Char Structural Evolution during Pyrolysis and Its Influence on Combustion Reactivity in Air and Oxy-Fuel Conditions

Abstract

Three kinds of Chinese coal and a biomass were pyrolyzed by N 2 and CO 2 in a bench scale fluidized bed reactor. Fourier transform (FT)-Raman/infrared (IR) spectroscopy was used to identify microstructure and to evaluate the structural evolution of chars generated in N 2 and CO 2 environments, which are the main diluting gases of air and oxy-fuel environments. The Raman spectra were fitted with five Lorentzian bands. The reactivities of the char were measured by a thermogravimetric analyzer from room temperature to 1373 K in air and oxy-fuel conditions with O 2 concentration of 21%. The derived activation energy for different samples was correlated with the Raman structural parameters. Results showed that more disordered char was formed with the pyrolysis in CO 2 than that in N 2 , and new O-containing functional structures would be introduced into the char structure in CO 2 atmosphere. The char structures became less ordered as the sample rank decreased. The reactivity of CO 2 char was higher than that of N 2 char, while the combustion atmospheres rarely affected the char reactivity, indicating CO 2 played a more important role on the devolatization process for coal than for char combustion. The activation energy had a good linear correlation for N 2 char with Raman characterizations, while the data points for CO 2 char were perfectly fitted with exponential functions. INTRODUCTIONO 2 /CO 2 combustion technology now has become a promising way for enrichment of CO 2 in the flue gas to levels as high as 95% by volume to capture CO 2 from the flue gas; 1 at the same time, it can also effectively reduce pollutants such as SO 2 and NO emissions. This novel combustion method has already received general concern all over the world. It has been reported that this technology has a great potential for retrofitting existing coal-fired power plants and building new power plants with several benefits. 2 As combustion generally comprises the process of devolatilization, followed by the combustion of the residue char, the devolatilization process exerts its influence throughout the life of the solid particles from injection to burnout. Therefore, differences in thermal properties and chemical action between N 2 and CO 2 make char combustion in oxy-fuel conditions quite different from conventional conditions. The presence of CO 2 in high concentrations during the initial pyrolysis stage will significantly change the distribution of alkali and alkaline earth metallic (AAEM) species. Then, these changes in the resulting char will substantially alter the coal reactivity and influence the combustion efficiency. 3 Scala et al. 4 found that, in the process of oxyfiring conditions, CO 2 gasification contributes to a comparative extent for carbon consumption at high temperatures and low oxygen concentrations. In addition to the different gas properties, the influence of CO 2 on the reactivity of coal chars is strongly dependent on the raw coal properties. 3,5 However, previous research has rarely examined the effects of structural var...

Effects of pyrolysis atmosphere on the porous structure and reactivity of chars from middle and high rank coals

“…The mass loss was higher for the coal sample of lower rank (HVBC) than for the higher rank (SA) sample. Additionally, the mass loss was higher in the CO 2 atmosphere than in the N 2 atmosphere, concordant with the results of other research (Su et al, 2015;B. Wang et al, 2012;Zeng et al, 2008).…”

“…non-pure pyrolysis in CO 2 ) is scarce and limited to particle sizes of a few micrometers (Angeles G. Borrego & Alvarez, 2007;Su et al, 2015;B. Wang et al, 2012;Zeng, Hu, & Sarv, 2008).…”

Changes in nascent char structure during the gasification of low-rank coals in CO2

“…The quantitative analysis of catalysts is relatively easy but the direct quantification of char structural features is a challenge. Raman spectroscopy has been used as a powerful technique to obtain information about the carbon skeleton in char during the pyrolysis/gasification of low-rank coals [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] and biomass 27]. The reaction pathways between gasification in O2, CO2 and H2O are different at 800 °C [9,18,19].…”

A study on high‐temperature co‐gasification reactivity characteristics and kinetics analysis of Hami coal and its liquefaction residue

“…[33][34][35][36] The higher I D3 /I G+D2+D3 value indicated the more amorphous carbon structure in tested samples. Thus, the D 1 band/G band area ratio (I D1 /I G ) and the G band/integrated bands area ratio (I G /I All ) were commonly used as quantitative parameters to assess the order degree of carbon structure of the tested samples.…”

“…The D 3 band mainly corresponded to the amorphous carbon structure vibration, so the area ratio of the D 3 band to the total of the G band, D 2 band, and D 3 band (I D3 /I G + I D2 + I D3 ) was commonly employed to characterize the amorphous carbon structure. [33][34][35][36] The higher I D3 /I G+D2+D3 value indicated the more amorphous carbon structure in tested samples. The Raman band ratios (i.e., I G /I All , I D1 /I G , and I D3 /I G + I D2 + I D3 ) of different samples are listed in Table 4.…”