R. P. Gupta

Impact of Mineral Impurities in Solid Fuel Combustion

Keynote Papers. Section 1: Mineral Matter, Ash and Slag Characterisation. Section 2: The Use of Low-Rank and Low-Grade Coals and Cofiring. Section 3: Case Studies in Conventional and Advanced Plant. Section 4: Studies at Rig Scale (Including Corrosion). Section 5: Developments in Advanced Coal Technologies. Section 6: Modelling.

Radiative heat transfer in pulverized-coal-fired boilers: Development of the absorptive/reflective character of initial ash deposits

Emission Fourier transform, infrare (FTIR) spectroscopy data provide in situ, time-resolved, spectralemissivity measurements for ash deposits generated from two U.S. Powder River Basin coals. The first 3h of deposit growth on a tube in cross flow in a pilot-scale furnace detail the development of surface emissivity with time. Measured emissivities vary significantly with wavelength, indicating the influence of the physical properties and chemical composition of the deposit. At long wavelength (>7 μm), emission features exhibit characteristics of silica, sulfates, and silicates. The spectral emissivity measured in this region approaches a steady value due to an increase in deposit thickness and the size of particles in the deposit. In contrast, deposits are not opaque at shorter wavelengths where the measured emissivity is influenced by the properties of the underlying metal surface. Theoretical predictions of the emsivity of a particulate layer were performed, and results are compared to the measured values. The theory adequately predicts the general features of spectral variation of the emissivity. The predicted trends in emissivity with particle size and deposit composition are also consistent with exerimental observations. Total (Planck-weighted) emissivities are calculated from the measured spectral values for the deposits at the tube temperatures. They increase with time from the clean tube value (0.2–0.3) to values typicals of deposits formed from western U.S. coal (0.45–0.55). Calculated total absorptivities are found to be lower than the correspoding emissivities.

System Accuracy for CCSEM Analysis of Minerals in Coal

It can be concluded from this international cooperative study that the main factor affecting the accuracy of CCSEM analysis is the coal sample preparation technique. The differences of the CCSEM systems including their operating conditions only play a minor role to account for the discrepancy of CCSEM measurements, as different laboratories gave similar CCSEM results on the same coal sample stub. The performance indices related to ash deposition determined from the CCSEM analyses from BYU and ATC were found to be similar.

The Thermal Conductivity of Ash Deposits

The thermal conductivity of granular or slag type deposits can be determined by correlations using two phase medium equations. At room temperature, the effective thermal conductivity appears to be independent of pore size or particle size. The influence of particle size or pore size is significant at higher temperatures when radiative transfer becomes important. Most of the equations for determining the radiative contribution indicate this transfer to be directly proportional to particle size and to the cube of the temperature of the deposit.

Oxy-fuel heat transfer characteristics and impacts on boiler design

Abstract: An outline of the basis of radiative transfer which dominates furnace heat transfer for coal-fired oxy-fuel furnaces is presented and illustrated by comparisons of the significance of the radiating species influencing heat transfer, and also differences between air-fired and oxy-fuel fired furnaces. Heat balances and heat transfer for three different oxy-fuel furnaces of 1.2 MWt, 30 and 420 MWe are given by a simple well-stirred reactor model and also comprehensive computational fluid dynamics models. It is concluded that oxy-fuel combustion can be operated at the same heat transfer rate as air-firing, but results in operational changes – lower volumetric gas flow rate, lower adiabatic flame temperature and lower flue exit gas temperature – associated with the higher gas emissivity of oxy-fuel firing and different heat capacity of the flue gas.