Studies on Direct Chemical Looping Combustion of Coal with Rice Straw using Electronic Waste Based Oxygen Carriers for Clean Energy Production

Abstract

Chemical looping combustion (CLC) is a promising technology paving the way for inherent CO2 capture. As Indian coals are either enriched in ash content or have high sulphur, their utilization in the CLC technology under direct fuelled conditions is challenging. Alternatively, the blending of biomass with these coals can enhance fuel reactivity. In this context, the present study is focused on the co-utilization of Indian coals (mainly varying in ash content) and rice straw (RS) in the CLC process using low cost metal oxides in a fixed bed reactor under CO2 based in-situ gasification mode of operation. A high ash coal (HAC) with 33% ash and a low ash coal (LAC) with 3% ash are used with RS. The intrinsic reactivity of ash, char, and volatile matter of the solid fuels with the oxygen carriers are assessed individually.

To commercialize the CLC based carbon capture technology, low-cost oxygen carriers are essential for economical operation. Electronic waste contains enormous metal components, which can be a potential source for the production of oxygen carriers. Thus, mixed oxygen carrier particles obtained from a printed circuit board (PCB) based electronic wastes are used in the present study. A series of thermochemical processes such as pyrolysis, gasification and combustion are conducted for the extraction of metals from the PCB board. The performance of the oxidized PCB based oxygen carrier (OPCB) is compared with commercial Fe2O3. The XRF analysis showed that the obtained OPCB contains 21% Fe2O3, 22.8% CuO and 3% NiO, 9.6% CaO and 33.6% inert supports (Al2O3, SiO2, TiO2). The experimental results showed that by using OPCB oxygen carriers, the CO2 yield has increased by 4.6% for LAC, 3.9% for HAC, and 4.2% for RS, compared to the commercial Fe2O3 metal oxides. This is due to the chemical looping oxygen uncoupling nature (CLOU) of CuO, which releases oxygen molecules combusting the fuel directly. Further, the generated OPCB based metal oxides have a higher surface area of 7.11m2/g, leading to higher reactivity with fuels, compared to the commercial Fe2O3 particles having a surface area of 2.52 m2/g.

The co-combustion of RS with HAC using commercial Fe2O3 metal oxides in the CLC process enhanced the CO2 yield from 81.2% to 86.3%. Similarly, using OPCB with the blend of HAC-RS, the CO2 yield is increased from 86.1% to 90.9% in the blend. Further, the char conversion is increased from 89.6% (HAC-OPCB) to 93.2% for the HAC-RSOPCB blend. The interaction between the coals and RS is further studied to evaluate their synergistic effects, the char-oxygen carrier interaction, and activation energy using a thermogravimetric analyzer (TGA) under N2 and CO2 atmosphere. Among the kinetic models considered, the shrinking core model is found to be the best fit for all the cases. The activation energy is reduced by 18-25% using the coals-RS blend for both the oxygen carriers. However, the OPCB further reduced the activation energy of the reaction mixture by 7-25%, compared to the commercial Fe2O3.

Using the OPCB based oxygen carriers, Aspen Plus simulations are performed to examine the feasibility of commercial implementation of this technology by integrating the CLC system with a combined cycle power plant having a net capacity of 150 MW for electricity generation. The effect of co-firing on the net thermal efficiency of the power plants is analyzed. Further, economic analyses are carried out for the proposed system, and the levelized cost of electricity (LCOE) is estimated for the power plants using various fuels. It is found that the use of electronic waste-based oxygen carriers achieved almost an equivalent net thermal efficiency of 42.4% based on the combined cycle power plants, compared to the commercial Fe2O3 based power plants. Further, the economic analysis has shown that the levelized cost of electricity (LCOE) of the CLC integrated combined cycle power system employed with electronic waste-based oxygen carriers is 85.9 $/MWh, which is the lowest among other power systems. The LCOE using the OPCB metal oxides is found to be lower than the commercial Fe2O3 and CuO by 8.9 $/MWh and 15.3 $/MWh, respectively.

It can be concluded that the oxygen carriers prepared from the discarded e-waste displayed higher reactivity with coal, RS, and their blends at 900°C than the commercial Fe2O3. Further, the blending of coal with RS demonstrated a better performance than that of the unblended conditions. Thus, the electronic-based oxygen carriers and the coprocessing of Indian coals with RS in the CLC process can be the alternative strategy to implement the technology economically feasible for large-scale applications.