Effect of CO₂ and H₂O on Char Properties. Part 2: In Situ and Ex Situ Char in Oxy-Steam Combustion

Abstract

The physical and chemical properties of H2O (oxy-steam combustion) and N2 (air combustion) are different, which has influence on the char properties in oxy-steam combustion. Measuring in situ char (without any cooling) kinetic parameters is very important, especially at oxy-steam atmosphere. Both in situ and ex situ char properties were investigated in this paper. Ex situ char was obtained at different cooling rates. Char properties were measured by X-ray diffraction, H/C ratio, Raman spectrum, and specific surface area. The effect of steam on in situ and ex situ char combustion kinetics were determined. We found that the combustion reaction rates of different chars are in order of in situ > rapid cooling (103–104 K/s) > medium cooling (10–100 K/s) > slow cooling char (0.1 K/s). The rapid cooling char structure is disordered and has a high active surface area, which results in the high reactivity. In situ char has a shorter burn-out time, a faster combustion rate, and a higher reaction constant. For example, at 903 K, it appears the reaction rate of 0.048 s–1 for ex situ char (Vientiane coal char) in oxy-steam atmosphere (30% O2 + 15% H2O), while that of in situ char is 0.07 s–1. The reactivity of in situ char is higher than that of ex situ char. The activation energy of in situ char (e.g., 20.9 kJ/mol) is much lower than that of ex situ char (e.g., 82.24 kJ/mol) in oxy-steam atmosphere (e.g., 30% O2 + 10% H2O). The reaction reactivity of both ex situ and in situ char increase in oxy-steam atmosphere when compared with those from O2/CO2 combustion.

Effect of CO2 and H2O on Char Properties. Part 3: Semi-Char from Continuous Sampling in a Microfluidized Bed

“…In our previous studies, it was reported that steam indeed influenced the char properties and combustion/gasification reaction rates in oxy-steam combustion. ,,− For instance, coal char is much developed and disordered after pyrolysis at a steam-containing atmosphere . As a result, the char combustion rate is higher at the oxy-steam atmosphere than that at the O 2 /CO 2 atmosphere . Furthermore, regarding the char combustion rate in O 2 /CO 2 /H 2 O, char-O 2 , char-CO 2 , and char-H 2 O reactions were found to compete for the carbon active sites. ,, Even in a high oxygen atmosphere, which means oxygen is excessive for char combustion, char-H 2 O still locally reacts with carbon active sites inside the char particles.…”

“…15 As a result, the char combustion rate is higher at the oxy-steam atmosphere than that at the O 2 /CO 2 atmosphere. 14 Furthermore, regarding the char combustion rate in O 2 /CO 2 /H 2 O, char-O 2 , char-CO 2 , and char-H 2 O reactions were found to compete for the carbon active sites. 8,13,16 Even in a high oxygen atmosphere, which means oxygen is excessive for char combustion, char-H 2 O still locally reacts with carbon active sites inside the char particles.…”

“…In our previous studies, it was reported that steam indeed influenced the char properties and combustion/gasification reaction rates in oxy-steam combustion. ,,− For instance, coal char is much developed and disordered after pyrolysis at a steam-containing atmosphere . As a result, the char combustion rate is higher at the oxy-steam atmosphere than that at the O 2 /CO 2 atmosphere .…”

“…In this case, oxy-steam combustion can save the investment in power plants. While oxy-fuel combustion has been investigated in recent decades, oxy-steam combustion has been attracting attention in recent years. ,− …”

“…In the present study, we will further investigate the effect of CO 2 /H 2 O in recycled flue gas on semi-char properties in oxy-steam combustion using a continuous semi-char particle sampling system. Basically, compared to the fixed bed reactor and drop tube furnace, the MFB reactor shows sufficient mass and heat transfer and is usually applied to the isothermal reactions for kinetic studies. ,− Hence, the MFB reactor with a combination of semi-char particle online sampling technique will be used in this study.…”