Question 7 of 8: Fuel characteristics

Question 7: Fuel characteristics (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.

The paper letters the third part ‘(b)’ a second time; it is treated as part (c) below, which is how its four marks are allocated.

(a) Heating value, and the difference between the higher and lower figures

The heating value, or calorific value, of a fuel is the quantity of heat released when unit mass (or, for gases, unit volume) of the fuel is burned completely with oxygen, the products being cooled back to the initial temperature of the reactants, conventionally 25 °C. It is measured in a bomb calorimeter at constant volume for solid and liquid fuels, and it is the single most important commercial property of a fuel, because it converts a tonnage into an energy input and therefore fixes both the price and the plant heat rate.

The distinction between the two values turns entirely on the water in the products. Every hydrocarbon fuel contains hydrogen, which burns to water vapour, and most fuels also carry some free moisture. The higher heating value assumes that all of this water is condensed back to liquid at the reference temperature, so it includes the latent heat of vaporisation, about 2442 kJ per kilogram of water at 25 °C. The lower heating value assumes the water leaves as vapour and therefore excludes that latent heat. The difference is the latent heat of the water formed plus the moisture evaporated, and its size follows the hydrogen content of the fuel: for pure carbon there is no hydrogen and no difference at all, which is why the table in Question 5 quotes one number for carbon; for methane, with a hydrogen-to-carbon ratio of four, the difference is 5480 kJ/kg, or about 10 per cent; for a bituminous coal it is 4 to 5 per cent and for a high-moisture lignite rather more.

Which value is used is a matter of convention, and the convention differs by region and by industry, so the basis must always be stated. North American utility practice, boiler guarantees and natural gas sales in Canada and the United States use the higher heating value, chiefly because it is what the calorimeter measures directly and because it represents the whole chemical energy purchased. European practice, most gas turbine ratings worldwide, and internal combustion engine ratings use the lower heating value, on the engineering argument that the latent heat is not recoverable: in a conventional plant the flue gas must leave the stack above its acid dew point, typically 120 to 150 °C, so the water never condenses and that energy is not available to the cycle. The practical consequence is that an efficiency quoted on the lower heating value is several percentage points higher than the same plant quoted on the higher heating value — about two points for coal and four to five for natural gas — and comparing the two without conversion is a common and expensive error. Condensing plant, where the flue gas is cooled below the dew point deliberately, is the exception that proves the rule: it is the one case where efficiencies above 100 per cent on a lower-heating-value basis are routinely and correctly quoted.

(b) Proximate and ultimate analysis of coal

Coal is not a compound but a heterogeneous rock, so it is characterised by two complementary standardised assays rather than by a formula. A proximate analysis is a set of four gravimetric determinations carried out by heating a weighed sample under prescribed conditions: moisture, driven off at 105 to 110 °C; volatile matter, the further mass lost when the dried sample is heated to 950 °C out of contact with air; ash, the residue after complete combustion at 750 °C; and fixed carbon, obtained by difference as 100 per cent minus the other three. Calorific value, sulphur and free-swelling index are usually reported alongside. The four fractions always sum to 100 per cent by construction, and results must be quoted on a stated basis — as received, air dried, dry, or dry ash free — because the same coal can differ by several percentage points between them.

An ultimate analysis is an elemental determination: the mass fractions of carbon, hydrogen, nitrogen, sulphur and ash are measured, and oxygen is normally obtained by difference. It is the chemist’s description of the same coal, whereas the proximate analysis is the operator’s.

The two serve different purposes and neither substitutes for the other. The proximate analysis is the routine quality-control and contract test, because it is quick, cheap and repeatable and because its four numbers map directly onto combustion behaviour: volatile matter governs ignition stability, flame length and the burner and furnace design (a high-volatile coal ignites readily and suits a low-NO_x staged burner, while an anthracite may need support firing); ash governs slagging and fouling of the heat transfer surfaces, erosion of the mills and tubes, precipitator sizing and disposal cost; moisture governs mill drying capacity, the mill air temperature and the flue gas loss; and calorific value with fixed carbon governs the fuel feed rate. The ultimate analysis is what combustion calculations actually need: from carbon, hydrogen and sulphur one computes the stoichiometric air requirement, the flue gas mass and composition, and hence the boiler efficiency by the heat loss method; the carbon and hydrogen fix the carbon dioxide emission per unit of heat, exactly as in Question 5; sulphur sizes the flue gas desulphurisation plant and sets the acid dew point that fixes the minimum stack temperature; and nitrogen contributes to fuel-bound NO_x. In short, the proximate analysis tells the plant how the coal will behave, and the ultimate analysis lets the engineer calculate what it will produce.

(c) Coalification: from peat to anthracite

Coal begins as accumulated vegetal matter — woody tissue, leaves and spores — deposited in a waterlogged swamp where standing water and acidity exclude oxygen and arrest ordinary aerobic decay. Anaerobic bacterial action partially decomposes the cellulose and lignin to form peat, a soft, fibrous, 75 to 90 per cent moisture material in which the original plant structure is still visible. This first stage is biochemical and takes centuries to millennia.

Everything after that is geochemical. Once the peat bed is buried under later sediment, compaction expels free water and the rising overburden pressure and, far more importantly, the rising temperature with depth (roughly 25 to 30 K per kilometre) drive a slow series of condensation and elimination reactions in the organic matter over tens to hundreds of millions of years. Time, temperature and pressure together determine how far the process runs, and the resulting position in the series is called the rank of the coal: peat, lignite (brown coal), sub-bituminous, bituminous, semi-anthracite and anthracite.

Three constituents change systematically and in a fixed order. Moisture goes first: from 75 per cent in peat, to 30 to 45 per cent in lignite, 15 to 25 per cent in sub-bituminous, 2 to 8 per cent in bituminous coal and 2 to 4 per cent in anthracite. Then the oxygen-bearing functional groups — carboxyl, hydroxyl and methoxyl — are eliminated as carbon dioxide and water, so the oxygen content falls from about 30 per cent in peat to under 3 per cent in anthracite. Finally, at higher rank still, the hydrogen-rich aliphatic side chains are cracked off as methane and the remaining aromatic sheets condense and grow, so volatile matter falls from about 65 per cent to under 8 per cent while fixed carbon rises from roughly 30 per cent to over 92 per cent on a dry, ash-free basis. Ash, being mineral matter, does not follow the sequence: it depends on the depositional environment, not on rank.

These compositional changes explain every practical property of the fuel. Because moisture and oxygen are both heat sinks — the first absorbs latent heat, the second is already partly oxidised carbon and cannot release energy — the calorific value rises steeply with rank, from about 8 MJ/kg for wet peat and 15 MJ/kg for lignite to 31 MJ/kg for bituminous coal and 33 MJ/kg for anthracite. High-volatile, lower-rank coals ignite easily and burn with a long luminous flame, so they suit pulverised firing and staged low-NO_x burners, but their moisture loads the mills and the flue gas loss, and their high oxygen content makes them prone to weathering and to spontaneous combustion in the stockyard. Anthracite is dense, hard, clean-burning and easily stored, but it is difficult to ignite, needs fine grinding, a hot furnace and often support firing. Between them, the medium-volatile bituminous coals develop the plastic, caking behaviour on heating that makes metallurgical coke, a property that peaks in a narrow rank window and is absent both below and above it. The engineer’s point is that rank is not a quality grade: the best coal is the one whose position in this series matches the equipment it will be burned in.