3 Process Parameters of Fluidized-Bed Burning of Graphite from HTGR Fuel Elements R. B Ö H N E R T , G . KAISER, and E . M E R Z
Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch003
Institute for Chemical Technology, Kernforschungsanlage Jülich, GmbH, Jülich, Germany
The only large scale method suitable to separate the residual nuclear fuel materials from the large amounts of graphite of HTGR fuel elements during reprocessing is burning the fuel with air, oxygen-carbon dioxide mixtures, or pure oxygen. The experiments to date for burning crushed graphite fuel elements in an inert-freefluidizedbed have demonstrated the technical feasibility of this process step. Combining known theoretical relationships with experiments in crushing the fuel elements, in fluidization, and in burning has led to a satisfactory means for predicting the operating points of a fluidized-bed burning process. This paper describes the studies defining the ranges of interest for the crushed particle size distribution, the gas velocities, and operating temperatures and pressures.
I
n commercial power-producing reactors, the fuel is generally in the reactor long enough so that fission products and neutron capture products, including bred fissile isotopes, have accumulated to where they significantly affect the economic optimization of fuel recycle. The purpose of reactor fuel reprocessing is thus to recover and purify fissile material from irradiated fuel. In general, reprocessing of spent fuels from high-temperature gas-cooled reactors (HGTR's) consists of shipping irradiated fuel elements from the reactor to a reprocessing plant, removing as much extraneous material from the fuel as possible by dissolving the fuel in nitric acid, separating and purifying the uranium and thorium by solvent extraction, disposing of the radioactive wastes, and decontaminating the gaseous radioactive effluents from the reprocessing steps. The feature which sets H T G R fuel apart from all other reactor fuels is the large amount of carbon which is directly associated with the fertile and fissile materials. Several fuel designs have been proposed by different designers and fuel element manufacturers; the most important are the hexagonal block elements of the Gulf General Atomic Co. (U.S.) and the spherical elements of several companies in the Federal Republic of Germany. The salient feature of both concepts is that the fissionable materials are contained in pyrocarbon (PyC) coated particles containing either sintered (U,Th)0 or ( U , T h ) C kernels. The coated particles are imbedded in a graphite matrix. About 95 wt % of the fuel elements is graphite. Thus, the basic problem in H T G R fuel 2
25
Hulburt; Chemical Reaction Engineering—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
2
Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch003
26
CHEMICAL REACTION ENGINEERING
II
reprocessing is the removal of this large quantity of carbon from the relatively small amount of fuel material—i.e., uranium and thorium. The preferred method for this task is burning the graphite in the so-called head-end step of reprocessing. This method of burning was first proposed in 1964 by the Oak Ridge National Laboratory (I). At that time it was recognized that fluidized-bed burning of the crushed fuel promised advantages over fixed-bed burning because of higher specific throughputs, easier heat removal and control, lower burning temperatures (thus minimizing off-gas contamination), and a considerably easier continuous operation. Most subsequent efforts have concentrated on the fluidized-bed burning concept (2, 3, 4). In the approach being studied in the United States, the material which is fluidized is finely divided alumina particles, which acts as an efficient heat transfer medium to carry the heat of combustion from the burning carbon to the wall of the burner. However, we at K F A in Germany are pursuing an approach without using an extra added inert fluidizing heat transfer medium (5). This measure should improve overall operating simplicity and economy, particularly since the later separation of fuel ash and alumina is eliminated. For several years we have been studying the feasibility of burning graphite in a fluidized bed, using the density difference between the heavy metal particles and the graphite to form a support and gas distribution zone for the burning bed by allowing the particles to settle out. Thus, the settled particles form the flow bottom plate, and they can be removed almost carbon-free as a free flowing product continuously or batchwise. For large reprocessing plants, carbon throughputs are of the order of tons per day. The desired throughputs should be accomplished with a minimum number of process steps and pieces of equipment. Experimental Fluidized-bed burning of H T G R fuel elements has the following process steps: (1) size reduction of the fuel elements by crushing and/or grinding, (2) transport of the crushed fuel to the fluidized bed burner, (3) burning of the graphite and pyrocarbon with oxygen, and (4) removal of the bare oxide particles, or ashes from the carbide particles, from the burner. Experiments with unirradiated and irradiated fuels were done. Only spherical fuel elements of German design (e.g., A V R type from the first prototype H T G R power plant) with a 60-mm diameter were used. However, no major differences are expected for the other fuel elements. Two hammer mills and a jaw crusher were available for the crushing tests. The average crushed particle size for the hammer mills was varied by changing the sieve plate openings while the jaw crusher had an adjustable slit width. As could be shown in preliminary experiments using quartz burning tubes, the size of the crushed product must be fixed somewhere between 0.6 and 3 mm. Three burners of different size and design were used (the results listed are mean values from all experiments): (a) A 60-mm diameter fluidizing tube equipped with an enlarged upper section containing sintered metal filters for cold and hot runs; air-cooled water vapor; average throughput, ca. 4 grams C/min. (b) A n 80-mm diameter fluidizing tube equipped again with an enlarged upper section but with external cyclones and filters for cold and hot runs; cooled with a recirculating water-steam carried by air to the burner; average throughput, ca. 15 grams C/min (see Figure 1).
Hulburt; Chemical Reaction Engineering—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
3.
BOHNERT ET A L .
Fluidized Bed Burning
27
Off- Gos
Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch003
A b s o l u t e Cell Filter Filter
)C0 ) 0 2
Figure 1.
Gas- Battery 2
Schematic for hot cell burning studies with the 80-mm diameter burner Secondary "Cooling
uff-Gas
-Water F nal Off - G a s P u r i f i co tion
Water Pump Cooling A i r Dust R e c y c l e to B u r n e r
Burner - Gos-
Figure 2. Generalizedflowsheetfor thefluidized-bedburning studies with 300-mm diameter burner. Bed temperature: < 800°C; carbon burning rate: 3 kg C/hr + 30%; burning gas: O ; off-gas composition: CO :CO ^ 5:1. t
t
(c) A 300-mm diameter fluidizing tube for cold runs only; cooled like b; average throughput, ca. 70 grams C/min (see Figure 2). Details of the hot cell arrangement of burner b and semitechnical arrangement with burner c are shown in Figures 1 and 2. The burner design with internal filters (a) did not give a complete burning of the dust trapped in the upper regions of the furnace. Much better per-
Hulburt; Chemical Reaction Engineering—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
28
CHEMICAL REACTION ENGINEERING
Π
formance was obtained applying external filtering devices to collect and recycle the dust carried out with the off-gas. Combustion was done for several operat ing conditions with pure oxygen and for 0 - C 0 mixtures using the crushed fuel and particles as the sole content of the fluidized bed. Crushed fuel is metered into the burner by a feed screw. The burner gas passes through a perforated entrance cone into the furnace. After the precooled off-gas has passed through the cyclone filtering device, samples are taken to determine C 0 , C O , and 0 content prior of its release through the stack after a final purification. The burners are brought to operating temperature either by external resistance heaters or with preheated combustion gas. It is possible to recycle the C 0 from the off-gas stream to the fluidizing gas stream. We decided to fix the operating temperature at less than 9 5 0 ° C to prevent ash melting or agglomeration. From fluidized-bed technology, the following parameters must be established: (a) particle size distribution of crushed fuel, (b) carbon burning rate, (c) composition of burner gas and gas velocities, and (d) average operating temperature and pressure. 2
2
2
2
Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch003
2
Theory, Results, and Discussion Permissible Sizes for the Crushed Fuel. The diameter of the coated fuel particles lies between 0.3 and 0.6 mm. The density of the graphite amounts to p = 1.7 grams/cm . Burning with pure oxygen yields at 8 0 0 ° C an off-gas with a C 0 : C O ratio of approximately 5:1. The average particle size of crushed fuel, d , should be ^ 2 mm to minimize particle breakage. The mini mum fluidization velocity, v , is given by Ergun (6) : 3
s
2
K
h
*
-
Re dK g ρs p ε η m
= = = « = = = =
g
, [ 1 5 θ α - 0
Ρ
+ 1 . 7 5 Η β ] ^
1
