Question 5 of 6: Boiler and Reactor Principles — pulverised coal firing and nuclear reactor components

Question 5: Boiler and Reactor Principles — pulverised coal firing and nuclear reactor components (15 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part I — Pulverised coal firing (8 marks)

Part I (a) — the coal firing process. A large utility boiler burns coal as a suspension of fine particles, in what is called a direct-firing system because the pulverised product goes straight to the burners without intermediate storage. Raw coal is held in a bunker sized for eight to twelve hours of full-load operation, and is drawn from it by a gravimetric feeder that weighs the coal continuously on a short belt and trims its speed to match the fuel demand signal from the combustion control system. Weighing rather than volumetric measurement matters because the bulk density of coal varies with moisture and size, and the boiler master needs the mass flow of heat, not a bucket count. From the feeder the coal falls into the pulveriser — on a station of this size typically a vertical spindle mill, in which large rolls or balls crush the coal against a rotating grinding table.

Part I (b) — coal size and how it is classified. Primary air is delivered into the mill body by the primary air fan, and it performs three distinct duties at once: it dries the coal, it fluidises and lifts the ground particles out of the grinding zone, and it transports them through the fuel pipes to the burners. Air leaving the mill passes upward through a classifier, a set of adjustable vanes that imparts a swirl; particles heavier than the design cut are thrown outward and fall back onto the grinding table for further reduction, while the fines are carried over. The classifier is therefore what actually grades the product, and it is set to deliver a fineness of roughly 70 to 75 % passing a 200 mesh sieve (75 micrometres) with less than about 2 % retained on 50 mesh (300 micrometres). That specification exists because burnout time scales roughly with the square of particle diameter: at 75 micrometres a particle burns out in well under a second, which is all the residence time a furnace offers, whereas coarse particles leave the furnace as unburned carbon in the ash and both waste fuel and spoil the ash for sale as a cement extender.

Part I (b) continued — air temperature and how it is controlled. Mill outlet air temperature is controlled by blending hot air taken from the air heater with cold tempering air taken from the primary air fan discharge, and the blend is regulated to hold a mill outlet temperature of about 65 to 90 °C for a bituminous coal, and lower — nearer 55 to 65 °C — for a high moisture lignite. The temperature must be high enough to evaporate the surface and inherent moisture, because wet coal will not grind, will not flow through the pipes and will not ignite promptly at the burner; it must be low enough that the coal-air mixture inside the mill cannot ignite or explode, since the mill is a confined vessel containing a combustible dust cloud. Mill outlet temperature is thus simultaneously a drying control and a safety interlock.

Part I (b) continued — why the air-fuel ratio differs at the pulveriser and at the burner. The air-fuel ratio differs deliberately between the pulveriser and the burner, and this is the heart of part (b). For a coal of 20 MJ/kg containing 60 % carbon, the stoichiometric requirement of the carbon alone is $0.60\times(32/12)/0.232=6.9\ \text{kg}$ of air per kilogram of coal, and with the hydrogen and sulphur included and 15 to 20 % excess air the burner needs roughly 9 to 10 kg of air per kilogram of coal. The mill, however, is supplied with only about 1.8 to 2.2 kg of primary air per kilogram of coal, that is around 20 % of the total combustion air. The pulveriser ratio is set by transport and drying duty, and is kept far below stoichiometric on purpose: a fuel-rich mixture inside the mill and the fuel pipes cannot propagate a flame, so a mill fire or a pipe fire is prevented by chemistry rather than by instrumentation. The balance of the air is supplied as secondary air, preheated in the air heater to 250 to 350 °C and admitted around each burner nozzle from the windbox, which is the plenum that distributes it evenly to all burners at all elevations. Because the secondary air arrives separately and with swirl, the burner can stage the combustion — a fuel-rich core stabilises the flame and suppresses the formation of nitrogen oxides, and the remaining air mixes in downstream to complete burnout. Splitting the air is therefore what makes a pulverised coal burner simultaneously safe, stable and comparatively clean.

Part II — Nuclear reactor components (7 marks)

Part II (a) — configuration of the reactor. A thermal fission reactor is an assembly of four functional materials whose geometrical relationship is as important as their chemistry. The fuel is uranium dioxide pellets, enriched to a few per cent U-235, sealed in zirconium alloy cladding to form rods; the rods stand in a square lattice at a pitch several times their own diameter. The moderator fills the space between the rods. The coolant flows along the rods in that same space — in a light water reactor the moderator and the coolant are the same water, which is why the lattice pitch is a compromise between neutronics, which wants more water, and heat transfer, which wants more fuel. The control rods are absorber elements, typically boron carbide or a silver-indium-cadmium alloy, that travel in guide tubes occupying lattice positions within the fuel assemblies themselves, so that when they are inserted the absorber sits directly in the neutron flux the surrounding fuel is producing.

Part II (b) — the fission process and the nuclear cycle from one fission to the next. The fission process runs as a cycle from one generation of neutrons to the next. Absorption of a thermal neutron by a U-235 nucleus produces a compound nucleus that splits, releasing two fission fragments carrying most of the roughly 200 MeV (about 32 pJ) of energy released, together with two to three fast neutrons at an average energy near 2 MeV. Those fast neutrons are almost useless for causing further fission, because the fission cross section of U-235 at 2 MeV is barely one barn, whereas at thermal energies it is several hundred barns. That is the moderator’s entire purpose: through elastic scattering off light nuclei — hydrogen in ordinary water, deuterium in heavy water, carbon in graphite — the neutrons lose energy in a succession of collisions until they are in equilibrium with the thermal motion of the moderator at about 0.025 eV. In light water about twenty collisions suffice, but hydrogen also absorbs neutrons, which is precisely why a light water reactor must use enriched fuel while a heavy-water-moderated CANDU can run on natural uranium. Of the neutrons born in one generation, one on average must survive slowing down, leakage and parasitic absorption in the coolant, cladding and structure, and go on to cause the next fission: that condition, $k_{eff}=1$, is criticality, and the control rods hold it there by absorbing the small surplus. A small fraction of the neutrons are emitted not instantly but seconds later from decaying fission products, and it is those delayed neutrons that slow the reactor’s response enough for mechanical control to be possible at all.

Part II (b) continued — how the fission heat is removed effectively. Heat removal is effective because the geometry deliberately maximises surface area and minimises conduction path. Fission energy is deposited almost entirely inside the pellet, so it must cross in series the pellet, the pellet-clad gap, the cladding and finally the clad-to-coolant film. Subdividing the fuel into more than forty thousand thin rods, as in the core of Question 4, gives some 4525 m² of heat transfer surface for 2838 MW, an average flux of 627 kW/m² and an average linear rate of 18.7 kW/m; the pellet is only about 8 mm across so its centreline stays below the melting point despite the low thermal conductivity of uranium dioxide. On the coolant side the primary circuit is pressurised to 15.5 MPa, which puts the saturation temperature at 344.8 °C and leaves the 325 °C core outlet nearly 20 K subcooled, so the water remains liquid, its heat capacity and its heat transfer coefficient stay high, and the departure from nucleate boiling that would insulate the clad with a vapour film is avoided everywhere including the hot channel.