A STIEFEL project UTO Uvrier case study

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Courtesy of Jakob Stiefel GmbH

The waste incineration plant UTO Uvrier is situated in the south western part of Switzerland. The Association which controls the plant was established in 1969 and treats waste from 44 municipalities with a combined population of 170,000 inhabitants.

The principal task of UTO Uvrier is the thermal heat recovery from urban and industrial waste. Apart from this, UTO also treats other types of waste like the composting of sludge, the sorting and crushing of bulk goods, as well as the conditioning and temporary storage of meat waste. Furthermore UTO manages the accumulation and sorting of glass and is also the official collection point for electrical and electronic equipment waste. It operates a collection point for hazardous household waste. Additionally, UTO produces wood pellets for the regional household market.

In 2003, the Swiss Law prohibited the use of sludge compost as fertilizer in farming; This provided the opportunity for UTO to co-incinerate the sludge in the two waste incineration lines. However incinerating the sludge in the two existing lines had a knock on effect of incinerating emissions and caused a reduction of reliability in the two furnaces.

As a consequence, the association concluded to construct a new combustion line with an integrated sludge combustion system. Its concept was based on a combination of conventional grate technology and a rotary kiln for compressed sludge. This lead to savings of not only capital investments in the required plant technology but also of running costs. Due to their innovative solutions, Jakob Stiefel GmbH was chosen as the main contractor.

Construction of the New Replacement Line
The existing combustion furnaces with a capacity of 3.5 t/h respectively 4.0 t/h were built in 1971 and 1974. Due to the extraordinary maintenance by the 27 employees, both lines had been in operation for almost 40 years.

The new unit consisted of a grate combination system, a steam generator and rotary kiln to replace both of the existing lines/units. The water cooled grate is the core component of STIEFEL’s innovative combustion system. A completely water cooled forward moving grate and an advanced combustion control system enabled an optimized combustion of various waste qualities and types.

The generated heat is used for steam production. 4 MW of electricity are produced by a condensing turbo generator. An electrostatic precipitator and a four stage flue gas treatment system enables an efficient cleaning.

Water Cooled Forward Moving Grate
The single track, forward moving grate consists of 24 grate steps and is divided into 5 primary air zones. The air for each zone is supplied by a frequency controlled fan. For this reason, each zone can be supplied with the required amount of primary air, without using any flaps or dampers. This innovative concept is characterized by stable operating behaviour and very low emissions.

Sewage Sludge Incineration in a Rotary Kiln
The waste incineration combined with combustion of sludge in a rotary kiln is a rarely used process on economic grounds. Hot gases with a temperature of 900 °C are taken from the first boiler pass and then fed through the rotary kiln. It operates in counter flow direction; gases exit the rotary kiln where the mechanically dewatered sludge enters. The sludge dries in the first zone of the rotary kiln, in the second zone the dried material ignites and incinerates. The emerging cooled down gases are fed back into the waste incinerator by a ventilator for complete combustion. The water filled ash discharger cools the ashes and the slag before they are discharged onto a conveyor belt.

In comparison to systems where the sludge is directly fed into the waste combustion, the rotary kiln system features the following advantages:

  • Both systems can be individually optimised.
  • The flue gases of the rotary kiln show the remarkable effect of denitrification (NOx reduction). Hence the operating costs of the DeNOx-plant are reduced significantly.
  • The ashe of the sludge is disposed together with the ashe of the waste incineration or is available for phosphorus recovery.
  • Mechanical draining of the sludge is sufficient, no pre drying is needed. No risks for the dry sludge in respect of dust and smouldering fire.
  • In comparison to a system with an independent sludge incineration, all components connected after the combustion chamber such as heat recovery, dust removal, flue gas purification and denitrification are not necessary.

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