Fuel properties

Heat Engines & Boilers

Contents• Classification of fuels • Definition of lower and higher

heating value• Analysis methods of solid fuels,

combustible and ash properties, solid fuel supply systems

• Analysis methods of liquid fuels, liquid fuel supply systems

• Analysis methods of gaseous fuels, gaseous fuel supply systems

Fuel classification

State of matter Fossil Renewable

• Solid Coal Biomass: black, brown, lignite wood, cane, grass, etc.

energy plants & waste materials

• Liquid Crude Oil Biomass:Petrol, kerosene, Vegetable oil & bio-dieselDiesel Oil Bio-ethanol

• Gaseous Natural gas Bio-gas

Digester gas,

pyrolysis-gas from gasification

Fuel features

• Fuels can be delivered, stored and burned at different power level, according to energy demands

Fossil Renewable

• Energy content High Low

• Delivery even for long only for shortdistance distance

• Storage capacity small large

Circulation of Carbon

Heating Value• Heating value (calorific value) is the heat released by the fuel when completely

burnt, and may be determined at constant volume or constant pressure, and flue gas is cooled back to the initial temperature (ambient temperature)

• - higher ( gross) heating value ( HHV ) - assumes that the water vapor in theproducts condenses and thus includes the latentheat of vaporization of the water vapor in theproducts.

• - lower heating value ( LHV ) - does not. contain the latent heat, the waterin flue gas remain in steam form at the initial temperature

possible dimensions: J/kg, J/m3, kWh/kg, kWh/m3

mw = mass of water vapor per unit mass of fuelhfg = latent heat of vaporization of water vapor / at its partial

pressure in the combustion products [J/kgH2O]mH2 = mass of original hydrogen per unit mass of fuel.

LHV HHV m h m hw fg H fg= − ⋅ − ⋅ ⋅92

Composition of solid fuels, and analysis methods

Original substance Green coal, damp wood, waste in original state

Incombustible, ballast

General composition

Combustible Mineral matter

Moisture, or Water content

Drying at ambient air Air-dry fuel

Water free fuel

Drying at 105 °°°°C Combustible Incombustible

Hygroscopic moisture

Free or surface

moisture,

Heating at 850 °°°°C

without air

Coke residue

Complete combustion at 850°°°°C

Result: Proximate analysis

Volatile matter Fix

Carbon

(fixC)

Ultimate analysis Sulphur

(S)

Nitrogen

(N)

Oxygen

(O)

Hydrogen

(H)

Total Carbon

(C)

Ash

(a)

Total moisture,

Water content

(w)

Properties of solid fuels

Properties of solid fuels

Properties of solid fuels

10.8390.71554Natural gas

10,20036,50087011.742Oil

4,500-9,10016,000-33,000800-1,1005.6-8.320-30Coal (lignite to

anthracite)

3,000-3,50010,800-12,600600-700518Wood pellets

2,300-4,6008,100-16,800450-8005-5.818-21Wood (solid -

oven dry)

1,300-2,3004,500-8,300300-5504.215

Log wood (stacked - air

dry: 20% moisture content)

600-1,0002,000-3,600175-3502-47-15

Wood chips (Very

dependent on moisture content)

Energy by volumekWh/m 3

Energy by volumeMJ/m3

Bulk densitykg/m 3

Energy by mass

kWh/kg

Energy by mass

GJ/tonneFuel

Water or moisture content• There are two methods used to calculate the moisture content, ‘Wet Basis’ and

‘Dry Basis’. The most common method in energy terms is wet basis, whilst foresters tend to use the dry basis. It is important to note that the two methods

will give a different result for the same piece of wood.

Example

A quantity of wood has a total mass of 10kg. It is dried in an oven so that all water is removed and then weighed. Its new mass is 8kg. The moisture content is calculated as:

WET BASIS

%20)10(

)2()( ==

kgwoodwetofmass

kgwaterofmassMCcontentmoisture

DRY BASIS

%25)8(

)2()( ==

kgwooddryofmass

kgwaterofmassMCcontentmoisture

Relationship between water content

and calorific value of wood

Wood as renewable

�Less suitable for automated systems (although some do exist)�Large storage space required to allow 1 – 2 years for seasoning

�Logs can be stored and transported conveniently when stacked�Ease of air passage through a log pile allows good drying�Can be easily produced on site or very locally

Logs

�More expensive fuel costs �Supply is less likely to be localised and so does not provide local economic impacts

�User input similar to conventional heating installations�Cheaper capital costs due to the drier and more homogeneous nature of the fuel�Denser fuel means reduced storage space and easier transport�Suitable for very small appliances

Wood pellets

�Can require greater user input, depending upon the quality of the fuel�System has a higher capital cost since wood chips require larger storage capacity and more robust fuel handling equipment�Need the services of a specialised chipper�Only suitable for larger appliances i.e. >25 kW

�Easy to produce locally from woodland thinnings etc. �Much cheaper fuel costs.�Expenditure on wood fuel can benefit the immediate local economy.

Wood chips

DisadvantagesAdvantagesFuel

Wood processing and woodchips collection

Pellet manufacturing from sawdust

Ash• Fuel contains incombustible parts,

known as ash, mainly potassium (K), sodium (Na), phosphorous (P), calcium (Ca),silicon (Si).

• Halogen content (Cl) and (F) also importantfrom environmental and corrosion viewpoint

• Waste materials can contain heavy metals and other pollutants

• Ash softening and melting properties are very important for combustion management

Bunte-Baum test

Bunte-Baum test result

Leitz type heated microscope

Leitz type heated microscope results

Composition of solid fuels• Proximate analysis is used for combustion

behavior evaluation• Ultimate analysis is used for combustion

(stoichiometric) calculations• Ash softening properties are important from

combustion chamber operation• Pollutants can be either neutral from combustion

viewpoint, (e.g. heavy metals)or can take part in reaction (e.g. Cl to HCl)but in both case are important from emission viewpoint

Fuel storage and supply system• Fuel size and size distribution

- from combustion viewpoint size distributionhas to be in narrow range

- but milling or pressing has certain energy demand

and increases fuel cost• Task so this system is to prepare and feed fuel for

reaction and for feeding into the combustion chamber.

• Furthermore fuel has to be available when it is needed, so storage or utility connection is also included in this system.

• In case preparation needs heat it is generally supplied from the boiler, which is called self consumption.

Fuel storage example

Photo: Beacon Stoves The log store should allow for plenty of air flow b ut should protect the logs from the rain

Wood pellet storage and feeding

Bagged wood pellets Pellet store using partition walls and auger

Pellet store and vacuum pellet feed

Pre-fabricated silo and auger

Wood chip storage and feeding

Rotary stirrer facilitates removal of chips

Wood chip bunker showing stirrer and

screw conveyor

Covered area for drying and storing large

quantities of wood chips

A ramp allows wood chips to be tipped

into the store

Wood chip storage and feeding

Wood chip storage and

feeding

Wood chip storage and

feedingbase plan

Straw-bale storage and feeding

Wood densities[kg/m3]

369627897297505722484823117745978111750

30755274724842160240368698038365193040

26444864021236151634658884032855879830

23139256018631645230351473528748869820

2173695271752974252854846922045965715

20534849816528140126945765425543462010

1843134481492533612424115882303905580

Moisture Content, Wet Basis, %

2.431.4312.431.4312.431.4312.431.431Packing

Ratio

ChipLogSolidChipLogSolidChipLogSolidChipLogSolidFuel type

PineSpruceOakBeechSpecies

Necessary fuel storage volumeIn order to calculate the store volume required the

following parameters are required:• Energy demand• Calorific value of fuel• Density of fuelThe two important equations are given below.

)/(

)()(

kgkWhvaluecalorific

kWhdemandenergyGrosskgnconsumptioFuel =

)/(

)()(

33

mkgdensityfuel

kgnconsumptiofuelmvolumeFuel =

Example for fuel storage calculation

Calculate the store volume for wood pellets required if:

- The annual heat requirement is 24 000 kWh and the monthly heat requirement during the coldest month is 4 000 kWh.

- Wood pellets density = 600 kg/m3

- Wood pellets calorific value 3.2 kWh/kg.

- Pellets are to be delivered at a maximum frequency of once a month.

1) Calculate the fuel consumption during the coldest month:

monthperkgkgkWh

kWhkgnconsumptioFuel 2501

)/(2.3

)(0004)( ==

2) Calculate the volume of fuel required:

33

3 08.2)/(600

)(1250)( m

mkg

kgmvolumeFuel ==

Properties of liquid fuels

• Ultimate analysis is used for composition investigation (similarly to solid fuels)

• Heating value is used for energy content evaluation (similarly to solid fuels)

• Liquid fuel can not be burnt in liquid formit has to be in gaseous form, so it has to be evaporated

• Liquid fuels can be stored in tanks• Liquid fuels can be delivered via pipelines

by means of appropriate pump

Properties of liquid fuelsPoint of solidification is understood as the temperature at

which the product no longer flows upon the effect of the gravitational force. Its value is significant first of all in respect of transportation.

Flash point is the temperature at which as much vapor generates from the liquid fuel under atmospheric pressure that, mixed with the ambient air, upon approach of flame it flashes over the whole oil surface. This value is used also for characterization of explosion and fire protection.

Firing point is the temperature at which vaporization of the liquid is of such extent, that with the approach of the flame for a short time it is ignited and the burning will be constant on the surface maintaining for at least 5 s. The firing point is characteristic to the inflammability of the fuel.

Conradson number The liquid fuel is heated and vaporized in an air-tight vessel. The retained coke part related to the initial amount of oil gives the Conradson number. The coking liability is an important characteristic.

Properties of liquid fuels

Properties of liquid fuels

20212012070Flash point [°°°°C]

285050- 6Pour point [°°°°C]

0.9160.960.950.84Density at 15°°°°C [kg/l]

0.3560.10.1-w water

0.350.30.3-N Nitrogen

10.161.01.0-O Oxygen

0.11.02.30.3S Sulphur

12,4711.011.113.4H Hydrogen

76.5686.685.286.3C Carbon

Composition [% w/w]

Animal Fat

Fuel Oil SA

Fuel Oil SFuel Oil EL

Properties of liquid fuels

42.143.3842.7645.76Higher Heating Value [MJ/kg]

39.340.9440.3842.82Lower Heating Value [MJ/kg]

0 (0.257)0.280.280.27CO2 emission [kg/kWh]

15.7116.0216.0015.31CO2max [% V/V]

1.121.000.971.20Water content in fluegas [kg/kg]

10.4811.3311.1711.86Wet fluegas volume [m3/kg]

9.0910.1610.0410.46Dry fluegas volume [m3/kg]

9.8110.7910.6511.22Air requirement [m3/kg]

Animal Fat

Fuel Oil SA

Fuel Oil S

Fuel Oil EL

Viscosity variation of liquid

fuels

Comparison

of bio-ethanol

with fossil fuels

74,271,871 - 7271,5CO2 emission, g/MJ

-7040-9016Vapor pressure, 38 ˚C, kPa

45-55108-148Cetane number

-10080-8892Motor octane number

-10088-98111Research octane number

---4,4Azeotrop water content, %

14,615,0714,78,97Stoichiometric air to fuel ratiomass / mass

42,743,54326,7Lower heating value, MJ/kg

45,847,847,229,8Higher heating value, MJ/kg

1,4-7,61,1-6,01,4-7,64,3-19,0Flammability range in air, V/ V %

250447495423Spontaneous ignition temperature, ˚C

704-43 (-) -3912,8Flammability point, ˚C

-0,2170,2510,662Evaporation heat, 20 ˚C, MJ/dm 3

0,2560,3140,3490,839Evaporation heat, 20 ˚C, MJ/kg

180-36099,225-22078,5Boiling temperature, ˚C

<3500<150<1Sulphur content, ppm

0,840,69190,720-0,7800,7893Density, g/cm 3

000-2,734,73O

13,915,8812-14,313,13H

86,184,1283-8852,14Ultimate analysis, %, C

208114,23110(average)46,07Molar mass

C15H28C8H18C4 -C12CH3CH2OHFormula

Diesel oilIsooctanePetrolEthanolCharacteristic data

Liquid fuel storage conditions

Properties of gaseous fuels

• Gaseous fuels can be handled as ideal gas mixture of different gas components

• Molecule analysis is used for composition investigation (instead of ultimate analysis)

• Heating value is used for energy content evaluation (similarly to solid and liquid fuels)but it is generally given by volume

• Gaseous fuels can be stored in tankseither in gaseous form under high pressureor in liquefied form under medium pressure

• Gaseous fuels can be delivered via pipelines by means of pressure difference

Important parameters of gaseous fuels• Relative density: d = ρgas/ρair [-]

(important from explosion safety viewpoints)

• Wobbe index(for assessment of gas exchange)

Extended Wobbe index

where: Hs - HHV higher heating value of the gas

d - relative density

∆p – pressure drop at a fuel nozzle

Flammability limits• Flammability limits, also called flammable limits, give the

proportion of combustible gases in a mixture, between which limits this mixture is flammable.

LFL The lower flammable limit describes the leanest mixture that is still flammable, i.e. the mixture with the smallest fraction of combustible gas,

UFL The upper flammable limit gives the richest flammable mixture.

• Increasing the fraction of inert gases in a mixture raises the LFL and decreases UFL.

• Flammability limits of mixtures of several combustible gases can be calculated using Le Chatelier's mixing rule for combustible volume fractions xi:

(and similar for UFLmix)

Gas Mixtures Having High Inert ContentGenerated from Biomass

Digester gas from anaerob fermentation:

• Main part: CH4 + CO2

• Pollutants: H2S, CO, H2O, particulates

• Heating value: 18 - 30 MJ/m3

Gasification or pyrolysis gas:

• Main part: CH4+CO+H2 + CO2+ N2+H2OPollutants: H2S, tar, particulates, coke

• Heating value: 5 - 20 MJ/m3

Biomass conversion types

Summary

You are already familiar with: • Properties of different fuels from combustion

viewpoint• Classification of fuels • Definition of lower and higher heating value• Analysis methods of solid fuels, combustible and

ash properties, solid fuel supply systems• Properties of liquid fuels,

liquid fuel supply systems• Properties of gaseous fuels,

gaseous fuel supply systems

Thank You for Your Attention !