Notes from Sweden #4: CHP and District Heating

[Clicking an image in this post will load a larger version of the image. A slideshow of the images in this post, and more, is also available. Previous posts in the "Notes from Sweden" series include #1: How They Get Around, #2: Western Harbor in Malmo, and #3: The Scandinavian Green Roof Institute in Malmo.] In Brattleboro, Vermont, I'm involved in an effort to establish a wood-chip-fired combined-heat-and-power (CHP) plant that will — if we can pull it off — generate power and provide district heating through a network of buried, insulated pipes. It's pretty exciting, really. While district heat is used in many large cities, university campuses, and medical complexes, there are no systems I am aware of that serve smaller towns. Brattleboro could be the first! Brattleboro could have the first small-town district heating system in the United States, that is. In Europe — especially Northern Europe — district heating is very common. Half of all buildings in countries like Sweden and Denmark are heated in this way. I've been learning a lot about this on my travels here. I had two in-depth tours of wood-chip-powered CHP plants that provide district heat to homes and businesses — and I stopped briefly at the Lund Energiplant that is a wood-chip-fired, heat-only plant used for district heating in Lund. At the C4 Energi plant in Kristianstäd (pronounced "cree quon staad"), I met with CEO Göran Thysén and was given a tour by him and a young plant engineer, Henrik Mattson. The company is 100% municipally owned, employees 95, and comprises three businesses: electricity production and distribution; district heating; and broadband Internet distribution through a fiber-optic network.

The heat and electricity production is accomplished with two wood-chip-fired boilers: one producing 50 MWt (megawatt-thermal) and 15 MWe (megawatt-electric); and the other producing 25 MWt and 6-7 MWe. (Wisely, Europeans use megawatt and kilowatt to refer to either electricity or heat, while in the U.S. we still refer to the British Thermal Unit [Btu] to refer to heat — see our discussion of that in a recent Back-Page Primer.) Total annual production by the plant is 334 GWt and 54.7 GWe. The boilers use "bubbling-bed" technology, which I'll try to describe later. Like most CHP plants in Europe, this is a "thermal following" plant in which the output of the boilers is governed by the heat demand, with the electricity output "following" that. Thus, the production is much lower during the summer months when thermal loads are much smaller. Sweden has a lot of hydro power and nuclear power; if biomass were to be depended on for a high percentage of year-round electricity production, more heating loads would have to be found for the summer months.

C4 Energi entered the district heating business in 1980 with oil-fired boilers. In 1980 biogas (methane produced from decomposition of organic matter) was added to the mix, and in 1994, most production was converted to wood-chips as the fuel source. Today, the plant is 97.6% fired by renewable energy sources — nearly all wood chips. The wood chips come from an 80-kilometer (50-mile) radius. Several days later, we took a two-hour a train trip from Lund to Växjö (pronounced sort-of like "vek' shua"), and I visited the Växjö Energi AB Sandvik plant. Before describing the Sandvik plant, let me say a little something about the city of Växjö. The municipality includes 80,000 residents, with 55,000 in the more densely populated city of Växjö. Ms. So Hie Kim-Hellström, the environmental coordinator for Växjö spent an hour describing the many initiatives that have led to Växjö being widely touted as "Europe's greenest city. " Strong interest in the environment here began in the 1960s, but the United Nations Rio Earth Summit in 1992 really galvanized this focus, especially through the "Agenda 21" Rio Declaration on Environment and Development. In 1995-97 the Swedish Society for Nature Conservation worked with Växjö to develop sustainability goals, which led to a unanimously passed decision by the Växjö city council to become fossil-fuel free. Okay, back on CHP and district heating. Monica Sandberg from the City of Växjö took me out to meet with Lars Ehrlén, the business manager for power and heat at Växjö Energi AB (VEAB), with whom I spent two hours learning about the company and then touring the Sandvik plant. The municipal power company VEAB was founded in 1887 — only five years after London and New York City began their own municipal power companies. In 1970, the company supplied its first district heat (produced by burning oil), and in 1980 it began burning wood chips, which supply the vast majority of energy today — along with some peat and oil.

In 1996, smaller, heat-only, wood-chip-fired boilers were installed in several outlying villages in the Växjö municipality to serve smaller networks of district heating. In 1998, a fiber-optic network was added to provide broadband for Internet, television, and phone — a third service in addition to electricity and heat.

VEAB is significantly larger than C4 Energi in Kristianstad. Including the output of the several small district heating plants in the municipality, annual production in 2006 totaled 580 GWh of heat and 190 GWh of electricity. The plant serves 29,000 electricity customers and 6,500 district heat customers (including 5,500 single-family houses). District heat is supplied through a 350-kilometer (220-mile) network. This compares with VEAB's electricity distribution network of 1,250 km (780 mi) and their fiber optic network of 420 km (260 mi) — fiber-optic cables are installed in buried conduit at the same time district heating pipes are laid. The company uses biofuels for 95% of its energy and has a staff of 171, including 45 who run the Sandvik plant I visited. There are actually four boilers of various sizes at the VEAB Sandvik plant, some older oil-fired boilers that are used for back-up and peak energy production. The plant uses about 60 tandem-trailer-truck loads of wood chips per day for a total of about one million cubic meters of wood chips per year. As with the C4 plant, this one is thermal following.

The largest boiler at VEAB's Sandvik plant produces 104 MW of total thermal energy. Like the C4 plant I visited, it is a circulating fluidized bed (CFB) boiler. Here's how this technology works, as described in a brochure from the company:

"Combustion takes place in a sand bed at a temperature of 800-900°C (1450 - 1650°F). As the sand bed becomes permeated with the combustion gases, it starts behaving like a boiling liquid. Some of the sand is swept up in the flue gases and is separated in the following cyclone and recycled to the combustion chamber."

This heat generates high-pressure steam — initially at a pressure of 140 bars (2,030 psi) and temperature of 540°C (1,000°F). This is used to generate 38 MW of electricity. Various heat exchangers, flue-gas condensers, and other components extract as much of the heat as possible to use for district heating. There are also extensive pollution-control components.

Rather than being distributed directly, it heats water in a 40,000 cubic-meter (10.6 million gallon) insulated tank (accumulator), from which hot water is drawn off to supply the district heating network. The insulated pipes leaving the plant (district heating is always provided through paired supply and return pipes) are 80 centimeters (31 inches) in diameter. Further from the plant, the pipe diameters drop as they branch and as feeds go off to heat buildings. At individual single-family houses, the pipe diameters are as small as 15 mm (0.6 in). There is typically a 50°C (90°F) delta-T (difference in temperature) between the outgoing and return water. Water leaves the plant at about 100°C (210°F) and returns at about 50°C (120°F). So that's a quick lesson in CHP and district heat. For more on this, visit our feature article on the topic (Vol. 16, No. 3). I was struck, on visiting the two plants, by just how organized and clean they were. Perhaps it has to do with power plants — I haven't visited others of this scale — but more likely I suspect it's because of the Swedish attention to detail, order, and cleanliness. You could have eaten off almost any of the floors in the two plants!

I was interested to learn, from Ehrlén, of the VEAB Sandvik plant, that the taxes in Sweden are a significant driver of biofuel use. Included in the operating cost for a company like VEAB is a carbon dioxide emission permit, a carbon dioxide tax, a sulfur tax, and the price of the fuel itself. With wood chips, the commodity fuel price (cost per tonne) accounts for 100% of the total fuel cost; with coal, the commodity fuel cost represents less than 40% of the total fuel cost; with heavy oil the commodity price represents about 75% of the total. When I asked about the minimum density of houses needed to make a district heating system economically viable, Ehrlén said that it's a complex issue, influenced by subsidies and laws that new housing be served by district heat, but they have a rule of thumb that you can put in about one meter of pipe if you can sell 1 MWh of heat per year for that meter of pipe. In the Växjö city region (with 55,000 residents) over 50% of single-family homes are served by district heat, as are 95% of multi-family buildings (also called "houses" here). Some useful resources for more information: http://www.c4energi.sehttp://www.veab.sehttp://www.vaxjo.se— Alex Wilson, Växjö, Sweden, 14 December 2007