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.

(a) Identification of the components. The tall left-hand shaft, lined on all four walls with closely spaced tubes and containing the flame, is the furnace. The horizontal cylinder at the top left, joined to the bottom of the furnace by the external downcomer and to the top of the water walls by the risers, is the boiler drum. In the rear pass, where the gas is coolest, the lowest horizontal serpentine bank carries the incoming feedwater and is the economiser; immediately above it, the first bank fed with saturated steam from the drum is the primary superheater. The two pendant banks hung in the furnace roof take the hottest gas: the one at the furnace exit, which finishes the main steam, is the secondary (finishing) superheater, and the one immediately downstream of it, fed from the cold reheat line returning from the high-pressure turbine, is the reheater. On the returned attachment these six labels are written directly against the corresponding features.

(b) Combustion system, fuel preparation and conveying. The arrangement is a pulverised-fuel suspension-fired boiler with wall-mounted burners on opposite furnace walls, which is what the burner ports drawn on both sides of the furnace indicate. Raw coal is delivered from the bunkers through gravimetric feeders to the mills, where it is ground until roughly seventy per cent passes a 75 µm sieve. Hot primary air, drawn through the air heater and tempered to control mill outlet temperature, is blown through the mill to dry the coal and then carries the pulverised fuel in suspension along the fuel pipes to the burners. At the burner the fuel-laden primary air is discharged into a swirling stream of secondary air supplied from the windbox by the forced draught fan; the swirl anchors the flame and completes the combustion in the furnace volume. Because the particles are so fine, they burn in suspension in one or two seconds, which is what allows a furnace of this size to release the several hundred megawatts of heat a large unit needs. Ash leaves in two streams: the heavier fraction falls into the hopper at the furnace bottom, while the fly ash is carried with the gas to the precipitator or bag filter downstream of the air heater.

(c) Radiation and convection surfaces. The furnace water walls, and the leading rows of the pendant secondary superheater that see the flame directly, absorb heat overwhelmingly by radiation from the luminous flame and the hot gas; radiation dominates wherever the gas temperature is high, because the exchange goes as the fourth power of absolute temperature. Everything in and beyond the furnace nose — the rest of the pendant banks, the reheater, the primary superheater and the economiser in the rear pass — absorbs mainly by convection from gas that is now cool enough and clean enough of luminous particles that radiation is secondary. The banks nearest the furnace exit are genuinely mixed, and that mixed character is exploited deliberately, as the next part explains.

(d) Purpose and placement of each surface. The economiser recovers the last useful heat from the flue gas into the feedwater. It sits at the cold end of the rear pass because the feedwater is the coldest fluid in the unit, so it is the only surface that can still extract heat from gas at 300 °C or below; putting it anywhere hotter would waste high-grade heat on low-grade duty and risk steaming in the tubes. Its practical limit is the acid dew point of the gas, which is why the gas is not cooled further until it reaches the air heater.

The superheaters raise the steam above saturation, which increases the enthalpy drop available in the turbine and, more importantly, keeps the expansion dry enough to protect the last-stage blading. They are split into a primary and a secondary section for control: the primary sits in the convective rear pass, where its absorption rises with gas flow and hence with load, while the secondary hangs in the radiant furnace exit, where its absorption falls relatively as load rises. Combining the two opposite characteristics, with an attemperator between them, is what holds the final steam temperature nearly constant over the load range. The secondary section must be last on the steam side and first on the gas side because it produces the hottest steam and therefore needs the largest driving temperature difference.

The reheater returns the steam exhausted from the high-pressure turbine to close to main-steam temperature before it enters the intermediate-pressure turbine. This adds several points of cycle efficiency and, again, keeps the low-pressure exhaust dry. It is placed just downstream of the secondary superheater, in gas hot enough to reach the required steam temperature but not so hot as to overheat tubes that carry steam at only a quarter of the main-steam pressure and therefore have thin walls and poor cooling at low flows. Its position also explains why the reheater must never be fired with the turbine off load: with no steam flow through it, the tubes have nothing to carry the absorbed heat away.