Fundamental aspects of coal-water fuel droplet combustion and secondary atomization of coal-water mixtures. Volume I, final report

Abstract

This Final Report is issued in two volumes, covering research into the combustion of coal-water fuels (CWF). Two separate but related tasks are discussed; the present report, Volume I, contains results obtained under Task 1 - "Fundamental Aspects of Coal- Water Fuel Droplet Combustion." Volume II describes the work of Task 2 - "Secondary Atomization of Coal Water Mixtures," which included experimental measurements in a Spray Test Facility and in the 1-3 MWth Combustion Research Facility.

The processes of devolatilization and char combustion were studied in a laminar flow reactor (LFR) by two experimental procedures. In the first of these, a CWF droplet generator was developed and used to feed CWF droplets directly into the LFR. The CWF droplet generator, which consists of a twin-fluid, internally-mixed atomizer and a series of skimmers to reduce the feed rate of droplets into the LFR, is capable of producing CWF droplets in the size range of 5-500 micrometers at feed rates of less than 3 mg/sec.

In the second parallel study, solid samples withdrawn from a CWF spray flame, close to the atomizing nozzle, were size graded and fed into the LFR in low particle concentrations. Their combustion history in the LFR was determined by monitoring the intensity of radiation emitted by individual particles during combustion (by two-color pyrometry) and by the use of high speed cinematography.

The study has established the importance of rotation induced by the volatile evolution on the break up of coal-aggregates and the release of ash particles. Particle rotation during devolatilization and char combustion generates centrifugal forces at the particle surface which can promote the separation of both weakly adhering char fragments and ash particles.

The results show that there is a competition between centrifugal forces which favors the break up of coal-aggregates and adhesive forces between coal particles during the plastic stage of coal pyrolysis. Based upon the theoretical model of agglomeration, the adhesive force on the process of coalescence of coal particles is strongly dependent on the duration of plasticity of the particles. It is also found that rapid heating reduces the tendency of coal particles to form aggregates during the CWF droplet evaporation. Therefore, whether particles burn individually or as aggregates can be influenced by the temperature history of the CWF particle and hence by burner design.