Page 76 - New Trends In Coal Conversion

42 New Trends in Coal Conversion

2.3.3 Pressurized oxy-combustion technologies

Pressurized oxy-combustion technologies have potential to increase plant efficiency,
reduce capital cost, avoid air ingress, and reduce oxygen requirements.
Flameless pressurized oxy-coal combustion (FPOC) occurs at high and uniform
temperature (volume combustion). FPOC has been demonstrated in Italy at 5 MW th
scale with high and low rank coals, accumulating over 18,000 h with low emissions.
Coal is fed as ground slurry with water to avoid dust pollution. Conventional ashes are
substituted by inert vitrified slag with near-zero carbon content. FPOC presents
competitive CAPEX, small footprint, flexible fuel, simple fumes treatment, easy
CO 2 capture, and low emissions. A 50 MW th prototype has been engineered to demon-
strate the technology at higher scale (Schmitt, 2017).
Oxy-fired pressurized fluidized bed (OPFB) has potential to reduce the size and
cost of conventional boilers by 1/3 and 1/2, respectively, providing affordable
energy with near-zero emissions from low-value feedstocks. An in-bed heat
exchanger provides an ultracompact combustor. OPFB with actual ASU technology
can provide affordable cost of electricity: 31% increase with existing ASU technol-
ogy for 98% capture (or 29.9% for 90% capture) and upgrade paths through high
efficient ASU development (27.5%) or supercritical Brayton cycle (18%) (Follett,
2016). Technoeconomic analyses suggest that OPFB exceeds the DOE goal of a
50% reduction in capture penalty. OPFB is being demonstrated at 1 MW th scale at
CanmetENERGY to reach TRL 6: coal ignition and burning in oxy-combustion
operation at full pressure with an average in-bed sulfur capture of 95% has been
achieved. The CPU consists of a direct contact cooler that cools the flue gas, con-
denses water vapor, recovers heat, and removes ash and HCl; a LICONOX unit
that removes sulfur and nitrogen oxides; and a DEOXO unit that removes O 2 and
recovers heat (Fitzsimmons, 2017).
Staged high-pressure oxy-combustion (SPOC) technology controls heat flux and
temperature with coal staging and burner design. This technology has potential to
improve efficiency over 25% with respect to first-generation atmospheric oxy-
combustion technology. A pressurized oxy-combustion facility of 100 kW is being
used to develop the technology under DOE-cofunded projects (Axelbaum, 2017).

2.3.4 Chemical Looping Combustion

Chemical looping combustion (CLC) makes use of oxygen carriers, which are metal
oxides that circulate between the fuel reactor, where they are reduced, and the air
reactor, where they are reoxidized, avoiding the need of an ASU. The oxides of com-
mon transition metals, such as Fe, Cu, Ni, Mn, and Ba, can be used as oxygen carriers,
with Fe 2 O 3 /Fe 2 O 2 , NiO/Ni, and BaO 2 /BaO being the most extensively studied. Theo-
retically, the overall energy of the CLC process is the same as for the direct combustion
of the fuel, and a very low efficiency drop could be achieved (z3% including
compression). However, the solid circulation constitutes a technological challenge
and a fundamental parameter to control the heat transfer from the air reactor to the
fuel reactor.