Improved Atomization Technology Produces Ultrafine Powders for

Fossil Fuel Power Plants

17th Aug, 2011 

The U.S. Dept of Energy’s (DOE) National Energy Technology Laboratory (NETL) has been

collaborating with the Ames Laboratory at Iowa State University, Ames, Iowa, to developadvanced gas atomization technology for the production of ultrafine iron aluminide and

precursor oxide dispersion strengthened (ODS) ferritic stainless steel powders used in fossil

energy (FE) power system components such as coal gasifier heat exchangers, hydrogenseparation membranes, and turbine blades.

NETL states that the new ferritic stainless steel powders developed in the ongoing researchproject offer high thermal conductivity, high creep resistance, and low thermal expansion (low

void swelling during neutron irradiation) allowing them to withstand the demanding high

temperature, high pressure, and often corrosive environments required in FE power plants.

Fig.1 Gas atomized powders produced in the NETL research project (courtesy NETL)

The new gas atomized oxide dispersion strengthened (ODS) powders based on Fe-Cr are also

considered to be a lower cost alternative to currently used mechanically alloyed ODS alloys forhigh temperature FE applications, allowing the FE plants to operate more efficiently with

reduced emissions and adaptability to carbon sequestration or capture.

One of the focuses of the work at NETL and Ames was to provide the level of atomized powder

size control needed for the discrete jet close-coupled atomization process. The nozzle and control

system developed for gas atomization involved two procedures, the first of which was to achievean increase in the efficiency of the melt disintegration process using a high pressure gas

atomization (HPGA) system to produce high yields of ultrafine powder. This was done by

adjusting the gas jet arrays to affect the supersonic gas flow pattern, and by choosing the

optimum atomization gas.

 

The developed HPGA technology allowed researchers to control the droplet size to maximize

ultrafine powder yield and increase intermediate powder yield. Particle sizes include ultrafine

 powders, with particle diameters of less than 10 microns (μm), and mid-range powders, with

 particle diameters ranging between 10 and 44 μm. This was accomplished by using gas flows of 

reduced energy with atomization nozzles of different array diameters and advanced orificedesigns.

The HPGA system was used to produce ultrafine powders which are said to have helpedeliminate a major barrier to the use of new concepts for fabrication of hydrogen membranes for

advanced coal fired power plants with CO 2 capture capability. A smooth micro-porous metallic

support surface was developed with Los Alamos National Laboratory (LANL), for example forfabrication of robust hydrogen separation membranes from Pd alloy thin films approximately 2

µm thick.

Fig.2 Schematic of the critical features of an improved hydrogen separation membrane*

(courtesy AMES Laboratory)

Recent work involved sintering of a 75 µm thick primary membrane support layer from ultrafine(<3 µm dia.) gas-atomized spherical Fe-16Al-2Cr (wt. %) powder onto a coarse (40 µm porosity)

stainless steel frit as a secondary support. Crack-free membrane support surfaces with 0.1-0.5

µm porosity resulted and a finished membrane was reported to have achieved encouraging

hydrogen separation performance.

NETL reported that previous collaboration with LANL on partially sintered ultra-fine iron

aluminide powders as porous supports for hydrogen separation membranes had resulted in USPatent No. 7,611,565 B1, issued on November 3, 2009.

Gas atomization approach to ODS alloys

A second focus of the research project involved ‘innovative gas atomization reaction synthesis’

(GARS). Here the addition of reactive gas components generated on precursor powders was used

 

to serve as an oxygen reservoir for subsequent formation of uniformly dispersed precipitates

during HIPing and heat treatment.

According to a report by Iver E. Anderson and colleagues* (Ames Lab) the precursor powder for

the ODS alloys was produced using GARS using a reactive gas mixture of Ar and O 2. Thereaction parameters (i.e., reactive gas content and inlet position) and the nominal chemical

composition of each alloy are shown in Table 1.

Table 1 Nominal alloy composition (wt %) and atomization processing parameters for

producing precursor powders for the ODS alloys by GARS* (courtesy AMES Laboratory)

The GARS process coats each individual powder particle in situ with an ultrathin (<100 nm)

metastable surface Cr-enriched oxide shell oxide shell during rapid solidification. The oxide

shell is later used as an oxygen reservoir for the formation of nano-metric yttrium-enriched oxide(Y-(Ti, Hf)) dispersoids during hot isostatic pressing. Hot isostatic pressing (HIP) at 850°C or

1300°C and pressure of 303 MPa for 4 hrs, fully consolidated the ODS precursor powder.

The first US Patent on the general approach to production of dispersion-strengthened alloys from

GARS powders was issued on April 20, 2010, with two additional patent applications under

examination. A project was begun in December 2009 with a major industrial partner tocommercialize this new simplified ODS alloy production technology.

Powder production tests have been performed in laboratory atomization systems to produceprototype batches of ultrafine high-temperature alloy powder in sufficient quantity for

subsequent powder processing trials; production of a batch of intermediate-sized alloy powder in

sufficient quantities for thermal spray trials; and mechanical testing and microstructure analysis.

The technology demonstrated steady-state operation and controls systems suitable for industrialuse, and is expected to extend the benefits of powder metallurgy within and beyond the fossilenergy field. The ability to produce high yields of specific sizes of powder particles by gas

atomization is additionally expected to help lower the cost of specialty metal powders.

www.netl.doe.gov  

 

*’Advanced Processing of Metallic Powders for Fossil Energy Applications’ by I.E. Anderson,

D.J. Byrd, J.R. Rieken at Ames Lab, Iowa University, and S.N. Paglieri at Los Alamos National

Laboratory, Los Alamos, New Mexico.