Volatilization of Arsenic During Coal Combustion Based on Isothermal Thermogravimetric Analysis at 600–1500 °C

Abstract

The volatilization characteristics of arsenic in different coals at 600−1500 °C were studied on an isothermal reaction system. Through thermogravimetric analysis similar to that used for coal analysis, the mass loss and mass loss rate of arsenic for coal samples were determined. Sequential chemical extraction method was used to measure the mode of occurrence of arsenic in the coals. TG-MS techniques were also carried out to study the relationship between sulfur and arsenic. Results show that the volatilization proportion of arsenic increases with temperature and 53− 99% of the total arsenic in coal is vaporized in the combustion zone at PC conditions (1500 °C). Coals with higher arsenic concentrations (As > 4 μg/g) tend to have larger arsenic volatility proportions than coals with lower arsenic concentrations (As < 4 μg/g) at a given temperature. In addition, three volatility zones with two mass loss rate peaks of arsenic are observed in all coals during coal combustion. Before 600 °C, the evaporation of organic-bound arsenic dominates; then the first arsenic mass loss peak at 800−900 °C is mainly from the decomposition or oxidation of arsenic in sulfide forms. The second peak after 1000 °C is probably generated through the decomposition of arsenates. For As > 4 μg/g coals, the first peak is higher than the second peak due to the larger proportion of sulfide-bound arsenic, while the second mass loss peak of arsenic is higher in As < 4 μg/g coals due to the larger proportion of arsenates. Furthermore, thermodynamic analysis by HSC chemistry6.0 software were carried out to prove that arsenates, like Ca 3 (AsO 4 ) 2 , FeAsO 4 , and Mg 3 (AsO 4 ) 2 , are thermally stable and could only decompose at relatively high temperatures.

Effect of H2O on the volatilization characteristics of arsenic during isothermal O2/CO2 combustion

“…The mixed gas flow rate was maintained at 2.33 L/min throughout the test. Preliminary experiments have verified that the flow rate used eliminate the influence of external diffusion [7,8]. were placed in a corundum boat, and the furnace was quickly pushed along the guide rail to ensure that the coal sample burned in the constant temperature zone of the furnace.…”

“…The volatilization and partitioning behavior of As is influenced by several factors, including the combustion temperature and combustion atmosphere [7][8][9][10][11]. In O 2 /CO 2 combustion, the CO 2 concentration in the furnace can reach 60%~80% or more [12][13][14][15], which is significantly higher than that in air fired combustion.…”

“…Given its nature as a semi-volatile toxic metal, most of the arsenic (As) in coal will volatilize during combustion and be found in fly ash, bottom ash and the gas phase as the flue gas cools down, and some of it will finally be emitted into the atmosphere [4,5]. The release of As is attracting increasing attention because it not only affects several human organs, but also interferes with the immune system [6].The volatilization and partitioning behavior of As is influenced by several factors, including the combustion temperature and combustion atmosphere [7][8][9][10][11]. In O 2 /CO 2 combustion, the CO 2 concentration in the furnace can reach 60%~80% or more [12][13][14][15], which is significantly higher than that in air fired combustion.…”

“…The temperature of the heated tube was controlled by a temperature controller, and the flow of H 2 O was controlled by an injection pump. More details about this testing can be found in our previous work[7,8,21,22].…”

Vaporization model for arsenic during single-particle coal combustion: Model development

“…In this work, the reaction kinetic parameters for the dominant arsenic species, along with the frequency factor and activation energy, were determined through a combination of previous experimental work (through data fitting to establish the vaporization yield model of each arsenic species) [26] and a single-particle combustion model. It is found that the orders of magnitude of the parameters can be obtained with reasonable accuracy allowing for the variations in coal samples and temperature conditions.…”