Fitter to promote clean coal power A thin metal filter material

developed at the US Depart-

ment of Energy’s (DOE’s)

Ames Laboratory may hold the

key to allowing power plants

to burn high sulphur coal

cleanly.

The technology to burn this

type of coal has existed for

some time, and demonstration

plants have shown that

pressurzed-fluidized com-

bustion and integrated gasifi-

cation combined cycles provide

high efficiency, low emission

power generation. However,

the resulting flue gases contain

fly ash, which is high in

sulphides, chlorides and

sodium compounds. This not

only poses a threat to air

quality, but can also damage the

turbines that drive plants’

generators. To prevent the

particles from reaching the

turbines and the atmosphere,

the hot gas is normally passed

through banks of cylindrical

ceramic candle filters. The

filters’ operating efficiency is

maintained by regular cleaning,

using an internal blast of

compressed air. Unfortunately,

ceramic materials are

susceptible to thermal shock,

so the abrupt change in tempe-

rature caused by the com-

pressed air can often crack the

ceramic material.

Researchers began looking

at developing metal filters

made from superalloys used in

the aerospace industry. They

chose a nickel-chromium-

aluminium-iron alloy because

it maintained its strength at

The thin, permeable sheets

of metal are formed by a pro-

high temperatures (850 “C)

cess called tap-densified loose

powder sintering. High purity

molten superalloy is converted

and was resistant to thermal

into a fine powder using a

high-pressure atomization

shock.

system. As the hot metal passes

through a nozzle, a high-

pressure jet of nitrogen gas

breaks the liquid into millions

of tiny metal spheres. The

resulting powder is sorted by

screening then spread out as a

thin layer (0.5 mm) and heated

in a vacuum furnace . The

sintering process bonds the

Tests have shown that the

material undergoes only a

particles together, forming

moderate drop in yield

strong and smooth joints

strength when going from

room temperature to high

between the spheres, but at the

operating temperatures. In a

series of bend radius tests the

material was also sufficiently

same time leaving air spaces.

ductile to be formed into cor-

rugated tubes; an important

feature for strength, as well as

for increasing the filter surface

area.

In the near future the scien-

tists hope to carry out the

sintering process on a com-

mercial scale, and to test the

filter in a DOE demonstration

power plant run by the

University of North Dakota.

Lighting up metal pollutants in water A team of US BrighamYoung

University scientists has

synthesized molecules that

glow in the presence of

specific metal pollutants.

High levels of metals in the

environment often result from

activities such as mining, fossil

fuel combustion and other

industrial applications. Expo-

sure to high levels of metals

such as mercury, cadmium and

zinc can pose serious health

risks. Methods of tracking

metal in water currently exist,

but they tend to be labour

intensive and relatively slow.

The latest technology uses

synthetic compounds that seek

out and bind metal ions.

Smaller synthetic molecules

then bind to the metal-binding

compounds and act as fluo-

rescent ‘reporters’, i.e. they

glow brightly under UV light.

If no metal ion is bound, the

compounds remain dark.

According to reseachers, the

technology is paving the way

for the development of an

early warning system for metal

contamination of drinking

water and waste streams.

Filtration+Separation October2OOl23