Advanced coal-fired power generation systems, such as pressurized fluidized-bed
combustors and integrated gasifier-combined cycles, may provide cost effective future
alternatives for power generation, improve our utilization of coal resources, and
decrease our dependence upon oil and gas. When coal is burned or converted to
combustible gas to produce energy, mineral matter and chemical compounds are
released as solid and gaseous contaminants. The control of contaminants is mandatory
to prevent pollution as well as degradation of equipment in advanced power
generation. To eliminate the need for expensive heat recovery equipment and to avoid
efficiency losses it is desirable to develop a technology capable of cleaning the hot gas.
For this technology the removal of particle contaminants is of major concern.
Several prototype high temperature particle filters have been developed, including
ceramic candle filters, ceramic bag filters, and ceramic cross-flow (CXF) filters.
Ceramic candle filters are rigid, tubular filters typically made by bonding silicon
carbide or alumina-silica grains with clay bonding materials and perhaps including
alumina-silica fibers. Ceramic bag filters are flexible and are made from long ceramic
fibers such as alumina-silica. CXF filters are rigid filters made of stacks of individual
lamina through which the dirty and clean gases flow in cross-wise directions.
CXF filters are advantageous for hot gas cleanup applications since they offer a large
effective filter surface per unit volume. The relatively small size of the filters allows
the pressurized vessel containing them to be small, thus reducing potential equipment
costs. CXF filters have shown promise but have experienced degradation at normal
operational high temperatures (close to 1173K) and high pressures (up to 24 bars).
Observed degradation modes include delamination of the individual tile layers,
cracking at either the tile-torid interface or at the mounting flange, or plugging of the
filter. These modes may be attributed to a number of material degradation
mechanisms, such as thermal shock, oxidation corrosion of the material, mechanical
loads, or phase changes in the filter material.
Development of high temperature optical fiber (sapphire) sensors embedded in the
CXF filters would be very valuable for both monitoring the integrity of the filter during
its use and understanding the mechanisms of degradation such that durable filter
development will be facilitated. Since the filter operating environment is very harsh,
the high temperature sapphire optical fibers need to be protected and for some sensing
techniques the fiber must also be coated with low refractive index film (cladding).
The objective of the present study is to identify materials and develop process
technologies for the application of claddings and protective coatings that are stable and
compatible with sapphire fibers at both high temperatures and pressures.