Formation of Calcium Chloride and Its Interaction With the Sand Particles During Fluidised Bed Combustion

Abstract

Fluidised bed boilers are well suited to burning fuels that have diverse characteristics such as refuse-derived fuels and biofuels. These fuels often contain various amounts of chlorine. In many countries, the emissions of HCl from waste material combustion are regulated by stringent laws. In the fluidised bed combustion of sulphur-containing fuels, the addition of a Ca-based sorbent into the bed is a widely used method for controlling SO2 emission. In this work, the absorption of HCl with limestone was studied in the laboratory at high temperatures, 650–850°C, by using a special thermogravimetric apparatus (TGA). In addition to the absorption rates, special emphasis was placed on studying the effects of molten phase formation during chlorination. We have recently reported our first results on the interactions between the calcium chloride being formed and the quartz sand. We proposed a chemical mechanism for the formation of the calcium silicate coatings frequently found on the sand particles in fluidised beds. Here, the proposed mechanism in which calcium chloride acts as a crucial intermediate is more thoroughly discussed.

Hydrogen Chloride Bonding with Calcium Hydroxide in Combustion and Two-Stage Combustion of Fuels from Waste

“…A significant role is played by CaCl 2 , which appears in the process of the chlorine bonding by the calcium-based sorbents. This issue is discussed more extensively in ref . from which the following example reactions are quoted: normalCaCl 2 ( s,c ) + normalSiO 2 ( s ) + normalH 2 normalO ( g ) = normalCaSiO 3 ( s ) + 2 normalHCl ( g ) , goodbreak0em1em⁣ c ( H 2 O ) = 20 % normalCaCl 2 ( s,c ) + normalSiO 2 ( s ) + normalH 2 normalO ( g ) = normalCaSiO 3 ( s ) + 2 normalHCl ( g ) , goodbreak0em1em⁣ normalc ( H 2 O ) = 1 % …”

“…Higher effectiveness of flue gas dechlorination in the two-stage process of SRF1 combustion can be explained by the low-temperature prevailing in the gas generator chamber (650 °C). It is claimed , that temperature is a control parameter in HCl bonding processes by means of calcium sorbents. The mentioned sources suggest that, when it grows above 750 °C, sorption capacity of the sorbents declines, leading to the weakening of HCl bonding reaction.…”

“…In the temperatures between 700 and 900 °C, there is only little reactivity of the sorbent toward HCl. The peak activity of CaO corresponds to the 500–600 °C temperature bracket, whereas, at 1000 °C, the sorbent is assumed to exhibit only minute activity . In the case of the two-stage combustion process, HCl can be secondarily formed in the reburning chamber in the environment of SO 2 , O 2 , and H 2 O, according to the reaction CaCl 2 (l) + SO 2 (g) + O 2 → CaSO 4 (s) + Cl 2 (g) or 2CaCl 2 (l) + 2SO 2 (g) + O 2 + 2H 2 O → 2CaSO 4 (s) + 4HCl­(g).…”

The effect of HCl and steam on cyclic CO2 capture performance in calcium looping systems

“…During this process, H 2 O and CO 2 release from the product layer could create additional porosity, lowering the CO 2 diffusional resistance to the unreacted CaO core. Alternatively, Partanen et al (2003) and Freidina & Fray (2000) considered the CaCl 2 -CaCO 3 product layer possibly forming a molten phase during carbonation at temperatures around 700 o C. This could reduce the diffusion limitations and explain the increased carbonation conversions in comparison to what is seen in Figure 2, where carbonation takes place at 650 o C. To investigate this phenomenon further a more detailed look at each carbonation cycle is required.…”