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Energy from Pre-sorted, Organic Household Waste

by H Ørtenblad, Herning Municipal Works and
F Hammer, Bruun & Sørensen Group AS, Denmark

INTRODUCTION

In 1993, in accordance with Danish national policy, the municipality of Herning decided to introduce separation of waste at source. This created a demand for treatment and recycling of the organic fraction.

A new way of pre-treating and gasifying this 'new' type of waste has been developed and demonstrated successfully during the past year in Herning. Support was obtained from the 'LIFE' programme of the European Union (a financial instrument for environment, nature and developing countries). The technology enables recycling of the waste for use on farmland without any restrictions, and energy for combined heat and power production is generated.

DEVELOPMENTS

For several years Herning Municipal Works have operated a large-scale biogas demonstration plant for treating manure and industrial organic waste in Sinding-Ørre. Some five years ago a small-scale pilot plant for special treatment of organic domestic waste was added. Very useful experience was gained from the operation of this plant. First of all, it was confirmed by thorough tests made by the veterinary authorities that the treated waste can be recycled to farmland without restrictions, and the farmers showed positive interest and confidence in the final product. Valuable experience in relation to the use of the equipment was gained and quality standards for the resulting fertiliser were formulated. In 1994 a full-scale 'second generation' pilot plant became operational.

Based on the experience gained at these plants in Sinding, it was decided to continue the development of this technology in another large-scale biogas plant, which was placed in Studsgaard. This new plant was inaugurated in November 1996. A new combined heat and power plant, 'Knudmoseværket', was commissioned in Herning late 1994 to incinerate the combustible fraction of the waste.

Herning Municipal Works owns and operates a city-wide district heating system supplying most of the demand for heating and hot water of the 56,000 inhabitants. The system comprises a variety of heat producing units based on traditional and, to an increasing extent, alternative energy sources such as biogas, wood chips, straw, landfill gas and waste. Thus, the low-temperature district heating network can exploit or recycle any source of heat energy, reducing the use of fossil fuels to the benefit of the environment.

Today, some 20% of the heat demand in Herning is covered by such local fuels. The biogas plants are integral parts of the regional energy infrastructure.

THE CONCEPT

The municipal waste is collected and brought to Knudmoseværket, where the combustible fraction is delivered to the incineration plant. The organic part, already placed in plastic bags by the consumers, is tipped into the silo of the pre-treatment plant.

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The flow of raw materials and energy in the concept

In this plant the waste bags are crushed and a first separation of the plastic and minimisation of the large organic items takes place. As this operation is extremely polluting, it takes place in a closed-off facility inside the plant. The most critical components are sealed and an advanced ventilation system extracts the contaminated air through a biological filter. The plant is designed to handle 8 tonnes/hour of waste which enables treatment of up to 22,000 tonnes/year.

The pre-treated organic waste is transferred, in closed containers, to one of the biogas plants. This fairly solid waste is then mixed with liquid manure to obtain a liquid solution. This liquid is deactivated at 70 oC for one hour and digested at 55 oC over a period of 16 days. Glass and metals are removed by precipitation prior to digestion; final separation of remaining plastics and other particles takes place after the treatment.

The digested material contains the same nutrients as the original material, and meets the following quality demands necessary for its use as fertiliser on farmland:

  • the content of non-degradable material such as glass, plastics and metal is negligible;
  • the content of heavy metals is well below the valid limit values;
  • digestion ensures suitable reduction of pathogenic micro-organisms.

Bacteriological analyses have shown that the untreated domestic waste sorted at source contains salmonella in considerable quantities. After digestion and biogasification the presence of salmonella could not be proved and, since the concentration of nutrients in the manure fraction corresponds to the fraction in ordinary liquid manure, there are no agricultural doubts about using the digested material.

The biogas generated is used in district heating stations in the villages of Sinding and Studsgaard; for combined heat and power generation in two stations located in the distribution systems of Herning; and, in one case, for separate power production. A minor residual fraction from the biogas plants is a solid used as solid fuel for local heat production only. The total maximum output from the plants supplied with biogas is 2.3 MWe and 3.1 MWth.

Annual capacity of the Two Biogas Plants (in 1,000 tonnes)

 

Sinding-Ørre

Studsgaard

Total

Liquid manure

35

110

145

Industrial waste

12

13

25

Domestic waste

4

7

11

Total

51

130

181

PERSPECTIVES

The project, consisting of the pre-sorting and biogas plants, has already attracted considerable world-wide interest as it proposes a solution to an important
environmental problem, frequently encountered in many countries. More than 1,800 people from 32 countries visited the plants during the first year of operation.

The project complies with the environmental legislation of the European Union, since the idea of such plants is to prevent environmental damage instead of curing it.
Furthermore, a high degree of recycling of organic material is achieved.

From an energetic point of view the concept is promising because it can supply clean energy in the form of both electricity and heat.

For more information contact the CADDET Danish National Team in Tølløse.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.