Posted on March, 01 2012

Virtuous circle of district heating, waste-to-energy and CHP

The increasingly urgent problem of climate change has focused attention on district heating, waste-to-energy (WtE) systems, and combined heat-and-power (CHP) as techniques that reduce greenhouse gas emissions while bringing other sustainability benefits. Many Danish municipalities have implemented these in combination.



Keywords: waste-to-energy (WtE), combined heat-and-power (CHP), district heating, carbon intensity, renewable energy, municipal solid waste (MSW), climate mitigation, landfill

Denmark is a leading user of district heating in Europe. District heat is used by approximately 2/3 of the Danish population – 60% of households (see also Stockholm). This is mainly in combination with CHP. And waste-to-energy, in the form of incinerating municipal solid waste, provides 20% of the heat for Denmark's over 400 district heating networks (see also San Francisco). WtE also provided some 4% of Denmark's electricity in 2007.

Waste is increasingly viewed as a renewable source of energy, not without debate (see below). In Denmark, some 3.5 million tonnes of waste are incinerated annually. The largest sources of renewable energy in Denmark – according to 2001 statistics – were biomass (wood/straw), followed by waste, and then wind energy. Renewable energy's share in the Danish energy system is growing rapidly: 30% of electricity production and 40% of district heating production in 2007 were from renewables, compared to 10% and 20% respectively in 1997.

Benefits from waste-to-energy
Waste-to-energy systems create a range of benefits. These include:

• WtE transforms waste from a problem into a resource.
• Energy generated by WtE contributes to primary energy savings from other energy sources. WtE also works symbiotically with CHP, increasing overall conversion efficiency and the performance of incinerators.
• WtE can reduce greenhouse-gas emissions when it replaces more carbon-intensive energy sources. Case-by-case analysis has found large variation in the carbon savings between municipal solid waste incineration techniques, depending on the local district heating system, the type of heat-producing plant, the additional fuels used in the system, the type of substituted heat technology, and interactions with the electricity system, according to a 2009 paper by Fruergaard et al.
• Waste to landfill is reduced heavily. This can be of major importance. For example, in the EU, municipal waste has been growing rapidly, reaching 522 kg per person per year in 2007 though sustainable development requires reduction of these waste flows.
• Waste treatment time is extremely short compared with landfills.
• WtE enables treatment of hazardous waste.
• Developing countries facing growing pressure on waste management and electricity supply – can deal with a large set of problems at once using WtE.
• WtE in temperate climates provides heat and electricity, but in warmer climates provides district cooling and electricity.

Benefits from district heating
The supply of heat and hot water to buildings through district heating has a range of benefits. Firstly, energy efficiency of space heating and hot water is improved, saving energy and greenhouse gas emissions. This is crucial for climate mitigation and other energy policy issues, given that a large part of total energy use is for heat and electricity in buildings. Secondly, district heating can be the basis for introducing key energy techniques, such as: (1) CHP, which also raises energy efficiency; (2) renewable energy for heat and electricity, e.g. waste incineration, geothermal energy, large-scale solar thermal energy; (3) productive use of excess heat production from industries. A third argument is that energy security is boosted through renewables and efficiency.

Criticisms
An important risk with WtE-based systems is that they become dependent on – and justify – societies' increasingly wasteful consumption. WtE developers should consider any possibility to recuperate heat from already existing industrial processes before expanding the capacity to incinerate waste for heat and electricity production.

Connected to these positions is the view that it may be too problematic to classify waste incineration – and thus waste – as a form of renewable energy. Mitigating this would the implementation of a Waste Hierarchy, where the various possible uses of waste are ranked, and where waste minimisation is given the first priority (highest ranking). Often, the conversion of waste to energy is ranked lower than re-use and recycling of materials in wastes.

Another criticism is that waste-to-energy may have unintended negative effects. These can include creating an incentive to increase – rather than decrease – the amounts of waste produced, thereby contributing to higher levels of energy and material use throughout a society, increasing upstream environmental impacts. Similarly, waste as a resource leads to the purchase of waste for energy, often requiring long-distance transport, which further increases environmental impact.


References
Sabina Andrén, 2010, Visions and Realities – Tensions in the field of urban sustainable development with Malmö and energy as a case study – Working report, Human Ecology Division, Lund University

Danish Energy Authority, 2003, Renewable Energy – Danish Solutions: Background, Technology, Projects, http://193.88.185.141/Graphics/Publikationer/Energiforskning_UK/VE_Brochure_web.pdf

T. Fruergaard, T.H. Christensen, T. Astrup, 2010, “Energy recovery from waste incineration: Assessing the importance of district heating networks”, Waste Management, 30, 1264–127

Leif Gustavsson, Anna Joelsson, 2007, “Energy conservation and conversion of electrical heating
systems in detached houses”, Energy and Buildings, 39, 717–72

H. Lund, B. Möller, B.V. Mathiesen, A. Dyrelund, 2010, “The role of district heating in future renewable energy systems”, Energy, 35, 1381–1390

Krushna Mahapatra, Leif Gustavsson, 2009, “Influencing Swedish homeowners to adopt district heating system”, Applied Energy 86, 144–154

Martin Pavlas, Michal Touš, Ladislav Bébar, Petr Stehlík, 2010, “Waste to energy - An evaluation of the environmental impact”, Applied Thermal Engineering, 30, 2326 – 2332

Elisabeth Rosenthal, 2010, “Europe finds clean energy in trash, but U.S. Lags”, New York Times, April 12, http://www.nytimes.com/2010/04/13/science/earth/13trash.html?scp=1&sq=trash&st=cse

Text by: Aaron Thomas