Process overview

The chemical looping combustion (CLC) process can effectively utilize solid, liquid and gaseous fuels. In the present study, the direct use of solid fuels in the CLC process is performed. The solid fuel based CLC process can be carried out by direct and indirect methods (Figure 2.1). The direct process is further categorized into in-situ gasification process (iG-CLC) and chemical looping oxygen uncoupling process (CLOU) process.

Figure 2.1. Classification of the CLC process.

2.1.1. Indirect CLC method

The indirect method involves two processes such as (i) gasification of solid fuel into

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is used to convert solid fuels into syngas (Eq. 2.1), which is further sent to the CLC system for the metal oxy-combustion process (Figure 2.2). The metal oxides readily react with syngas in the fuel reactor (FR) (Eq. 2.2), and the obtained metal particles in the reduced form are transported to the air reactor (AR) for regeneration.

Gasifier: CoalCO₂/H₂Osyngas (2.1)

Fuel reactor (FR): syngasMeOMeCO₂H₂O (2.2)

Figure 2.2. Schematic diagram of an indirect method in the CLC process.

2.1.2. Direct CLC method

The direct method involves the simultaneous occurrence of both the processes such as (i) coal gasification and (ii) syngas metal oxy-combustion in a single reactor. Chen et al.

(2016) discussed the chemical reactions that occurred between solid fuels and metal oxides. These reactions include (a) devolatilization/gasification of fuel, (b) interaction of metal oxide with syngas and char and (c) homogeneous water gas shift reaction. This method can be carried out in two ways, which are based on the choice of metal oxides.

Air Gasifier

Fuel Reactor (FR)

Air Reactor (AR) CO₂

H₂O Solid Fuel

O₂ depleted air

Ash

Syngas

Oxygen carrier (OC) CO₂, H₂O

LITERATURE REVIEW

As discussed earlier, the direct method is classified into (i) chemical looping with oxygen uncoupling (CLOU), and (ii) in-situ gasification chemical looping combustion (ig-CLC).

Leion et al. (2009) found that the use of oxygen carriers such as nickel, ilmenite, etc.

could result in a low conversion rate of solid fuel due to the rate-limiting step of char gasification reaction. In the CLOU technology, the metal oxides release their oxygen molecule in the gas phase at higher operating temperatures and, this gas phase oxygen burns solid fuels directly. Metal oxides such as Cu, Mn, Co possess the CLOU characteristics.

2 1 y x y

xO 2Me O O

2Me  _  (2.3)

Char Volatiles

Coal  (2.4)

O H CO O

Volatiles ₂  ₂ ₂ (2.5)

2

2 CO

O

Char   (2.6)

This technology is highly efficient due to the direct combustion of solid fuel with the gas phase oxygen. However, the following drawbacks are reported for the CLOU based metal oxides in the literature.

(i) Cobalt oxide is expensive and toxic (Frick et al., 2015).

(ii) Mn-based oxides release oxygen at a lower rate. The re-oxidation of Mn2O3 occurs at lower temperatures where the reaction rate is too slow (Rydén et al., 2011).

(iii) Cu-based metal oxide is expensive and has a short lifetime due to high attrition rate (0.04 wt%/h) (Adanez et al., 2012). Furthermore, Cu has the low melting point (1085℃) that could lead to agglomeration issues in the CLC reactors (Qin et al., 2012).

Another method of the CLC technology is referred to as in-situ gasification technology (iG-CLC). Due to the absence of an additional gasifier, iG-CLC process captures CO at

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relatively low cost (Abad et al., 2018). The solid fuel is gasified by a gasifying medium (steam or recirculated CO2) to convert the char into CO, since char gasification is the limiting step. The produced CO then subsequently gets oxidized by the oxygen carriers into CO2 and H2O. However, an oxygen polishing step is usually required due to the presence of unconverted volatiles and char (Wang et al., 2021). A carbon stripper is usually employed to prevent the transportation of unconverted char from FR to AR (Abad et al., 2015). In this method, sequential chemical reactions such as pyrolysis (Eq. 2.7), char gasification (Eq. 2.8), metal-oxy combustion of syngas (Eq. 2.9) etc., occur simultaneously. As this technology eliminates the use of a gasifier, the operational cost of the CLC process can be reduced. Ohlemüller et al. (2015) and Adanez et al. (2012) explained the iG-CLC reaction mechanism as follows,

Coal (s) →volatiles (g) + char (s) (2.7)

C + H₂O/CO₂ →CO + H₂ (2.8)

Volatiles (g) + MeO (s)→ CO₂(g) + H₂O (g) (2.9)

As discussed earlier, carbon capture and storage (CCS) technologies will reduce the CO2

emission from the industries. However, transport sector for CO2 capture technologies is quite complex. H2 when used as a fuel reduces the CO2 emission. Further, this can be utilized in power generation and for the production of various chemical feedstocks such as ammonia, methanol etc. (Adanez et al., 2012). The conventional way to generate syngas from hydrocarbons is usually through steam reforming (Mattisson et al., 2018).

Steam methane reforming (SMR) was the most widely used for H2 production and requires enormous heat input for H2 purification (Luo et al., 2018). However, CLR has superior characteristics over SMR since the former can capture carbon inherently (Tang et al., 2015). Further, high CO/H2 ratio is achieved using the CLR technique, which can further be used to produce chemicals. Hydrogen production using chemical looping

LITERATURE REVIEW

processes can be categorized into two sections: (a) chemical looping gasification (CLG) and (b) chemical looping reforming (CLR). Figure 2.3 shows the overview of CLR and CLG using solid fuels. The main difference between CLG and CLR is that former process produces syngas from solid fuels (Figure 2.3a), while pure H2 is obtained using CLR as shown in Figure 2.3(b). CLG produces a gas mixtures (CO, H2, CH4 etc), thus requires additional gas separation technique to separate H2. However, CLR concept employs three reactors and produces rice stream of H2. They both follows the same principles as CLC.

However, heat management for CLG and CLR cases will be difficult since they release less heat than the CLC case due to incomplete combustion (Yu et al., 2019).

(a) (b)

Figure 2.3. Schematic diagram of (a) CLG and (b) CLR.