BioMatNet Item: ERK5-1999-00004 - Straw gasification for co-combustion in large CHP plants

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ERK5-1999-00004
Straw gasification for co-combustion in large CHP plants

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Agricultural Residues : EESD (Energy, Environment and Sustainable Development) : Electricity : Thermochemical Conversion

Proposal No:	ERK5-1999-00004
Date Prepared:	February 2004
Source:	European Bio-Energy Projects (EUR 20808)

Introduction

Straw gasification technology, including gas cooling and cleaning, was further developed in this project by Energi E2 and Foster Wheeler Energia Oy during 1999-2001. The high alkalinity and chlorine content of straw requires removal of these harmful components from the gas before it is burned in the integrated boiler. Three pilot-scale trials were executed on straw gasification in a 3 MW_th CFB gasifier. Furthermore, filter ash treatment was tested and developed at pilot scale.

In addition to the pilot-scale testing, process validation and design study covered gasification of 100% straw and a fuel mix of straw and wood. In the design study a full-scale straw gasification plant of 100 MW_th and its integration with an existing large CHP plant was investigated. The practical solutions of all unit operations were developed. The budget for a complete plant was calculated and consequently the overall project economy was assessed.

Activities

As a first phase, a test programme comprising four separate test series for straw gasification in a 3 MWth atmospheric CFB gasifier with gas cleaning, was carried out by Foster Wheeler Energia Oy and ENERGI E2. The project also included development of the straw feeders, based on the ideas of, and carried out by TK-Energy from Denmark. The test series consisted of three gasification trials on straw and one on the burning of filter ash in a CFB combustor. Secondly, in parallel with the testing programme, a design study was conducted with a view to creating a decision basis for erecting a 100 MW_th demonstration plant.

The plant designed in the study is a complete plant covering:

straw storage, conveying and preparation facilities;

feeding system for straw;

feeding system for wood chips;

CFB gasifier;

silos and feeding systems for make-up materials;

inert gas system;

syngas cooler;

baghouses for syngas cleaning;

ash handling systems;

ash incineration system;

syngas burners in existing PC boiler;

instrumentation and control; and

powering.

Progress

Pilot-scale tests Four test series were conducted:

Running in of test pilot plant with straw pellets;
Tests with loosely cut straw and a specially designed cutter;
Tests with loosely cut straw; long-term testing with gas cleaning, etc.;
Burning of filter ashes in a low-temperature CFB.

During the three gasification trials, more than 220 tonnes of pelletised and loose straw were gasified during over 400 operational hours. Detailed test reports have been prepared for all test periods. The following conclusions can be drawn from the pilot test programme:

Loose straw gasification is technically feasible;
In spite of high alkaline fuel, smooth, stable operation could be achieved;
Wood and straw may be gasified together and trouble-free operation can be reached;
Carbon conversion was in the range of 95- 97%;
Gas cooler could be kept clean both by soot blowing and by spring hammering;
3 M's filter operated well without blinding by tars, and it also removed also alkalis and chlorides quantitatively at 350-370°C from the synthesis gas;
PAH were formed in the gasification conditions but dioxins and furans were not; and
The optimal gasification conditions were validated in the project.

The actual gasification process with gas cleaning had already proved technically feasible during testing.

Commercial-scale design study In the study, straw price was fixed at #5.6/GJ or #83.1/tonne and the price of wood chip price was #4.3/GJ or #40.2/tonne. The price of saved coal was fixed at #1.3/GJ. The market price of electricity was #21.4/MWh and that of heat #1.7/GJ. The expected CO₂ and SO₂ taxes and biomass incentives were included in the evaluation. After these, the income from sales of electricity would be #53.6/MWh for the first ten years, and #34.8/MWh thereafter. The predicted income from the sale of heat would be #8.2/GJ. The total investment was calculated at M#42.3 and the annual straw consumption would be about 150 000 tonnes. After incineration, the ashes could be recycled back to the fields as -a fertiliser so as to recover the nutrients and minerals.

The design study results indicated that the economy of a 100MW_th gasifier integrated with an existing large CHP plant is sensitive to fuel and energy prices and, at present, biomass fuel which is slightly cheaper than straw should be available for profitable operation in Denmark.

To summarise, it could be concluded that straw gasification is technically feasible. However, with the fuel and energy price levels in year 2001, investment in a 100 MW_th straw-fired CFB gasifier would not have been profitable in Denmark.

Impact and exploitation

The results of the process validation and design study could be the technical and economical basis for a decision to build a demonstration plant and, furthermore, to commercialise gasification and co-combustion technology based on the utilisation of straw as a fuel in the gasifier. Since straw is a major biomass resource in large parts of Europe, this project can help to increase the share of renewable energy in the energy system, and to improve economy and employment in the agricultural sector.