decision to go ahead with this has yet to be made by the council.

With an installed boiler capacity of 60 MWth and steam turbine generator output of 13 MWe, the Torsvik plant will require about 20 tonnes of waste per hour. The electrical output is variable dependant on what the heat needs are. Once up and running by late 2006, Torsvik phase one will cover almost half the current DH needs of Jàƒ¶nkàƒ¶ping and almost double the current power output. It will replace much of the oil-burning capacity at the Munksjàƒ¶ CHP plant, bringing down oil to less than 10% of the fuel mix for the company. Annual production is estimated to be in the region of 340 GWh (thermal) and 80 GWh (electrical), treating up to 160,000 tonnes of municipal and industrial waste. Although Torsvik is in many respects a conventional WTE plant, it does have some interesting unconventional features. The design and architecture itself is noteworthy as is the unique heat pump and flue gas condensing stage which increases the overall thermal efficiency of the plant.

Challenging site location

The chosen site is situated about 10 km south of Jàƒ¶nkàƒ¶ping just by the E4 motorway. The Torsvik area is a designated industrial zone with light manufacturing and engineering industries along with warehousing. The Swedish furniture giant IKEA has for instance their Nordic distribution point located here. With its undulating forest-covered terrain, the 16-hectare large greenfield site posed an interesting design challenge for the architects. Not least because of the 18-metre height difference between the highest and lowest point. The overall aesthetical ambition has been to retain the horizon by designing and placing the building so that it drops into the landscape rather than tower up over and above it.

The undulating forest terrain posed a plant design challenge. This aerial view, taken January 2006, shows the roundabout traffic flow and the proximity to the motorway. the heat storage tank can be seen on the far left-hand side(Leif Gustavsson)
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The exterior of the building is clad with silver-coloured corrugated sheeting with large glass facades facing the motorway. The visible process equipment is painted a pale yellow and the whole interior is lit at night to give a warm glow, providing an interesting contrast to the dark pine forest surrounding the site. The 120-metre-tall flue gas stack is tear-drop shaped rather than round. It has a glass slit that runs the entire vertical length on the side facing the motorway, the lift shaft housing. The end result is that the flue stack looks rather like a gigantic thermometer, especially at night once the lighting is on.

The Torsvik process flow

As with any such plant, transport logistics was a key factor in site location and design of the building surrounds. Easy access to and around the plant for the anticipated volume of 40 trucks per day is a crucial issue, not least given further expansion plans should phase two become a reality. The traffic flow is modelled on a roundabout with the possibility of entering the building on three levels. On arrival, refuse collection trucks are weighed before proceeding to unload at the storage bunker in the unloading hall. The unloading hall has seven back-tipping stations and one side-tipping station enabling quick turn-around times. The bunker can hold up to 20,000 m3 of material for intermediate storage and is equipped with crane and material grabs.

Figure 1. Torsvik process flow chart
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From the storage, bunker material is lifted into the boiler feed-in funnel. Supplied by FISIA Babcock Environment, the heart of the facility is of course the main boiler, along with a water-cooled forward-moving grate with a maximum throughput of 22 tonnes MSW per hour. This type of moving grate is becoming increasingly popular for new WTE plants and plant refurbishments or upgrades. By water-cooling grate bars in the main combustion zones, thermal stress is greatly reduced, thereby increasing the service life of the grate. This is achieved without any adverse effects on the incineration performance. Combustion temperatures reach about 1000à‚°C inside the boiler and unburnt material at the end of the grate is removed as slag with magnetic separation of any ferrous materials. The slag is then transported to specialist plants for other metal recovery after which the remaining material is used in controlled environments such as landfill linings. The amount of slag is estimated at 10%-15% of the fuel depending on the composition.

The steam production is up to 79.5 tonnes of 380à‚°C steam per hour at 41 bar in a closed-loop circuit. The steam generated is passed through a BVI (now MAN Turbo) generator drive steam turbine before returning via the turbine condenser to the boiler to be reheated to steam again. The steam passing through the turbine condenser is cooled by the incoming return water from the DH network. A unique feature of the plant is that the incoming return DH water can either come directly from the network or first via the flue gas condenser or both. Either way, the water has a temperature in the range of 45à‚°-50à‚°C, and it is heated by the cooling steam to the outgoing temperature of 75à‚°-110à‚°C depending on the season (higher during the winter).

On leaving the combustion chamber flue gases pass through a series of cleaning and condensing stages, all supplied by Alstom Power, before entering the flue stack. The entire system has a gas flow rate of 60,000-127,000 Nm3 per hour. The first stage is the patented Novel Integrated Desulphurization (NID) system for the removal of the acidic components along with dioxin and heavy metals such as mercury from the flue gas. The system consists of a mixer/humidifier, a reactor and textile filter.

The flue gas is led into the NID reactor where it is mixed with humidified dust consisting of lime, activated carbon, fly ash and re-circulated dust from the textile filter. From the reactor the mixture of gas, reaction compounds and dust is passed through the textile filter which separates the dust and reaction compounds from the outgoing flue gas. The dust is re-circulated with more lime and activated carbon whereas the discharged reaction compounds are removed to a residue silo. An estimated 4000 tonnes of residue per annum will be produced which, due to its nature, can only be placed in special landfill sites. At present this has been contracted to a site in Norway, however plans are underway to secure a suitable site within the region.

Downstream of the NID system is the wet scrubber stage. This consists of both an acidic and neutral part removing any residual ammonia, hydrochloric acid or sulphur dioxides. The flue gas then enters the condenser to be cooled to 36à‚°C, preheating the incoming return DH water if needed prior to the main turbine condenser. The bleed from the wet scrubbers is reused within the plant whereas the condensate from the condenser is led through an on-site wastewater treatment facility before discharge to the recipient. Finally having been cooled by the condenser, the flue gases are led to the stack via an induced draft (ID) fan. The stack itself has two exhausts, one for the main boiler and the other for the auxiliary boiler, with room for a third exhaust for phase two.

As with any heating system, there are user variations throughout the 24 hours. To keep an even incineration rate in the boiler during demand peaks and lows, Torsvik has a 6000 m3 hot water storage tank. This 35-metre high tank also doubles as an expansion tank as it is located on the highest point of the entire DH network. Connecting Torsvik to the existing network also proved to be a challenge as it was necessary to prospect a suitable route for the 12 km-long distance, of which half was to go through built-up areas. The pipes themselves are 80 cm in diameter, so the channel had to be substantial in order to accommodate both the incoming and outgoing pipe systems. Their size also meant having limited curving options, so the channel sections had to be as straight as possible.

The Torsvik project is just one example of how a mid-sized municipality looks towards combined heat and power as a way forward to resolve waste management, environmental and energy issues for their citizens.

Alan Sherrard is with Elmia AB, Jàƒ¶nkàƒ¶ping, Sweden. Fax: +46 36 16 46 92 e-mail: alan.sherrard@elmia.se

District heating in Sweden

District heating (DH) in Sweden is well established. The country’s first district heating plant was inaugurated in Karlstad in 1948. Since then DH has grown significantly, now supplying around 50 TWh per annum an average year in climatic terms. DH is available in 570 of about 1930 urban areas and has a market share of approximately 50% of all home and commercial space heating in Sweden today. And it continues to grow. According to the Swedish District Heating Association, its members plan on expanding existing networks as well as bringing DH to additional 100 or so new urban areas.

DH has also considerably reduced Sweden’s carbon dioxide (CO2) emissions. In 1981, fossil fuels such as oil and coal were the dominant fuel source for DH plants, at a 90% share. By 2001, these fuels had dropped to a 10% share while DH production expanded 75% during the same period. This simultaneous market growth of DH and fuel switch to renewable sources has decreased CO2 emissions from heating by an estimated 11 million tonnes (cumulative), which is equivalent to about 20% of Sweden’s total CO2 emissions in 2004. Noteworthy too is that unlike many other countries, industrial waste heat is widely used as a fuel in Swedish DH production. In 2004, industrial waste heat accounted for approximately 6.1 TWh or a 10% share of the Swedish DH fuel mix.

However, in comparison to its Nordic neighbours Denmark and Finland – who also display high market shares for DH, at 60% and 48% respectively – Sweden lags behind in terms of combined production connected to DH plants. Both Denmark and Finland have very high percentages of cogenerated heat, at around 75% compared with Sweden’s 12%. This has been attributed to unfavourable taxation conditions for DH plants, especially those fired on fossil fuels, as increased energy and CO2 taxes have weakened the competitiveness of fossil-fuel-based CHP to an extent that imported electricity generated in, for instance, coal-fired stations has been cheaper.

The situation has now changed with a more favourable holistic view, equating municipal DH with industrial CHP plants such as at pulp and paper mills. The introduction of tradable electricity certificates has also dramatically improved the terms of competition for biofuel-fired CHP, using fuels such as municipal solid waste (MSW), peat, biomass and biogas. As a result there has been extensive investment into CHP both by industrial, commercial and municipal DH companies. Combined production is expected to double from about 5.7 TWh in 2002 to 12.7 TWh by 2010. The lion’s share of this increase, or 5.5 TWh, is expected from biofuel-based production including MSW, with the remainder from natural gas.

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