Producer Gas Cleaning Process From Biomass Gasification And Its Impact On Solid Oxide Fuel Cells Performance
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Date
2018-05-01
Authors
Zia, Ud Din
Journal Title
Journal ISSN
Volume Title
Publisher
Universiti Sains Malaysia
Abstract
Biomass derived producer gas (PG) can be efficiently converted into
electricity in solid oxide fuel cells (SOFC) at their operating temperature between
700–900 °C if sufficiently cleaned from PG contaminants especially tar. Amongst
the tar cleaning options, thermal cracking offers the advantage of increasing the PG
heating value by cracking tar into useful gases and the heat available can be utilized
as heat source for SOFC operation. However, current tar thermal cracking systems
are at lab–scale and are based on less efficient and expensive electric furnaces.
Alternatively, thermal cracking system based on Microwave (MW) heating is rather
more efficient and cost effective and has the potential for process scale–up. In this
work, a modified industrial MW oven was developed and characterized for tar
thermal cracking and integrated with a 10 kWth downdraft gasifier. The sensible heat
of PG from MW tar cracking reactor was preserved in a stainless steel (SS) chamber
for the operation of a 60 W single SOFC. PG was then subjected to a cooling process
prior to compression in a compressor as a further tar cleaning mechanism of biomass
tar. Particulates and traces of alkali metals and HCl were removed via cooling and
filtration processes. The remaining contaminant H2S was removed using urea
impregnated coconut shell activated carbon (CSAC) before feeding the cleaned
compressed PG to a SOFC in a thermally insulated stainless steel chamber. The
experimental work showed that MW tar cracking reactor converted 95% of
particulates and 93% of tar into combustible gases resulting in the highest heating
value of 5.53 MJ Nm-3 at 1250 °C. Tar was reduced from 1703 mg Nm-3 to 140 mg
Nm-3. Kinetic studies revealed that tar conversion rate was 1.7 times faster under
MW heating as compared to conventional heating. The high temperature PG exiting
MW reactor of above 800 °C is suitable for SOFC operation thus omitting an electric
furnace otherwise required to maintain SOFC operating temperature. The
compression of PG in a compressor further reduced tar to 22 mg Nm-3 exhibiting
84% removal efficiency. All PG contaminants were successfully reduced below the
probable tolerance limits for SOFC using the designed cleaning system. SOFC
exhibited the stable voltage of 0.865 V for the tested duration of 300 min under
current density of 260 mA cm-2 without showing any significant degradation under
PG operation with low S/C=0.3 but still under thermodynamic carbon deposition free
conditions. A single SOFC delivered the power of 23 W with the electrical efficiency
of 24% at the low fuel utilization factor of 36%. PG as fuel did not cause any
deteriorating effects on anode microstructure. The SOFC exhibited comparable
performance under thermal profile sustained by hot PG with those of the cells
operated in temperature controlled electric furnaces in similar conditions.