Boiler Tube Deposition - Full scale performance trials have shown that the high

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1 ~ IEA Workshop, Cambridge, June Working Group 2. i.- I'dBORATORY STUDIES INTO THE COMBUSTION OF REFUSE DERIVED FUEL IN CONVENTIONAL STOKERED BOILERS Dr T W A Rampling Warren Spring Laboratory Stevenage, Hertfordshire UK Refuse Deri red Fuel (RDF) is recovered from municipal waste whi h consists of a mixture of paper and plastics with minor quantities of other combustibles such as wood, leather, rubber and textiles, putrescible material and non-combustibles such as glass etc. Over the past 5 years the Department of Energy has been funding combustion trials to test the suitability of conventional boilers for use with RDF. These trials have tested a variety of stokered boilers and fluidised bed combustors. Based on general combustion performance the chain grate stokered boiler has been shown to be one of the most practical units for RDF combustion. RDF contains far more organic and inorganic volatile components than bituminous coals. The volatile organics make combustion of RDF a lot simpler than coal bur, the volatile inorganics are causing problems in terms of boiler tube depositior and emissions to the atmosphere. Warren Spring Laboratory (WSL) has been studying ways of resolving these problems, particularly for the chain grate stokered boiler. The problems and the proposed studies to find ways of resolving them are now given in more detail. Boiler Tube Deposition - Full scale performance trials have shown that the high temperature boiler deposits contain over 90% wt water soluble alkali sulphates and the rate of deposition is such that boiler availability, under standard operating conditions without incorporating any means to inhibit deposition, is around 200 hours in comparison with approx hours for a normal coal. The tendancy for deposition has been shown to be dependent on the alkali metal content of the fuel ash. RDF contains over 10 times as much alkali metal as normal coals. Furthermore, a high proportion of the alkali metals in RDF are present as simple salts, whereas much of the alkali in coal is in the form of non-volatile alumino-silicates. WSL has been studying two possible ways of decreasing the rate of deposition: inhibition of the volatilisation of the alkali metals from the fuel bed and prevention of the condensation of the low melting point salts on contact with the relatively cool heat transfer surface. There are two cycles to the eombustion on a chain grate stoker - ignition and burn-out. Past work has shown that the alkali metals are released during the high temperature burn-out phase. The two main factors that control volatilisation are time and temperature

2 With the assistance of DTI and DOE support procedures and facilities for dioxin determinations at Warren Spring Laboratory have been substantially upgraded. a re-appraisal of extraction, clean-up and analytical methods resulted in for example changes: from a clean-up based on normal phase high performance liquid chromatography to one using open column mixed acid/base impregnated silica - - alumina; from medium to high resolution gas chromatography; and from a magnetic sector mass spectrometer service to a dedicated quadrupole mass spectrometer. Quality control has been greatly strengthened by the use of recovery and syringe internal standards, and by full participation in the BCR dioxin collaborative trials. It is now intended to provide a more extended isomer specific 'screening', makitg use of the injection to results automation now in place. This development is in line with current thinking in the UK which is that a limited range of selected isomer specific determinations provides the best option. The urgea need, hovever, remains to agree which isomers should be chosen. Widening the scope of the trace organic determinations made on combustion products may be a higher priority than obtaining comprehensive data on one group. namely PCDDs and PCDFs. To this end, Warren Spring Laboratory has now put in place the analytical resources for measurements of brominated dioxins and furans, polyaromatic hydrocarbons, and volatile organic compounds. 5. Sampling System Developments In general, pollutant emissions from refuse combustion may be placed in three categories: 1. Gaseous, such as HC1, Sop and NO,. 2. Particulate, incorporating most metals. 3. Intermediate, including mercury and trace organics. Intercomparability data for the most commonly used sampling systems is scarce ard generally of poor quality. With funding from DOE and DTI, some preliminary comparative evaluation of the BCURA and Method 5 (EPA) systems for particulate and dioxin collection has been done at Warren Spring Laboratory. Further work is required to characterise properly differences in the collection efficiencies of these. A priority area is the continuing development of robust heavy metal and HCl sampling systems. In view of the 'unstable' emission characteristics of refuse burning systems, on-site work of sufficient extent is costly, and so co-operative exercises couli be a useful means of sharing cost burdens

3 To islhibit the volatilisation, studies have been undertaken: i) to find a chemical additive to the fuel that reacts with the alkalis to form a more stable alkali compound, ii) iii) to determine operating conditions for "equilibrium burning" where the rate of combustion equals the rate of ignition and the duration of the burn-out is reduced, to test the effects of humidification of the primary air upon fuel bed temperature and the rate of combustion. These studies are being carried out using a standard pot furnace test rig. Several chemicals will be tested but from past studies the additive most likely to react with and retain the alkali metals in the fuel ash is kaolin (Al2O3, 2SiO2, 2H20). The two main operating parameters in chain grate combustion are bed depth and primary air flowrate. Test runs are being carried out on 100% RDF with bed depths ranging from 4 to 10 in. at air flowrates between 100 and 400 lb/h/ft2. Comparative trials have also been carried out with bituminous coal and RDF/coal co-firing. During these tests the following data can be obtained: the rates of combustion and ignition as combustion progresses, the durations of the ignition and burn-out stages, the fuel bed temperatures at various bed heights throughout combustion, the clinker build-up. A comprehensive analysis of the results should reveal the operating conditions that promote "equilibrium burning" and minimise clinker build-up. Results so far indicate that the optimum operating conditions for firing RDF on a chain grate stoker are with a 6 in. bed depth at a primary air flowrate of 275 lb/h/ft2. A proven way of reducing fuel bed temperature and boiler tube deposition is humidification of the primary air. The endothermic water gas reaction will fonn CO and H2 which will then burn above the bed in no contact with the alkali material. The object of the current test programme is to determine the required rate of water supply and the ideal position along the grate to supply it. The mechanism of deposition in RDF burning seems to involve the same steps as in coal burning. The main step is the release of the alkali metals from the fuel bed, which then recondense as a salt onto the cool heat transfer surfaces to form the bulk of the deposits. Bench-scale work at WSL has shown that the temperature at which the alkali salts become '*sticky" and prone to adhere to the metallic surfaces is 650'C. Other routes to inhibit deposition are to reduce temperatures on entry to the heat transfer area to below 6SO'C or to convert the alkiali salts to compounds of a much higher melting point. 100% excess air will be needed to cool the flue gas to 650'C. This is a fairly easy procedure but calculations have shown that there will be a drop of 4 to 5 percentage points in the thermal efficiency of the boiler. Several additives including MgO.Ca0 and dolomite have been tested in a bench scale simulation of the boiler but the temperatures at which the product become "sticky" have only increased by SO to 1OO'C

4 Emission to the Atmosphere - The inorganic material released during combust'ion also causes problems with emission to the atmosphere. The emissions of most concern are the sub-micron particles and HC1 which arises from the combustion of both inorganic and organic chlorinated compounds., RDF contains up to 1% w/w chlorine in comparison with % for most normal coals. Measurements taken during full-scale chain grate stoker firing trials have recorded flue gas levels of HC1 in the range 200 to 500 ppm. Tests are being carried out to find ways of reducing the HC1 content to below the proposed EEC limits. So far these have involved adding chemical compounds to the fuel, but no success has been obtained. There is some correlation between deposition and particulate emission in RDF combustion. Most of the particulates are between 0.1 and 1.0 micron and contain at least 60% alkali chlorides. As discussed, the alkali metals are volatilised from the high temperature burn-out stage of combustion. The conventional inertial particulate abatement units fitted to most combustors are designed to arrest the coarse "fly-ash" particles produced in coal combustion and are unsuitable for arresting the sub-micron fumed particles. The tests WSL are carrying out to find ways of inhibiting volatilisation of metals from the fuel bed relate to particulate emission as well as deposition

5 GROUP 2. DISCUSSION Four areas were regarded as having potential for useful co-operative R&D effort: (a) STANDARDISATION OF SAMPLING, ANALYSIS AND REPORTING Standardisation of sampling and analytical procedures was considered important with respect to existing and planned national programmes. A project was proposed, the initial phase of which would involve compilation of an inventory of existing techniques used on both raw MSW and all effluent streams (i.e. fly ash, bottom ash, water and flue gas). This task was expected to take 2-3 months of effort spread over one year and it was suggested that this should be a secretariat role. This phase of the work could lead on to a more detailed evaluation of procedures (perhaps by "round robin" testing). (b) DEFINITION OF IMPORTANT PARAMETERS IN THE DEGRADATION OF PCDD/PCDFs AND THEIR PRECURSORS An understanding of the important parameters in the degradation of PCDD/PCDFS and their precursors was considered important in terms of providing support to regulators (e.g. in specifying gas residence times at different temperatures), particularly in the setting of design criteria for small units not justifying sophisticated flue gas cleaning systems. Each country was able to identify existing data or relevant R&D programmes. It was suggested that a meeting should be convened in order to explore the scope for co-ordination of national programmes. This could lead to a co-operative effort over 2-3 years at a cost of US $15,000 (the research itself being funded from national programmes). (c) MULTIVARIATE ANALYSIS OF INCINERATION PARAMETERS Work has been undertaken at the University of Umea in Sweden using multivariate analysis to identify the key parameters for optimisation of MSW combustion performance in terms of energy production and emissions quality. It was suggested that this work could be extended by submission of UK and Canadian data to Umea for similar analysis. Approximately 3-6 months of effort would be involved at a cost of US $10,000-20,000, over a one year period. The potential of this analysis technique could be explored by way of a pilot study

6 (d) FORMATION PATHWAYS FOR ORGANO-CHLORINE COMPOUNDS INCLUDING PCDD/PCDFs An understanding of the formation pathways of organo-chlorine compounds was considered fundamental in making decisions on appropriate control strategies for MSW combusters. The aim of this project would be to capitalise on the existing Swedish and Canadian programmes and co-ordinate and extend activity in order to further knowledge on both reactions occurring in the high temperature combustion chamber and catalysed reactions occurring at lower temperatures. The work would involve additional funding, estimated to be of the order of US $60K, by the countries involved, spread over a two year period. An IEA secretariat role could be to co-ordinate input by other interested countries such as the UK. Other projects considered by working group 2, but not recommended for adoption at this stage, were: (e) THEORETICAL MODELLING OF COMBUSTION Programmes on modelling the MSW combustion process were considered as being currently at an early stage of development in the UK and Sweden. (f) INSTRUMENTATION FOR CONTROL AND MONITORING A review of available options for instrumentation was regarded as valuable but was not recommended at this stage. (g) NON-CHLORINATED POLY-AROMATIC HYDROCARBONS This subject area was suggested by an observer country (Finland). It was considered that a review should be conducted in 1989 following further assessment of UK and Canadian (Ontario) interest together with work currently being sponsored by other organisations including the World Health Organisation (WHO). (h) METAL EMISSIONS Exchange of information on RDGD related to metal emissions and control was considered an area to be kept under Secretariat review