Development of a Reactivity Test for Coal-Blend Combustion:  The Laboratory-Scale Suspension-Firing Reactor

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  • Development of a Reactivity Test for Coal-BlendCombustion: The Laboratory-Scale Suspension-Firing

    Reactor

    D. Peralta, N. P. Paterson,* D. R. Dugwell, and R. Kandiyoti

    Department of Chemical Engineering and Chemical Technology,Imperial College (University of London), London SW7 2BY, U.K.

    Received June 14, 2001. Revised Manuscript Received October 27, 2001

    A laboratory-scale technique has been developed for assessing changes in the combustionreactivity of coal blends with blend composition. The test consists of the incomplete combustionof a batch of coal particles in a novel suspension-firing reactor. Residual chars are analyzed forcomposition and reactivity by nonisothermal thermogravimetry. Results from the bench-scalecombustor were compared with observations from a full-sized power station and a single-burnerpilot installation. Using the same sets of coals, the suspension-firing reactor gave the same orderof reactivities as that observed in larger-scale equipment. Systematic preferential combustion ofthe higher-reactivity coal in blends has been identified; the extent of preferential combustionhas been quantified from the thermogravimetric profiles of the residual chars. Our findingsindicate that the reactivity of residual coal-blend chars is determined by the extent of enrichmentof the lower-reactivity component in the blend. The coupled use of the new suspension-firingreactor and nonisothermal thermogravimetric analysis provides a powerful tool in comparingthe combustion performance of different coal blends. Initial indications are that this simple systemcan be used as a predictive tool.

    Introduction

    Coal-blend combustion is increasingly being seen asan attractive route for alleviating problems of fuelselection for pulverized-fuel (pf) power stations. Blend-ing introduces greater fuel flexibility and provides a wayof minimizing costs, e.g., by the use of several lower-grade coals to achieve desirable average blend proper-ties. Coal blending may also provide a useful approachto controlling pollutant emissions, e.g., by mixing high-sulfur and low-sulfur coals to limit SO2 emissions toacceptable levels.1

    Although some utility companies have already gainedoperating experience with coal blends in pulverized-fuelcombustors, the underlying mechanisms are still poorlyunderstood. Problems have been reported in plant trials,including high levels of unburned carbon in fly ash,flame instability, increased slagging and fouling, COemissions, and plume opacity. The success of coal-blendfiring seems to be strongly dependent on individualplant operators experience1. To date, there has been noaccepted test method for assessing or predicting thepotential performance of blends, other than in large-scale combustion trials.

    The outcomes of coal-blending trials are not straight-forward to predict. When higher- and lower-reactivitycoals are mixed, higher local temperatures and moreintense radiative heat transfer would result from the

    more rapid combustion of the higher-reactivity compo-nents. One likely outcome would be the assisted ignitionand the more complete combustion of the less reactivecoals/chars. However, it is also possible to speculate thatfaster combustion of higher-reactivity components mightinduce partial oxygen starvation of the lower-reactivitycoals/chars. Outcomes of plant-based coal-blend com-bustion trials suggest that the latter may be thepredominant effect; declining burnout rates appear tobe particularly pronounced when low-NOx technologiesare employed.2

    Three types of laboratory-scale devices have previ-ously been used to study the combustion of coal blends:thermogravimetric analyzers (TGA), drop-tube furnaces,and a bomb-calorimeter-based test. TGA-based coal-blend combustion studies have shown additive3 as wellas synergistic4 (i.e., non additive) effects. In the case ofbinary blends, additivity of behavior may be identifiedby the presence of two independent peaks in the TGAsignal. Possible synergistic effects were indicated by thenonadditivity of certain characteristics, such as igni-tion temperature and burnout time. It has also beenclaimed that the initial ignition temperature of anonisothermal TGA method could be correlated with thecombustion efficiency of a research boiler. The resultscould be used as an empirical indicator of the relative

    * Corresponding author. E-mail: n.paterson@ic.ac.uk.(1) Carpenter, A. M. Coal Blending for Power Stations; IEA Coal

    Research: London, 1995; pp 11-14.

    (2) Smart, J. P.; Nakamura, T. J. Inst. Energy. 1993, 66, 99-105.(3) Artos, V.; Scaroni, A. W. Fuel 1993, 72, 927-933.(4) Rubiera, F.; Fuente, E.; Arenillas, A.; Pis, J. J. In Prospects for

    Coal Science in the 21st Century; Li, B. Q., Liu, Z. Y., Eds.; ShanxiScience & Technology Press: Taiyuan, China, 1999; Vol. 1, pp 531-534.

    404 Energy & Fuels 2002, 16, 404-411

    10.1021/ef010127p CCC: $22.00 2002 American Chemical SocietyPublished on Web 01/17/2002

  • combustion characteristics of coals and coal blends whenlarger tests were difficult to do.5

    In drop-tube furnaces, the combustion performanceof coal blends is additive.3,6 This result is expected, asnormal drop-tube operation does not allow high enoughparticle densities. In sparse particle density environ-ments, particles appear to burn independently, inconfigurations where volatile clouds of different particlesdo not necessarily overlap.

    In a more recent investigation, a standard bombcalorimeter has been used to investigate the combustionof coal blends.7 Coals and coal blends were partiallycombusted in the bomb, using lower oxygen pressures(than the usual 30 bar), where volatiles and charparticles can interact in the reaction zone. Two sets ofcoal blends previously combusted in a single-burnerpilot plant (U.K.) and in a power station (Chile) weretested in the bomb calorimeter. Relative orders ofreactivities observed in the bomb calorimeter were foundto reproduce trends observed in the larger-scale trials:the degree of burnout of blends followed the trendspreviously obtained in both the pilot plant and powerstation trials. The preferential combustion of the higher-reactivity coal in blends was identified. However, themethod did not perform as expected when testing high-swelling coals; in the face of partial melting of thesample, uniform distribution of oxygen could not bemaintained throughout the calorimeter crucible. As inthe case of the crucible test for volatile matter deter-minations, the method does not attempt to reproducethe hydrodynamic conditions of a pf burner. Neverthe-less, it provides a readilysand commerciallysavailablebench-scale test for estimating relative reactivities ofcoal blends, which plant operators could use for plan-ning and preparing their feedstocks.

    The present paper describes the more recent develop-ment of a novel laboratory-scale method based on asuspension-firing reactor, for analyzing the combustionof coal blends under conditions relevant to pulverized-fuel combustion. The reactor provides some of theoperating conditions typical of pf burners that arerelevant to blend combustion: fast heating rates in awell-defined reaction zone, where coal particles (sus-pended in the oxidizing gas) interact with evolvingvolatiles during combustion. The design has evolvedfrom an earlier apparatus constructed to investigate thedecomposition of limestone and fuel burnout in precal-ciners of cement works.8 A similar reactor configurationis currently being used to investigate toxic trace-elementreleases during cofiring of coal and biomass.9

    Experimental Section

    Description of the Suspension-Firing Reactor. Figure1 presents a schematic diagram of the suspension-firingreactor. The reactor is made of quartz (5 cm ID, 115 cm long).

    The top and bottom chambers are electrically heated using twoindependent coil heaters (1 and 2 kW, respectively) to amaximum of 1000 C. The bottom chamber of the reactorserves to preheat incoming air, prior to delivery to the (top)combustion chamber through a constriction. The narrow entryserves to increase the inlet velocity of air into the upperchamber and prevents sample coal particles from falling down,out of the reaction chamber. During an experiment, a batchof coal particles (fed from the top) is suspended in preheatedair and partially combusted in a cloud of released volatiles(1000 C, atmospheric pressure). Visualization tests at roomtemperature, with the air flow scaled up to simulate theconditions at high temperature, have been carried out toensure that coal particles are in suspension during theexperiments. These tests were done using an air flow equal to1.5 times that used in the high-temperature experiments soas to provide the same particle terminal velocity at bothtemperatures.

    Nitrogen is used to transport fuel particles through aslightly pressurized plug valve (0.1 bar above atmosphericpressure) and a water-cooled probe into the reactor. The probeis cooled to avoid the premature oxidation of coal. A sintereddisk holds the thermocouple holder and the cooled-probe inplace; it also serves as a screen, preventing particles from beingpushed out of the reactor from the top.

    The experiment itself is based on determining extents ofchar survival for different coal blends, under conditions ofincomplete combustion. This is achieved by reversing thedirection of flow and flooding the reactor with gaseousnitrogen, after a preset period (2-5 s) following fuel injection.Two three-way solenoid valves (valves 1 and 2 in Figure 1)switch automatically and nitrogen floods the reactor throughvalve 1, sweeping the particles out of the heated zone.Eventually, particles are trapped in the ash collector; thenitrogen stream leaves the reactor through valve 2.

    (5) Pisupati, S. V.; Scaroni, A. W. In Proceedings of the 9thInternational Conference on Coal Science; Zi