Hongwei Wu

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part I. Volatilisation of Na and Cl from a set of NaCl-loaded samples

Abstract A set of NaCl-loaded coal samples was prepared by physically impregnating NaCl into a Victorian (Loy Yang) brown coal. This set of brown coal samples was pyrolysed in a thermogravimetric analyser and in a novel fluidised-bed/fixed-bed reactor. The latter reactor has some features of both a fluidised-bed reactor and a fixed-bed reactor. The reactor configuration allowed the volatilised Na to be swept away by carrier gas from the bed of char particles, avoiding the re-condensation of the volatilised Na on the char particles at lower temperatures. The volatilisation of Na and of Cl during pyrolysis was quantified simultaneously. The results indicated that a significant proportion of Cl could be volatilised at temperatures around 200°C. The volatilisation of Cl increased drastically with increasing temperature, from 200 to about 500°C. At higher temperatures with a fast heating rate, Cl could interact with the nascent char to be retained in the char. The volatilisation of Na followed a different trend from that of Cl and increased monotonically with increasing temperature. The loading of NaCl into the brown coal had negligible effects on the total volatile yields and on the volatilisation of Mg and Ca during pyrolysis. It is concluded that NaCl in the brown coal was mainly released as Na and Cl separately rather than as NaCl molecules. Reactions involving radicals play important roles in the volatilisation of Na and Cl.

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IV. Catalytic effects of NaCl and ion-exchangeable Na in coal on char reactivity☆☆

Abstract The purpose of this study is to investigate the catalytic effects of Na as NaCl or as sodium carboxylates (–COONa) in Victorian brown coal on the char reactivity. A Na-exchanged coal and a set of NaCl-loaded coal samples prepared from a Loy Yang brown coal were pyrolysed in a fluidised-bed/fixed-bed reactor and in a thermogravimetric analyser (TGA). The reactivities of the chars were measured in air at 400xa0°C using the TGA. The experimental data indicate that the Na in coal as NaCl and as sodium carboxylates (–COONa) had very different catalytic effects on the char reactivity. It is the chemical form and dispersion of Na in char, not in coal, that govern the catalytic effects of Na. For the Na-form (Na-exchanged) coal, the char reactivity increased with increasing pyrolysis temperature from 500 to 700xa0°C and then decreased with pyrolysis temperature from 700 to 900xa0°C. The increase in reactivity with pyrolysis temperature (500–700xa0°C) is mainly due to the changes in the relative distribution of Na in the char matrix and on the pore surface. For the NaCl-loaded coals, when Cl was released during pyrolysis or gasification, the Na originally present in coal as NaCl showed good catalytic effects for the char gasification. Otherwise, Cl would combine with Na in the char to form NaCl during gasification, preventing Na from becoming an active catalyst. Controlling the pyrolysis conditions to favour the release of Cl can be a promising way to transform NaCl in coal into an active catalyst for char gasification.

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part II. Effects of chemical form and valence

Alkali and alkaline earth metallic (AAEM) species (Na, Mg and Ca) exist in Victorian brown coal mainly as carboxylates forming a part of the coal organic matter or as dissolved salt (NaCl) in the coal moisture. The experimental results in this paper show that the chemical and/or physical form of sodium in the brown coal is an important factor influencing the volatilisation of sodium during pyrolysis. Significant amounts of light species containing carboxyl or carboxylate groups such as formate, acetate and oxalate were found in the volatiles from the pyrolysis of the brown coal. It is believed that the release of AAEM carboxylates is an important mechanism for the volatilisation of AAEM species, particularly at low temperatures (<600°C). The carrier gas flow rate passing through the coal bed can greatly affect the volatilisation of AAEM species through this mechanism. Another mechanism for the volatilisation of AAEM species is the breakage of bonds between AAEM species and char matrix at high temperatures. Under our experimental conditions, the sodium in the form of NaCl in the coal substrate seems to volatilise more easily than the sodium in the form of carboxylate in the coal substrate. The monovalent species (Na) is volatilised much more easily that the divalent species (Mg and Ca) during pyrolysis.

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part III. The importance of the interactions between volatiles and char at high temperature

Abstract A novel two-stage fluidised-bed/fixed-bed reactor was designed to investigate the effects of volatile-char interactions on the volatilisation of alkali and alkaline earth metallic (AAEM) species during the pyrolysis of Victorian brown coal at 900xa0°C. With the two-stage reactor configuration, the AAEM-free volatiles generated from the pyrolysis of the H-form coal in the fluidised bed came into direct contact with the char from NaCl-loaded or Na-form coals in the fixed bed. The results indicated that the interactions between the volatiles, especially free radicals in the volatiles, and the char particles enhanced the volatilisation of Na from the char drastically. However, such radical–char interactions resulted in little volatilisation of Mg and Ca, indicating the importance of valence of the AAEM species. The degree of the volatile–char interactions was also related to the ageing of the char and the chemical form of AAEM species in the coal substrate. The volatiles interacted more strongly with the nascent char than the aged char, indicating that the AAEM species existed in the aged char in more stable forms than in the nascent char.

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part V. Combined effects of Na concentration and char structure on char reactivity

Abstract A set of NaCl-loaded Loy Yang brown coal was pyrolysed in a thermogravimetric analyser between 600 and 900 °C. The char sample after pyrolysis was cooled down directly for in situ reactivity measurement with air. The results indicated that the volatilisation of Na during pyrolysis is an important reason for the existence of catalyst loading saturation level with Na as a catalyst in char because the char prepared at high temperature had a limited holding capacity for Na. Under the experimental conditions in this study, the char reactivity showed good linear correlation with the Na concentration in the reacting char. Peak pyrolysis temperature, affecting the release of Cl and distribution of Na in char, is an important factor governing the correlation between the char reactivity and Na concentration in char. The catalytic activity of Na is a result of the interaction between Na and char and thus is greatly dependent on the char/carbon structure. At high char conversion levels where the char structure is more inert and highly condensed, the catalytic activity of Na is reduced compared with its activity at low char conversion levels. The catalytic activity of Na depends on the structure of char.