Simulation of a New Chemical-Looping Combustion Process without Sulfur Evolution Based on Ca-Based Oxygen Carrier

Abstract

Calcium sulfate (CaSO 4 ) is a promising oxygen carrier for chemical-looping combustion system. The release of sulfurous gas in the circulating of Ca-based oxygen carrier is a crucial problem. In this paper, it is found that the released amount of sulfurous gas is greatly affected by the partial pressure of reductive gas. If the partial pressure of the mixed reductive gases, composed of H 2 and CO, maintains higher than 40 kPa, the sulfurous gas can be completely eliminated even at the reacting temperature higher than 1,000°C. Therefore, a new chemical-looping combustion system based on CaSO 4 and NiO oxygen carrier, without any sulfur evolution, is simulated using Aspen Plus software. In the system, the syngas is generated from the steam gasification of coal and reacts with CaSO 4 and NiO consecutively. The sulfur release occurred in the circulating of Ca-based oxygen carriers can be prevented because the concentrations of CO and H 2 keep higher than 45% in the presence of Cabased oxygen carriers. The suitable operating regime, in which auto-thermally operating of the system, zero-sulfur release and easy sequestration of CO 2 from the flue gas are realized, is proposed in this paper. The effects of fuel reactor temperature, air reactor temperature and the recycle rate of oxygen carrier on the released heat from the system and the concentration of CO 2 and H 2 O are also studied in the paper.

In situ gasification Chemical-Looping Combustion of coal using limestone as oxygen carrier precursor and sulphur sorbent

“…It was found that CaS was the dominant compound after CaSO 4 reduction at 900 ºC, whereas the presence of CaO was more relevant at 950 ºC. Further works have also observed the same effect of the temperature on side reactions [27][28][29][30].…”

“…Several authors have evaluated the use of anhydrite ore, a mineral which is mainly composed of CaSO 4 , for CLC [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42]; but not sulphated limestone. Song et al [26] observed that both reduction R1 and side reactions R4 and R7 were enhanced by increasing temperature.…”

“…Some studies have reported that the addition of Fe 2 O 3 to CaSO 4 by impregnation method [31] or mechanically [32,33] or physically mixed [34] may contribute to inhibit undesired side reactions. Tian et al [28,35] observed by thermogravimetric analysis that CaSO 4 presented higher reactivity with H 2 than with CO and that solid reactive products were almost pure CaS when the partial pressure of CO or H 2 was 40 kPa or higher. They proposed to carry out a partial combustion of the fuel in a CLC unit with CaSO 4 and subsequently, the complete combustion of product gases might be performed in another CLC system using nickel oxide as oxygen carrier [28].…”

Coal chemical-looping gasification of Ca-based oxygen carriers decorated by CaO

“…The effect of CaO on the concentration of sulfurous gas during the CLG of coal was evaluated through the following experiments. According to side reactions R6-R13 [19][20][21], sulfurous gasses, such as H 2 S, COS and SO 2 , may be formed during the CaSO 4 reduction process in the fuel reactor. During the experiment, the concentration of SO 2 was so low that these gasses could be neglected (i.e., their concentrations were below the detection limit of 0.001%).…”

“…First, the gasification of coal is considered to be the rate-limiting step, and CaSO 4 oxygen carriers inefficiently catalyze coal gasification. Second, the release of sulfurous gas and the formation of calcium oxide (CaO) are unavoidable during the CaSO 4 reduction process in the fuel reactor due to side reactions R6-R13 [19][20][21]. If CaO is formed, the oxygen transport capacity and heat release of CaSO 4 oxygen carriers in the CLG progress are eliminated.…”