1. Sources

The present Combustion File is part of the cluster of CFs produced within the literature survey phase of the industrial sponsored research and development project PowerFlam1 and is confidential to the participants registered for that project.

This CF is specifically concerned with the research project:

Billotet, T. (1994): Combustion tests with dried sewage sludge in a coal-fired power plant, which is published within Clean Coal Technology Programme 1992-1994 Vol. II, Combined Combustion of Biomass/Sewage Sludge and Coals: Final Reports, EC-Research Project: APAS-Contract COAL-CT92-0002, Institute for Process Engineering and Power Plant. 18 + 12 pages. Editor – Hein, K.R.G.

2. Background

·         This combustion file is concerned with the provision of combustion related data to the sponsors.

·         In this html file the source of the data is summarised in section 4 below, in order to give the reader a general overview of the way the data was collected.

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3. Abstract

Extensive investigations have been conducted recently in various plants in order to examine the possibilities offered by the joint combustion of bituminous coal and sewage sludge. The additional matter used for these tests is mechanically dewatered or dried sewage sludge. Each result has to be individually evaluated due to the extensively varying side conditions, such as coal qualities, combustion technique, waste-gas cleaning, etc.

The large-scale test series with the combustion of sewage sludge, which have been started in the Federal State of the Saarland in 1993 by the Saarbergwerke AG as the operator of the Weiher II Power Plant, are being conducted within the course of the APAS Clean Coal Technology Programme and are based on a preliminary licence granted for a two-year test period. The sewage sludge used has been subjected to a preparatory treatment in the sewage-sludge drying plant. Drying plant and Power Plant are of compound design.

Power Plant Weiher II consists of two units, each having a capacity of 150 MW_e. Each unit is equipped with two boilers for the combustion of low-grade coal. The dust contained in the flue gas is collected in an electrostatic precipitator (ESP); the gas is then subjected to a desulphurisation process in the wet scrubber before it is catalytically denitrified.

On comparing the contents of the coal burned in this Power Plant with the contents of the sewage sludge, the most extensive deviations can be found with phosphorus, zinc, and cadmium. Sewage sludge also shows increased values of other trace elements as compared to coal.

The problem to be solved by the test programme with the co combustion of sewage sludge was the quantitative assessment of the effects on the emissions, on the power-plant by-products, as well as on the power plant itself, besides establishing an evaluation of the results obtained.

Within the course of these large-scale test series, more than 4000 tons of dried sewage sludge have been burned in one of the four boilers within more than 2000 hours of plant operation. In the various test phases, the portion of dried sewage sludge added to the coal in the boiler was gradually raised from initially two percent to approx. twenty percent. The results obtained with these test series can be summarised as follows.

No operational problems have been encountered with the individual power-plant units on account of the use of sewage sludge

–          Special attention must be paid to problems had in connection with the handling of the dried sewage sludge, such as dust formation, wear and tear, self-ignition and health aspects. After the first few trial runs, a conveyor system has been installed to facilitate a direct transfer of the dried sewage sludge from the drying plant into a beater mill of the power-plant boiler.

–          The flue-gas cleaning process is influenced by the introduction of vapour coming from the adjacent drying plant. The dust collection in the ESP is positively influenced by the increased water-steam content of the flue gas. This reduces the introduction of trace elements into the subsequently arranged flue-gas cleaning plant.

–          Extensive investigations of the ESP ash performed at present are aiming at the attainment of a new test-mark certificate for the utilisation of the ash in concrete.

During power-plant operation with the co combustion of sewage sludge, the granulate showed increased values of phosphorus and zinc. The eluation behaviour of the trace elements did not change. In this respect, the utilisation of granulate coming from sewage sludge combustion is not affected.

Effects on the gypsum and/or the emissions values remain within the normal range found in power plants burning bituminous coal.

4. Synopsis

Data on combustor

Schematic fig.

Stepped slag-tap furnaces for firing low-grade coal

Schematic fig.

Sewage sludge drying plant

Company/Institute

Saarbergwerke AG

Combustion type

Co-firing

Pulverised coal

Main fuel

Low-grade coal (Göttelborn)

Substitute fuel

Sewage sludge (mechanically dewatered, dried)

Fuel data

All: proximate analysis; ultimate analysis; ash fusibility; trace elements; ash analysis;

Experiments

Sewage sludge % in thermal input: 0, 2.1, 2.4, 4.2, 4.4, 7, 9, 10

Results

Operational aspects:

–          handling of dried sewage sludge

–          influence on combustion

Power plant by-products: granulate; fly ash; gypsum; waste water; granulate water

Emissions: trace element; PCDD; PCDF; PCB; PAH

Detailed analysis (data): granulate; fly ash; gypsum; waste water; granulate water; trace element emission;

Comments

Intensive examination of co-firing bituminous coal and dried sewage sludge. Fuel, combustion and power plant by-product analysis. Findings of 20 months test operation.