Page 78 - New Trends In Coal Conversion

44 New Trends in Coal Conversion

H 2 and CO reduce the iron oxide, producing CO 2 . The reduced oxygen carrier is reoxi-
dized in the combustion reactor, where the heat released can be used for electricity
production. CDCL has been demonstrated in a 25 kW th subpilot unit for 800 h. Full
coal conversion was achieved during 200 h of continuous operation with nearly
100% carbon capture (Velazquez-Vargas, 2016). B&W built a 250 kW th pilot unit
to demonstrate the technology at higher scale; with over 200 h of operation accumu-
lated, initial coal testing showed nearly 100% CO 2 purity. A large pilot of 10 MW e has
been designed under a project cofunded by DOE (Tong et al., 2017).
Chemical looping with oxygen uncoupling (CLOU) makes use of oxygen carriers
that release gaseous oxygen that reacts directly with the coal, which is 50 times faster
than the gasification reaction. Mn, Cu, and Co oxides have been identified as potential
oxygen carriers for CLOU. The CLOU concept was first demonstrated at ICB-CSIC, in
Spain, in a continuous CLC unit at bench scale (1.5 kW th ) that achieved complete com-
bustion with different rank coals with high capture efficiency and low oxygen carrier
inventories (Ad anez-Rubio et al., 2013). The technology will reach TRL 5 by demon-
stration at 200 kW th scale in a DOE-cofunded project. The semipilot unit, designed
and built by the University of Utah, consists of two interconnected circulating fluidized
beds with a 175 kg bed inventory that has already demonstrated with over 600 h of hot
circulation operation (Whitty and Lighty, 2017).
Limestone CLC (LCL-C) based on CaS/CaSO 4 , has been validated in a 3 MW th
prototype, which is the largest CLC facility worldwide. This has been operated for
over 350 h (>75 h in autothermal mode), confirming chemical looping reactions
and performance potential. However, during coal firing operation, some technology
gaps were identified: solids flow instability, carbon loss in cyclones, carbon carryover
to the oxidizer, sulfur loss, and incomplete fuel conversion. A 100 kW th pilot scale test
facility was used to develop the technology; the results and solutions obtained were
planned to be validated in the 3 MW th prototype in 2017, with the aim to demonstrate
LCL-C at 10e25 MW e scale. Technoeconomic studies indicate that LCL-C has the
potential to be the lowest cost option for coal-based power generation with CO 2 cap-
ture (Levasseur et al., 2016). LCL-C with 97% capture installed in a 550 MW e plant
would increase LCOE by 19.5% compared with the supercritical pulverized coal
base case (Levasseur, 2015).

2.4 Power generation from coal gasification with pre-
combustion capture

Coal gasification involves the reaction of the coal with oxygen or air and/or steam to
produce a synthesis gas (syngas) composed mainly of CO and H 2 . The main advan-
tages of this technology are that it can handle low-grade fuels, such as lignite, biomass,
heavy oils, or wastes, and that it offers the potential to coproduce power, chemicals,
and low carbon fuels, such as hydrogen. On the other hand, it is not responsive in terms
of meeting daily load changes (which might be overcome storing H 2 in salt caverns), is
more demanding to operate, and there is limited experience of building (ETI, 2016).
PRECC involves the reaction of CO with H 2 O in a catalytic reactor, called water