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Jack D. Spain Southern Research Institute Engineering Research Center 757 Tom Martin Drive Birmingham, Alabama 35211 Jack D. Spain
Southern Research Institute
Engineering Research Center
757 Tom Martin Drive
Birmingham, Alabama 35211
(205)581-2323
spain@sri.org
Characterization of Filter Elements for Service in a Coal Gasification Environment
Key Words: Hot gas filters, Metal filter elements, Ceramic filter elements
Introduction
The Power Systems Development Facility (PSDF) is a joint Department of
Energy/Industry sponsored engineering-scale facility for testing advanced coal-based
power generation technologies. High temperature, high pressure gas cleaning is critical
to many of these advanced technologies. Barrier filter elements that can operate
continuously for nearly 9000 hours are required for a successful gas cleaning system for
use in commercial power generation. Since late 1999, the Kellogg Brown & Root
Transport reactor at the PSDF has been operated in gasification mode. This paper
describes the test results for filter elements operating in the Siemens-Westinghouse
particle collection device (PCD) with the Transport reactor in gasification mode.
Operating conditions in the PCD have varied during gasification operation as described
elsewhere in these proceedings (Martin et al, 2002). Nominal operating conditions
during the most recent gasification runs were:
PCD Inlet Temperature
371 427 Deg. C (700 800 Deg. F)
System Pressure
1.10 1.65 MPa (160 240 psig)
Face Velocity
0.015 0.026 m/s (3 5 ft/min)
Baseline Pressure Drop
126 302 mbar (50 120 inH
2
O)
In addition to these normal operating conditions, the elements are subjected to system
upsets that result in more severe, but transient conditions. Thermal transients are
sometimes caused by oxygen breakthrough to the PCD causing ignition of char on the
surface of the elements. During several of these transients, temperature measurements on
the surface of some elements have indicated a temperature increase of approximately
300°F in one minute. There was evidence, such as the response of some elements during
thermal transients, that the transients were more severe on other elements that were not
instrumented with thermocouples.
Twenty-two different types of filter elements have been tested in gasification operation at
the PSDF. The types of elements tested are shown in Table 1. The hours of operation
accumulated on each type of element are shown in Figure 1. The value plotted on the y-
axis of Figure 1 is the total hours accumulated on all elements of a particular type. The
maximum hours accumulated on any individual element is shown above the bar for each
element type. Pall iron aluminide (Fe
3
Al) and Hastelloy X metals, Pall 326 ceramic, and Pall/Schumacher ceramics with the T binder, T05-20, T10-20, and TF20, have seen the
most operation and are the subject of this paper. A brief description of each of these
element types is given below.
Pall Fe
3
Al filter elements were manufactured of sintered metal powder with a
composition of 2 at.% Cr,-15.9% Al,-balance Fe. The elements were seamless cylinders
with nominal dimensions of 60 mm (2.36 in) O.D. and 56 mm (2.22 in) I.D. The
elements were manufactured in sections of approximately 0.5 m (20 in) length with
individual sections joined by welding the porous Fe
3
Al to solid 310 SS support rings.
Most elements tested at the PSDF consisted of three sections for an overall length of
1.5 m (60 in) but some elements had four sections for an overall length of 2 m (80 in).
All elements were preoxidized by Pall to form a protective layer of alumina over the
particles.
Pall Hastelloy X filter elements were manufactured of sintered metal powder to nominal
dimensions of 60 mm (2.36 in) O.D. and 53 - 56 mm (2.1 - 2.2 in) I.D. The elements had
an axial seam weld. These elements were manufactured in sections of approximately
0.5 m (19.5 in) length with individual sections joined by welding the porous Hastelloy X
to solid Hastelloy C-276 support rings. All elements tested at the PSDF consisted of
three sections for an overall length of 1.5 m (59 in).
Pall/Schumacher TF20, T10-20, and T05-20 were membrane-coated, clay-bonded SiC
particle filter elements. The structural support wall of these materials consisted of
individual SiC particles connected by clay or glass bridges. The elements had a nominal
I.D. of 40 mm (1.58 in) and a nominal O.D. of 60 mm (2.36 in). Mechanical and thermal
properties of the elements were controlled by the structural walls and the thin (~100 µm)
membrane layer provided filtration. The structural walls of TF20, T10-20, and T05-20
were the same but the filtration membranes were different in chemical composition and
pore size. Since mechanical and thermal properties of these materials were controlled by
the structural walls, the properties are presented together on one graph. Pall/Schumacher
N10-20 is similar to T10-20 except that the binder has a lower cristobalite content and
improved bonding between the binder and SiC particles.
Pall 326 was a membrane-coated, bonded SiC particle filter element. The structural
support wall consisted of SiC particles in an alumino-silicate binder. The elements had a
nominal I.D. of 40 mm (1.58 in) and a nominal O.D. of 60 mm (2.36 in). Mechanical and
thermal properties of the elements were controlled by the structural walls and the
relatively thin membrane layer provided filtration.
Objective
The objectives of this work were to understand the performance of filter elements in the
coal gasification environment and identify filter materials with the best chance of meeting
the demand of 9000 hours of continuous operation under the conditions given above. Approach
The approach used in this effort has been to measure the physical, mechanical, and
thermal properties of the filter materials and relate those properties to performance in the
coal gasification environment. Based on operating conditions measured at the PSDF, in-
service performance of filter elements tested so far, and anticipated requirements in
future systems, several material issues have been identified that are critical to the
performance of filter elements. Test matrices used for testing as-manufactured elements
and elements after gasification operation are shown in Tables 2 and 3. Tensile tests were
conducted in both the axial and hoop direction. Axial tests were conducted on dog-bone
shaped specimens cut from the wall of the filter elements. Load was applied through pins
in the grips. Hoop tensile tests were conducted on 50.8 mm (2 inch) long rings loaded
internally by hydrostatic pressure. Tensile testing was used to determine if a material had
sufficient strength to withstand all operating conditions including handling, installation,
and backpulse cleaning. Tensile testing was also conducted on filter elements removed
after operation. The tensile strength and tensile strain-to-failure, an indication of
ductility, were compared to those measured on as-manufactured elements to assess
degradation during operation. Unit thermal expansion (unit thermal expansion refers to
length change divided by the original length) and thermal conductivity were measured on
as-manufactured materials to predict performance during thermal transient conditions.
Thermal conductivity testing was not feasible on the metal filter materials because of
their thin walls and relatively high thermal conductivity; however, these same
characteristics also made thermal stress failure unlikely in these elements. Optical
microscopy at low magnifications, <200X, was also conducted as needed.
Results
As of June, 2003 Pall iron aluminide (Fe
3
Al) elements have been in gasification operation
at the PSDF for a total of ~150,000 hours with some individual elements in operation for
over 2000 hours. The operational results with Fe
3
Al elements have been very good with
no element failures during normal operation. The elements have survived several thermal
transients where the temperature measured at the element surface increased ~167°C
(300°F) in 60 seconds and at least one thermal transient where the surface temperature
increased ~333°C (600°F) in 180 seconds during off-coal operation. However, there
have been three element failures, all of which occurred under severe circumstances
during off-coal operation. The first was during gasification run TC06A when char in the
PCD, apparently bridged material, ignited because of oxygen breakthrough to the PCD.
The temperature on one side of the element most likely exceeded 982°C (1800°F), the
temperature where a phase change accompanied by a permanent length decrease occurs
in Fe
3
Al. Since only one side of the element reached that temperature, the element
tended to bow toward the side with the decreased length. Since the bottom support rods
prevented the element from bowing enough to accommodate the length decrease, it
cracked. The effects of this thermal transient were localized. Inspection of the elements
indicated that only 5 Fe
3
Al elements were affected. Two elements failed during an off-
coal period of gasification run TC07 and the cause of these failures was not determined.
There were s