Behaviour of bed material during fluidized bed gasification: the effects of mineral matter distributions

Behaviour of bed material during fluidized bed gasification: the effects of mineral matter distributions

Conference Abstracts to a modified Mettler balance. The gas flow of a mixture of O,+N, directed through the sample bed. A quartz probe placed -2cm a...

175KB Sizes 0 Downloads 34 Views

Conference

Abstracts

to a modified Mettler balance. The gas flow of a mixture of O,+N, directed through the sample bed. A quartz probe placed -2cm above the bed of narticles oermits us to follow. durine the combustion of the char, the’evolutio; of the main gaseous species and trace species as NO, NO,, N,O and SO, through FT-i.r. spectroscopy. Among other results we determine that, increasing the pressure decreases sharply the maximum concentration of all the species analysed. This decrease is very strong between 0.1 MPa and 0.6 MPa and smoother between 0.6 MPa and 2.1 MPa. A correlation between reactivity and evolution of trace gases is given. This work was supported by Commission of the European

Behaviour of bed material during fluidized bed gasification: the effects of mineral matter distributions

Communities, grant no. JOUF 0052.

The British Coal Topping Cycle is one of a number of advanced coal conversion technologies for power generation currently under development. The process is designed to give improved conversion efficiencies compared to pf combustion, with reduced emissions. A key element of the process is the pressurized fluidized bed gasifier, which produces a low calorific gas for combustion in a gas turbine, and a residual char which is burnt in a circulating fluidized bed combustor. The process is designed to take a wide range of coals of varying ash composition with minimal coal preparation. In order to establish the process flexibility it is necessary to be able to predict an operating window for the range of coals which may be used. This study is concerned with the mineral matter interactions which may occur in the gasifier for coal with varying amounts and proportions of clays, quartz, pyrite and limestone. Several coals and their high temperature ashes have been size fractionated and the chemical composition and the mineral matter of each size fraction determined. When particle size distributions for each coal were compared to the size distribution of the respective high temperature ashes, an increase in the proportion of fine particles was observed. This indicates that the larger coal particles produce many finer ash particles. A chemical analysis of the ash from each size fraction shows a decrease in SiO, content with increases in Fe,O, and CaO as the size fractions decrease. Thus potential fluxing oxides are concentrated in the finer ash fractions. A normative analysis for the mineral matter in each coal fraction highlights the importance of the size distribution of pyrite in the original coal, which is concentrated in the finer fractions. Bed material from the test facility has been examined optically and with a CCSEM fitted with an EDS probe. The bulk of the bed material consists of char and large (l-3 mm) clay derived particles. Agglomeration tests have shown that an initial bond between particles is formed by the adherence of a thin layer of a partially oxidized iron sulfide which coats the surface of the aluminosilicate particles, allowing one particle to bond to another. Further reactions may lead to the formation of an aluminosilicate rich in iron oxides. However only traces of liquid phases seem necessary to form cohesive agglomerates. The study has also shown the effects which part washing of a run of mine coal may have on the mineral matter distributions and subsequent effect on the particle interactions. The results suggest that run of mine coal may be less problematic than part washed coal due to a smaller proportion of fine pyrite in the finer sized fractions and relatively more clays in the coarser sized fractions.

was totally

Session

3: Effects

of Mineral

Matter

Combustion and fate of mineral matter Klaus R. G. Hein Technical

University,

Delft, The Netherlands

Recent political developments in the Middle East have led world-wide to an increased awareness of the dependency on oil imports. Also, the decaying availability of cheap fuels has forced the majority of countries either to reconsider their own primary energy resources or to initiate an intensive search for their own possible resources in order to satisfy their future needs for electrical power generation, as well as to serve their industrial and municipal purposes. In particular, solid fossil fuels of a variety of origins, ranks, and/or grades are available in most parts of the world but were until quite recently not competitive with fuel oil or its derivatives. The various solid fuel types differ from each other in the appearance of the originally organic carbonaceous material and their degree of purity. Common fuel impurities are moisture and solids, sometimes both in considerable amounts. Due to the geologically very different situation of the various coal mining areas and limited fuel blending facilities strong variations in boiler fuel composition, in particular with regard to the mineral matter and some organically bonded components, are common which can lead to severe deposit formation on heat transfer surfaces inside the boilers. This may result in considerable negative effects on operation and, hence, on the availability of the plants. In order to alleviate this problem, many years’ research has been carried out world-wide, in which characteristic fuel components and combustion parameters have been determined. Therefore, this paper will survey the main results of small- and large-scale research and also describe recent findings with special emphasis on the effect of various key impurity components for typical time/temperature histories in boiler operation.

Tracing emissions from coal-fired power plants with enriched rare earth isotopes

J. Williamson, S. S. West and M. K. Laughlin* Department of Materials, Imperial College, London SW7 2BP, UK * British Coal, Coal Research Establishment, Cheltenham GL52 4RZ, UK

Z.-C. Lin, J. M. Ondov and W R. Kelly* Department of Chemistry, University of Maryland, College Park, MD 20742, USA * Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA A definitive technique for tracing primary particulate emissions from high temperature combustion sources using enriched, stable, rare earth isotopes has been developed and field tested. The particulate emissions from a 100 MW(e) coal-fired power plant in the Washington, DC, metropolitan area were successfully tagged with ld5Nd and 14’Nd using a thermal degradation apparatus. Ambient samples were collected with dichotomous samplers at 14 sites along a 72” arc centred 20km downwind from the plant. Fly ash samples were dissolved and the rare earth elements chemically purified using column chromatography and the isotopic composition of the Nd fractions was determined by thermal ionization mass spectrometry using pulse counting detection. The isotopic composition of Nd in coal and air is constant. A measurable enrichment at mass 145 and 148 in the Nd composition is definitive evidence that the fly ash from the tagged source has been collected. The results were in good agreement with Gaussian plume model predictions. Tracer concentrations measured in ambient particles smaller than 0.65 pm corresponded to a signal-to-noise ratio of more than 4500. The experimental and preliminary results demonstrate the feasibility of using enriched rare earth isotopes as intentional tracers over distances of hundreds of kilometres.

Evidence of the reciprocal organic matter-pyrite interactions affecting the sulfur removal during coal pyrolysis Jose I/: Ibarra, Jos& M. Palacios*, Rafael Moliner and Ana J. Bonet Instituto de Carboquimica, CSIC, PO Box 589, 50080 Zaragoza, Spain * Instituto de Catalisis y Petroleoquimica, CSIC, Madrid, Spain In this paper the mutual influences of the organic matter and pyrite concentration on sulfur removal during coal pyrolysis have been studied. Several high-sulfur, low-rank Spanish coals with different organic and pyritic sulfur contents were pyrolysed in a swept fixed-bed reactor at different temperatures (300-800°C) with a heating rate of S’Cmin-‘. The H,S evolved from the reactor was continuously measured by a sulfide selective electrode. Hydrogen disulfide was retained in an antioxidant buffer (SAOB) and the integral response of the electrode was then mathematically derivatized. Graphs of sulfur evolution as a function of temperature were obtained. The graphs showed the presence of two complex peaks centred over 500 and 630°C which were assigned to the decomposition of organic sulfur structures and pyrite, respectively. Coals and chars were also studied by scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDX) and X-ray

Fuel 1993

Volume

72 Number

5

697