On the Effects of High Pressure and Heating Rate during Coal Pyrolysis on Char Gasification Reactivity

Abstract

Effects of pyrolysis pressure on char reactivity remain a poorly understood aspect of the coal gasification process. In an attempt to address this problem, effects of pyrolysis pressure on char structure and reactivity are being investigated. In this paper, chemical reactivities to O2, CO2, and H2O of chars made from three Australian black coals were measured in a pressurized thermogravimetric analyzer, under conditions where chemical processes alone controlled reaction rates. These chars were prepared at a range of pyrolysis pressures, using pressurized flow reactors and an atmospheric pressure tube furnace, to ascertain any effects of devolatilization pressure and heating rate on the chemical reactivity of the resultant coal chars. It was found that while the apparent (as measured) reaction rate can be affected by pyrolysis pressure, the rate normalized to the char surface area (intrinsic rate) is much less affected, because of large effects of pyrolysis pressure on char micropore surface area. This finding was supported by measurements of char carbon crystallite dimensions that were unaffected by pyrolysis pressure increases. These results indicate that effects of pyrolysis pressure and heating rate on char gasification rates are more likely to be due to effects of structure and surface area and (depending on reaction conditions) the consequent effects on diffusion of reactants to the char surface, rather than on the intrinsic reactivity of the coal chars.

Fundamental Investigation of Fuel Transformations in Pulverized Coal Combustion and Gasification Technologies

“…This decrease in reactivity with pressure differs from results reported previously in the literature [8].…”

“…Figure 1: Swelling vs. heating rate at 1 atm [7] Significant pressurized coal pyrolysis research has been performed in low temperature reactors such as Thermogravimetric Analyzers (TGAs) and wire-mesh reactors. Some research has been performed using medium temperature drop tube reactors [8]. The heating rates of these reactors range from 1 to 10 4 K/sec, which are lower than in entrained-flow industrial applications.…”

“…burnout [8,14,16,17]. In the results presented here, the entire TGA reactivity curve as a function of burnout is presented.…”

Effect of operating parameters on coal gasification

“…2.6 Effect of pressure on yield products during coal gasification Sha et al (1990), Roberts et al (2003), Canel et al (2005) and Yu et al (2007) have investigated the effect of pressure on yield products during pyrolysis and gasification. Sha et al (1990) and Roberts et al (2003) have reported that the pyrolysis pressure of coal affects the morphology of the produced char.…”

“…Sha et al (1990) and Roberts et al (2003) have reported that the pyrolysis pressure of coal affects the morphology of the produced char. It is also reported that with the increase in pyrolysis pressure tendency of char to become sponge is increased due to opening of large void areas and accessible porosity (Roberts et al 2003). Sha et al (1990) reported that the char reactivity is decreased when it is produced by high pressure pyrolysis.…”