Question 6 of 8: Coal Fired Boiler

Question 6: Coal Fired Boiler (10 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.

This is a descriptive question about the architecture of a large pulverised-coal utility boiler. The four parts are answered in turn, with the annotated arrangement below serving as the marked-up version of the Page 15 diagram.

Part (a) — identification of the components. The tall rectangular shaft on the left of the diagram, with the flame in its lower half and its walls formed of closely spaced tubes, is the furnace. The large cylinder at the top left, outside the gas path and connected to the furnace walls by the long unheated pipe running down the outside, is the boiler drum; that outside pipe is the downcomer and the wall tubes themselves are the risers. The two tube banks hanging in the gas stream immediately at the furnace exit, where the gas is hottest after leaving the flame, are the secondary superheater (the nearer one, fed from the primary superheater and delivering to the turbine) and the reheater (the second one, whose steam lines leave the diagram separately at a lower elevation because it takes cold reheat steam back from the high-pressure turbine exhaust). The horizontal serpentine bank in the upper part of the rear pass is the primary superheater, and the bank below it, at the cold end of the gas path with the feedwater line entering it, is the economiser. The convention on the drawing settles any ambiguity: dashed lines carry steam and solid lines carry water, so the only bank fed by a solid line is the economiser and the only large solid-line circuit is the drum, downcomer and waterwall loop.

Part (b) — the combustion system and fuel handling. A boiler of this arrangement burns pulverised coal in suspension, fired through wall or corner burners into an open water-cooled furnace. It is not a stoker or a fluidised bed: there is no grate at the bottom, only a hopper for ash falling out of the gas stream, and the flame occupies the free volume of the shaft rather than sitting on a bed. Raw coal arrives from the bunkers by gravity onto gravimetric feeders, which meter it by weight to each mill so that the fuel/air ratio can be controlled. It then passes into a pulveriser — typically a vertical-spindle bowl mill or a ball-and-race mill — which grinds it so that about 70  % passes a 200-mesh (75 µm) screen. Hot primary air, drawn from the air heater and tempered with cold air to hold the mill outlet at roughly 65–90 °C, is blown through the mill; it dries the coal and carries the pulverised fuel out of the classifier at the top of the mill. The classifier returns oversize particles for regrinding. The fuel-and-air mixture is then conveyed through pipes to the burners, where it meets the much larger secondary air flow admitted through registers around each burner throat, which supplies most of the combustion air and imparts the swirl that stabilises the flame. Because the particles are so fine they burn essentially as a gas, with residence in the furnace of only one or two seconds. In a Canadian utility context the ancillaries that go with this system are equally characteristic: an induced-draught fan and forced-draught fan pair balancing the furnace at a slight negative pressure, electrostatic precipitators or fabric filters for particulate control, and — where provincial regulation or the federal coal-fired electricity regulations demand it — low-NO_x burners with overfire air, and flue-gas desulphurisation for higher-sulphur fuels.

Part (c) — radiant and convective heat transfer. The split follows the gas temperature. The furnace waterwalls receive their heat almost entirely by radiation from the flame and the incandescent particle cloud, at gas temperatures of 1400–1700 °C where radiation varies as the fourth power of absolute temperature and swamps convection; roughly half the total heat absorption of the boiler occurs here. The secondary superheater and reheater at the furnace exit are mixed: hung as widely spaced platens or pendants in a gas stream at 1100–1300 °C, they see the furnace radiantly as well as taking convective heat, and are often described as radiant or platen surfaces for that reason. The primary superheater and the economiser are convective, being closely pitched banks buried in a gas stream that has already fallen below about 900 °C, where the gas is largely transparent and heat moves by forced convection across the tube bundle. The air heater downstream of the economiser is purely convective. The practical consequence is that radiant and convective surfaces respond in opposite directions to load: radiant absorption per unit of steam flow falls as load rises, convective absorption rises, and boiler designers exploit that opposition to hold the final steam temperature flat across the load range.

Part (d) — purpose and placement of each surface. The economiser recovers the last useful heat from the flue gas by warming feedwater from the final feedwater heater up towards saturation before it enters the drum. It must sit at the coldest end of the gas path because that is the only place where a cold stream still has a temperature difference to work with, and putting it there is what pulls the stack temperature down to 130–150 °C and lifts boiler efficiency by several points. It is placed last also to protect it: water inside it is the coolest fluid in the unit, and it would steam locally if exposed to furnace-exit gas. The superheaters raise dry saturated steam leaving the drum to the design throttle temperature, which increases the work per kilogram of steam, raises the mean temperature of heat addition and — most importantly for the turbine — keeps the expansion out of the wet region for most of its length so that blade erosion is controlled. They are split into a primary and a secondary section with the attemperator between them: the primary sits in the convective pass where the duty is moderate and the tube metal stays cool, and the secondary, which carries the highest steam temperature in the plant and therefore the most expensive alloy, is placed at the furnace exit where the gas is hot enough to deliver the last 100 K of superheat with a sensible temperature difference. Splitting them also gives the control system somewhere to inject spray water. The reheater returns steam that has already expanded through the high-pressure turbine back to a high temperature before it enters the intermediate-pressure turbine. Its purpose is to add a second slug of high-temperature heat, which raises cycle efficiency by four or five percentage points and keeps the low-pressure exhaust dry enough to avoid moisture damage. It sits alongside the secondary superheater at the furnace exit because it too needs a high gas temperature, but it operates at a much lower pressure — a few megapascals rather than sixteen or more — so its tubes are thin-walled and cool poorly, which is exactly why it must never be placed where it could be starved of steam while the furnace is firing.