Pellet fuels, production

Fabrication. Processes for fabricating solid fuel pellets from a variety of feedstocks, particulady RDF, wood, and wood and agricultural residues, have been developed. The pellets are manufactured by extrusion and other techniques and, in some cases, a binding agent such as a thermoplastic resin is incorporated during fabrication. The fabricated products are reported to be more uniform in combustion characteristics than the raw biomass. Depending on the composition of the additives in the pelletized fuel, the heat of combustion can be higher or lower than that of the unpelletized material. 

The heating value depends on the moisture and ash contents of the densified material and is usually in the range of 15 to 17 MJ/kg. The use of asphaltic binders or pelletizing conditions that result in some carbonization can yield densified products that have higher heating values. Pellets, briquettes, and logs have been manufactured by densification methods from biomass for many years. Prestologs made from waste wood and sawdust were marketed before 1940 in North America, and the market for pellet fuels made from wood... 

Pellet fuel is made of wood residues which are left over from lumber production. The material is taken to a pellet mill where it is dried, compressed and formed into small, cylindrical pellets. Pellets are supplied directly to the storage device by specialised tanker trucks. 

In an alternate fuel fabrication technique, crushed or spherical fuel particles are vibratory-packed directly into the cladding tubes, thereby avoiding the problems of pellet production. The fabrication operations can be carried out automatically by remote operation at room temperature. Dust contamination is avoided, as well as radiation exposure of personnel. Irradiation tests of these fuel rods indicate improved performance over pellet fueled rods for equivalent exposures " . 

Nuclear fuels require surface protection to retain fission products and minimize corrosion. Also, pelletized fuel requires a tubular container to hold the pellets in the required physical configuration. The requirements for cladding material to serve these different purposes will vary with the type of reactor however, some general characteristics can be noted. This chapter will discuss the general characteristics associated with cladding and reflectors. 

Cladding is used to provide surface protection for retaining fission products and minimizing corrosion. Cladding is also used to contain pelletized fuel to provide the required physical configuration. 

The fuel used in most nuclear reactors consists of uranium dioxide (UO2), a ceramic material. This is in the form of small pellets (less than an inch in diameter) contained within metal tubes, usually of an alloy of zirconium (as in the RBMK) or of stainless steel. These tubes are collected into bundles (fuel elements) and, in the RBMK, are inserted into the pressure tubes to form the core. The vast majority of the radioactive material produced by the fission process is held by the ceramic material itself and what little does escape from the ceramic matrix is retained by the metal tube surrounding the pellets. Fission products can only be released from the fuel elements if these overheat. This is a three-stage process ... 

Fuel fabrication facilities are located in many parts of the world, as shown in Table 12.2, where the countries and the annual fuel production capacihes are listed. These facilities range from sites which can handle all steps including UF processing, UO2 powder blending, pelletizing lines, and specialized component manufacture, to sites that are responsible for final assembly only. 

Raman imaging has been used successfully to help elucidate mechanisms for removal of trace contaminants from actinide mixed-oxide (MOX) nuclear fuel cells. MOX fuel production consists of converting weapons-grade plutonium metal to PUO2, which is then mixed with depleted UO2 and pressed into a fuel pellet to be utilized in conunercial reactors. The production and use of MOX commercial fuel is one strategy available for consuming weapons-grade plutonium that presents risks to security and the environment [62]. 

One of the routes to dense, fine-microstructured, polycrystaUine abrasive minerals considered by 3M at that time (-1974) was sol-gel technology. Prior to work on abrasives, 3M s Dr. Harold Sowman had been successful utilizing sol-gel approaches for the synthesis of nuclear fuel pellets and production of ceramic fibers. He, along with Dr. Melvin Leitheiser, initiated an effort to develop new aluminous abrasive minerals based on sol-gel technology. They focused their effort on the conversion of colloidal boehmite, y-AlOOH, into dense, polycrystalline alumina. Since that time, boehmite has become, not only the precursor of choice for sol-gel abrasive grains, but the precursor of choice for virtually every sol-gel alumina process. 

Completely continuous coking systems (Continuous Contact and Fluid Coking ) were developed during 1949 to 1955, and these promise to revolutionize the coking art (over 500,000 bpd capacity, 1956) and possibly the economics of residual fuel production. In these processes, coke is built up on pellets or on fluidized coke particles until a size suitable for removal is attained. Meanwhile, the smaller particles are retained within the system. In the Fluid process shown in Fig. 19-20, about 5 per cent of the coke yield (on feed) is burned in a fluidized coke bed to dry the coke particles and to heat them (1100 to 1200°F) for the reaction chamber which is also maintained in a fluidized state. The reactor operates at 900 to 1050°F, and the pitch-type feed is also introduced and heated in the reactor. Reaction products are separated in a fractionator system situated just above the reactor. Fluidized coke has a gross heating value of about 14,300 Btu per lb and its size distribution is ... 

AH operating facilities shear the spent fuel elements into segments several centimeters long to expose the oxide pellets to nitric acid for dissolution. This operation is often referred to as chop-leach. The design and operation of the shear is of primary importance because (/) the shear can be the production botdeneck, and (2) the shear is the point at which tritium and fission gases are released. 

Confusion as to what constitutes municipal waste is presenting an obstacle to the use of packaging waste as a fuel in cement kilns. Whilst cement kilns can bum hazardous waste, they cannot bum a wide range of non-hazardous materials, it is reported. The case of Castle Cement is described which planned to bum a range of non-hazardous commercial and industrial wastes. Some waste-fired combustion processes, however, such as UK Waste s Fibre Fuel operation have been granted derogations where fuel is manufactured by advanced mechanical processes, which includes the production of fuel pellets. This latter process would be pointless for the cement industry since their fuels have to be pulverised. The problems are further discussed with reference to current European legislation. 

The ANL catalyst (not identified, but presumably a Pt supported on Gd-doped ceria) was also successfully used for ATR of diesel fuel. Tests of three different types of diesel fuels (n-Cie, low-sulfur diesel, and regular diesel) showed complete conversion of hydrocarbons at 800°C. The diesel surrogate n-Ci6 yielded 60% H2 on a dry, N2-free basis at 800°C, whereas the other two diesel fuels required higher temperatures (>850°C) to yield similar levels of H2 in the product gases. Similar or improved H2 yields from diesel ATR were observed with a microchannel monolith catalyst, compared with extruded pellets in a fixed-bed reactor. ... 

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