Fuel characteristics

Fuel & Combustion2/2006

Dr. Suneerat Pipatmanomai

FuelFuels are any materials that can be burnt to release thermal energyMost familiar fuels consist primarily of C and H – called hydrocarbon fuels, denoted as CnHmFuels can be broadly classified as

Form of fuel Primary (natural) Secondary (synthetic)

Liquid oil liquid biofuel

Solid coal, oil shale, biomass

charcoal, coke, MSW

Gas natural gas biogas, refinery gas

Fossil Fuels

Natural gas, oil and coal are the three (fossil) fuels that are abundantly used. This energy is a stored form of solar energy that accumulated over millions of years, and at the current and projected rates of consumption, fossil fuels will be used up in a fraction of time compared to the time it took to collect the energy from the sun.

Natural gasNatural gas like petroleum is generally believed to be derived from deposits of plant and animal remains from millions of yearsago. It may be found along with oil or by itself as in many gas fields where little or no oil is found. As supplied is the cleanest fuel with sulfur removed (except forsmall amounts of odorants added)Simplest in term of composition and being a gas mixes immediately in the combustor. Along with methane which is by far the major combustible constituent of natural gas, other light hydrocarbons, namely ethane, propane, and butane are present.No ash and only molecular nitrogen, and a high H/C ratio which minimizes the greenhouse gas CO2 emission.

Coal Is the least clean (fossil) fuel containing sulfur, elemental nitrogen, low H/C ratio and ash Coal has a very complex structure and being a solid is more difficult to burn. Coal combustion undergoes devolatilisation and combustion of the released gases, char combustion and fly ash formation which are particles 10 microns in size (the low visibility around certain coal fired power plants is due to the fly ash).

Almost all of the coal consumed in the world is for electric power generation by combusting the coal in boilers and generating steam to power a turbine.

Coal is being used to a limited extent in gasification based plants to produce gas to fuel gas turbine based combined cycles (IGCCs) and in some countries such as China for chemicals synthesis. With more advanced gas turbines under development, coal based IGCC will have a strong economic and environmental basis to compete with boiler based power plants.

Coal is classified into the following four types according to the degree of metamorphism:

Anthracite which is low in volatile matter (which forms tars, oils and gasses when coal is heated) and consists of mostly carbon (fixed carbon) Bituminous which contains significant amounts of the volatile matter and typically exhibit swelling or caking properties when heated Sub-bituminous is a younger coal and contains in addition to the volatile matter, significant amounts of moisture Lignite is the youngest form of coal (when peat is not included in the broader definition of coal types) and is very high in moisture content resulting in a much lower heating value than the other types of coal.

Oil Represents an intermediate fuel in terms of quality. Petroleum oil is a mixture of a number of hydrocarbons with some sulfur, nitrogen and organo-metallic compounds also present. A number of processing steps are involved in producing the various high value salable fuel streams such as gasoline, dieseland jet fuel from the petroleum. Oil which contains more than 300 molecular species needs to be atomized (less than 10 microns to provide large surface area), and within the combustor it has to vaporize and mix before combustion can occur).

Oil shaleThe organic solids in oil shale rock are a wax-like material called kerogen. Kerogen is extracted by heating in retorts in the absence of air where it decomposes forming oil, gas, water and some carbon residue. Production of gasoline or jet fuel from the oil produced from the oil shale, however requires more extensive processing than most petroleum feedstocks. The shale oil also contains more nitrogen than petroleum does which if left in the fuels produced from the shale oil would result in significant NOx emissions.

Non-fossil fuels

BiomassIs all plant and animal matter on the Earth's surface including trees, crops, algae and other plants, as well as agricultural and forest residues plus other wastes, e.g. MSW, industrial wastes, wastewaterRenewable (produced sustainably)Considered carbon neutral fuel. When using biomass to displace fossil fuels, CO2 emissions are largely avoided and the overall system is often carbon neutral or close to it.Multiuse – food, energy, materialsDistributed nature and can be grown close to where it is used

Fuel contains combustibles, which should be known for stoichiometric calculations.Analyses of various solid fuels are conducted for

Proximate analysis: Moisture, Volatile matter (VM), Mineral matter (or ash), Fixed carbon, Calorific valuesThe value of proximate analysis

Identifies the fuel value of the as-received materialProvides an estimate of ash handling requirementDescribes something of the burning characteristics

Ultimate analysis: C, H, N, O, SDescribes something of the burning and product characteristics

Fuel properties

Calorific values = Heat of combustion of fuelDefined as “the total heat produced when a unit mass of fuel is completely burnt with pure oxygen”Two terms of calorific values

NCV or LHV: when water vapour is present in the flue gas (the latent heat of vapourisation is lost)GCV or HHV: when water vapour is condensed and therefore this latent heat is added

NCV = GCV – (% mass of hydrogen) x 9 x λv

λv = latent heat of vapourisation at reference temperature= 2442.5 kJ/kg at 298.15 K (25°C)

MoistureWater expelled from fuel in its various forms (when tested under specified conditions)Normally moisture content is determined by drying sample of known mass at 110°C until no further weight loss is observed.Depends on a combination of its origination and treatment/storage

Biomass: harvesting method, climatic conditions, time of year when harvesting takes place and the length and method of storageCoal: coal rank, method of storage, pre-treatment

Moisture content has a significant effect on many of the energy conversion processes. For example,

The percentage of solids present in the digestate when biogas is obtained from an anaerobic digestion process affects the gas yields For dry biomass fuels, such as wood or straw, the amount of water present has a considerable effect on the proportion of the total heat content of the material that is possible to recover as a result of combustionHigh moisture fuel makes feeding system difficult, render agglomeration, incomplete combustion

Volatile matter (VM) and Fixed carbonVM = Total loss in the weight minus the moisture in fuel when heated under specified conditionsFixed C is normally obtained by difference

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0 10 20 30 40 50 60 70 80 90 100 110Time (min)

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TGA result for proximate analysis of solid fuel

(1)

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(1) moisture

(2) Volatile matter

(3) Fixed carbon

(4) Ash

Generally biomass fuels are highly volatile (= low fixed carbon) and need to have specialized combustor designs to cope with rapid gas evolution when heatedFuels with low volatiles, such as coal, need to be burnt on a grate as they take a long time to burn out unless they are pulverized to a very small size

Mineral matter or often referred to as ash contentInorganic residue left over when fuel is incinerated (completely combusted) in air to constant mass under specified condition Characterization of ash by elemental analysis and fusion temperatures is an important aspect of utilizing biomass fuelsAsh analysis provides

Information on how much ash there will be to disposeInformation on whether special ash treatments are needed before disposal Information on slagging, fouling and clinker formation in the burner and boiler to be predicted

Ash management presents both a problem and an opportunity

Removal of ash from the furnace and disposal in landfill areas incurs costs for power plantsAsh can be recycled in the forest ecosystem, depletion of plant nutrients (other than nitrogen) and acidification associated with intensive biomass removal, is then radically reduced

Examples of slag problemFor pure wood combustion, the combustion temperatures are likely to be low, ash fusion does not usually represent a problem; however, when wood is co-fired with coal, combustion temperatures are considerably higher and may reach a level where slagging could occur

In the case of straw or palm EFB combustion, ash fusion and the resulting slagging represent a considerable problem which has to be solved by special boiler designsThe combination of some mineral matters in coal also increase slagging potential

Bottom ash Slag

Ultimate analysis: C, H, N, O, SFor ultimate analysis, fuel sample is burnt in a current of oxygen producing water, carbon dioxide, nitrogen oxide and sulfur dioxide, which are measured to determine the amount of the original elementsThe results are normally presented on air-dried basis Converting to as-received basis by

As-received basis = Air-dried basis x (100 – moisture)100

Attempts have been made to correlate the ultimate analysis of a fuel with its calorific value. One of the most commonly used relationships is that given by Dülong

GCV (kJ/kg) = 33950 C + 144200 [H – (O/8)] + 9400 S

Where C and S = mass fraction of carbon and sulfurH – (O/8) = mass fraction of net hydrogen

= total hydrogen – 1/8 (oxygen)

Calderwood equation is relating total carbon content with the proximate analysis and the GCV

mass % of carbon = 5.88 + 0.00512 (GCV – 40.5 S) ± 0.0053 [80 – 100 (VM/FC)]1.55

If 100 (VM/FC) > 80, the sign is (-) and vice versa

Calculate NCV at 298.15 K of crude oil having following properties:

Ultimate analysis: 87.1% C, 12,5% H and 0.4% S (by mass) GCV at 298.15 K is 45,071 kJ/kg oilLatent heat of water vapour at 298.15 K = 2442.5 kJ/kg

The GHV of gaseous propane is 2,219.71 kJ/mol at 298.15 K, calculate its NHV

Exercise

Combustion

Is a chemical reaction during which a fuel is oxidised and a large quantity of energy is releasedFor any combustion reaction, oxygen is the agent which will combine with carbon, hydrogen and sulfurIn normal practice, air is used since it is the cheapest source of oxygen (about 21 mole% of air)One drawback of air utilisation is the presence of nitrogen (79 mole%), which reduces the flame temperature considerably and also accounts for the high heat loss of stack Oxygen has much greater tendency to combine with hydrogen than it does with carbon, therefore hydrogen is normally burned to completion forming H2O. Some of carbon, however, ends up as CO or just as plain as C particles (soot) in the products.

It should also be mentioned that bringing a fuel into intimate contact with oxygen is not sufficient to start a combustion process. The fuel must be brought above its ignition temperature to start combustionMinimum ignition temperatures of various substances in air

Gasoline 260°CCarbon 400°CHydrogen 580°CCarbon monoxide 610°Cmethane 630°C

Moreover, the proportions of the fuel and air must be in proper range for combustion to begin, e.g. natural will only be burn in air in concentration between 5-15%

Combustion equations are balanced on the basis of the conservation of mass principle: The total mass of each element is conserved during a chemical reaction

2 kg of hydrogen 16 kg of oxygen 2 kg of hydrogen 16 kg of oxygen

H2 + ½ O2 = H2O

Theoretical or stoichiometric amount of air = the minimum air required to burn fuel completely so that C, H and S are converted into CO2, H2O and SO2, respectively

Theoretical/ stoichiometric air

Consider combustion reactionsmole of O2 needed/ 1 mole of reactant

C + O2 = CO2 1H2 + ½ O2 = H2O ½S + O2 = SO2 1

Theoretical air demand (in moles) = Theoretical oxygen demand (in moles)/ 0.21

mole of air needed/ 1 mole of reactantCH4C6H12O6

Stoichiometry

For a hydrocarbon fuel given by CxHy, the stoichiometric relation can be expressed as

Where a = x + y/4Composition of air is 21% O2 and 79% N2

Each mole of O2 in air, there are 3.76 moles of N2

CxHy + a(O2 + 3.76N2) xCO2 + (y/2)H2O + 3.76aN2

F = = (A/F)stoi

(A/F)

(F/A)(F/A)stoi

for fuel-rich mixtures, F > 1 fuel-lean mixtures, F < 1stoichiometric mixture, F = 1

Where A/F = mass ratio of air to fuel

Equivalence ratio

The equivalence ratio, F, is commonly used to indicate quantitatively whether a fuel-oxidizer mixture is rich, lean, or stoichiometric.

In actual practice, theoretical air is not sufficient to get complete combustion. Excess air supply (or, in the other words, excess oxygen supply) is essential for complete combustion. % Excess air

= (actual air supply – theoretical air demand) x 100theoretical air demand

The actual percentage excess air depends on the fuel used for combustion. Normally gaseous fuels require very less excess air, i.e. 5-15% excess air, than liquid and solid fuels, which require 10-50% excess air.Excess air can reduce the flame temperature and increase the heat losses through the flue gases

Excess air

Theoretical as well as actual air requirements are expressed in kg/kg of fuel by multiplying with the average molar mass of air m3/kg of fuel by multiplying with specific volume of air at that condition

Normally, flue gases contain CO2, CO, H2O, O2, SO2 and N2, with very low concentration of SO3.Water in flue gases

Interferes with the gas analysis, it is removed prior to the analysis of dry gases.Comes from three sources: water vapour product, evaporated moisture in fuel, water vapour accompanying air for combustion

Exercise

One kmol of octane is burned with air that contain 20 kmol of O2. Assuming the products contain only CO2, H2O, O2, and N2, determine the mole number of each gas in the products and the air-fuel ratio for this combustion process.

Exercise

The ultimate analysis of a residual fuel oil sample is given below:C: 88.4 %, H: 9.4%, and S: 2.2% (mass)

It is used as a fuel in a power-generating boiler with 25 % excess air. Calculate

(a) the theoretical dry air requirement(b) the actual dry air supplied(c) composition of flue gases