Studies on Direct Chemical Looping Combustion of Coal with Rice Straw using Electronic Waste Based Oxygen Carriers for Clean Energy Production
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Date
2022
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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.
Description
Supervisor: Prabhu V
Keywords
CHEMICAL ENGINEERING