Fossil Fuels

Fossil Fuels

K.C. Yadav, AVP, Noida Technical Training Centre

Contents     

Definition, type and resources Analysis of combustibles and impurities Heating value Combustion character Tariff

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Definition Chemical Fuels can loosely be defined as the substances, containing the significant quantities of the elements, which oxidize exothermally 

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The fuel elements essential involve in exothermic reaction but all the element involve in exothermic reaction are not essentially fuel elements. Fossil fuels essentially contain hydrogen and carbon but all the substance contain hydrogen and carbon are not essentially fossil fuels

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Definition Fossil fuels may be defined as the remains of plants and animal, preserved from an earlier era inside a rock or other geological deposit. In other words fossil fuels are energy rich substances that have formed from long buried plants and micro-organisms

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Classification Gaseous Fuels  Liquid Fuels  Solid Fuels 

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Gaseous Fuels  

Natural Gas (CNG & LPG) Men made gaseous fuels (Coal Gas, Producer Gas, Biogas)

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Liquid Fuels       

Aviation Fuel Kerosene Petrol Diesel LSHS Heavy Fuel Oils Non conventional (non fossil) fuels

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Solid Fuels   

Coal Crude oil residue Non fossil fuels (Biomass, Industrial Residue, Animal Dung)

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Coal Coal is a mixture of the degradation products of plant and animals. It is associated with different proportions of inorganic matter. Coal is thus an intimate mixture of complex organic mass and inorganic matter. Coal is also known as combustion organic rock composed of primarily of carbon, hydrogen and oxygen

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Chemistry of Coal Ultimate constituents of pure coal are in conformity with those found in plants i.e. Carbon, Hydrogen, Oxygen, Nitrogen, Sulpher and other minor elements. Carbon & Hydrogen: Real factors for heat production. In coal carbon exists in combination with Hydrogen or a free residual carbon – Fixed Carbon.

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Classification of Coal It is classified depending on the degree of coalification a guide to its carbon content.

Type

Name

%VM

%M(MF basis)

Anthracite

a)Antharacite b)Semi-Anth.

3 – 10 10 - 15

1–3 1-2

Bituminous

Low volatile (caking) MediumVol. High Vol. High Vol. (semi-cakin) High Vol. (non-cak.)

15 – 20

0.5 – 1.5

20 – 32 32+ 32+

0.5 – 2 1–3 3–7

32+

7 - 14

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Classification of Coal Cont. Type

Name

% VM

%M(MF basis)

Sub-Bituminous coal

Non caking Slack on weathering

32+

10 - 20

Lignite or Brown coal

Lignite

45 - 55

10 - 15

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Important Terms •

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Coal Rank: The rank of coal is indicative of its maturity in the process of coalification as adjudged by its chemical and physical parameters. Calorific value, Net: The Gross Calorific Value less the latent heat of evaporation of the water originally contained in the fuel and that formed during its combustion. Calorific Value, Gross: The number of heat units measured as being liberated when unit mass of coal is burned in oxygen saturated with water vapour in a bomb under standard condition. Proximate Analysis: The analysis of coal, expressed in terms of Fixed carbon, moisture, volatile matter and ash. Ultimate Analysis: The analysis of coal, expressed in terms of its carbon, hydrogen, nitrogen, sulphur and oxygen contents

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Analysis of Coal

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In the power plant, Boilers are designed based on the following characteristics: Proximate Analysis Ultimate Analysis GCV HGI Fusion Behaviour of ash of coal. 14

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Proximate Analysis 

Proximate analysis serves a quick characterization of coal in respect of its quality, rank and type and thus broadly indicate its suitability for a particular mode of use.

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Moisture:- Different forms – Free Moisture Bed/Seam/capacity Moisture Air-dried Moisture Total Moisture

d) e) f) g)

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Proximate Analysis 2. Composition of Typical Ash

Constituents

Formula

Percentage

Silica

SiO2

55-64

Alumina

Al2O3

25-35

Iron Oxide

Fe2O3

4-10

Calcium Oxide

CaO

1.4-12

Magnesium Oxide

MgO

0.4-4

Sodium Oxide

Na2O

0.2-0.4

Potassium Oxide

K2O

0.7-2.4

Titania

TiO2

0.7-2.9

Sulfur Trioxide

SO3

0.2-1.2

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Proximate Analysis • •

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Ash & Mineral Matters: (Previous slide) Volatile Matter – Responsible for flame size. It contains mainly CO2, CO, CH4, H2, unsaturated Hydro Carbon, Water, Tar Vapours, H2S and Ammonia, etc. Fixed Carbon : 100 –(A+M+VM)% - On DMF basis FC contains carbon 97%, H2 – 0.6%, N2-1.4%, S-0.5% and O2 – 0.5% Approx. irrespective of the rank of coal. Total Moisture: X+Y(1-X/100)

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Ultimate Analysis 

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Analysis of coal for its elementary constituents, carbon, hydrogen, nitrogen, sulphur and oxygen is called ultimate analysis. It is done to predict the extent of coalification and weathering affect. It is important for efficiency calculation and adjustment of air for combustion. From lignite to anthracite, the carbon in the pure coal substance increases progressively. A weathered coal will be deficient in carbon and hydrogen, but richer in O2 resulting in low CV. 18

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Gross Calorific Value 

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This is the basic property of fuel indicating the quantity of heat evolved by its complete combustion. In fact, design of boiler is based on gross calorific value of coal. GCV is employed to find out : Thermal efficiency of a combustor Coal equivalent of any fuel for operational and commercial purpose. Coal consumption per KWH. Thus it is a quality control parameter. Useful heat value of coal which has been accepted as an index of price fixation of high moisture coal. 19

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Physical Chemistry of Fuel Elements 2H2 + O2 = 2H2O (W) + 143050 KJ/Kg Hydrogen 2H2 + O2 = 2H2O (S) + 121840 KJ/Kg Hydrogen C + O2 = CO2 + 33820 KJ/Kg carbon 2C + O2 = 2CO + 10200 KJ/Kg carbon 2CO + O2 = 2CO2 + 10165 KJ/Kg CO S + O2 = SO2 + 9304 KJ/Kg Sulfur

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Physical Chemistry of Fuel Elements 

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Hydrogen is best fossil fuel. Its chemical reactivity is very high, which makes pure hydrogen unavailable in nature. Carbon is much inferior fuel element in comparison to hydrogen and also highly reactive. Availability of pure carbon is also poor. Sulfur is bad fuel element, contribute very little to heat value and leads to hostile environment for the combustion control system and surrounding 21

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Gross Calorific Value GCV = [33820*C + 143050*(H-O/8) + 9304*S] KJ/Coal C, H, S & O are in Kg/Kg Coal

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Net Calorific Value 

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GCV determined in lab. includes the latent heat of condensation of moist. Formed due to the presence of Hydrogen and moisture in coal. Under conditions of boiler, moisture formed remain in vapour form hence CV available is lower than GCV. This is known as NCV. Net CV = GCV – (9H+IM) x 5.86 in Cal/g.

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Useful Heat Value:

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UHV – 8900 – 138 (A+M), where A and M represents Ash & Moisture percent at 60% RH and 40 degree C.

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Hard Grove Grindability Index 

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HGI of coal indicates hardness, strength and its easiness towards pulverization of coal. It is also related to the power consumption for pulverization of coal. It measures the increase of surface produced by the application of a standard amount of work and express the result as HGI which ranges between 20 – 100 for most of the coals. HGI = 13 + 6.93 W , where W = gram of coal passing through 200mesh after grinding of 50g of coal of size 10 – 30 mesh. 24

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Gradation of Coal 

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For the purpose of prices, the grades of coal on the basis of UHV are being are as under: Grade UHV A >6200 B >5600 – 6200 C >4940 – 5600 D >4200 – 4940 E >3360 – 4200 F >2400 – 3360 G >1300 - 2400

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Petrographic Parameters   

Organic Macerals Inorganic Minerals Moisture

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Organic Macerals    

Vitrinite Exenite Inertinite Associations of two/three

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Combustion behaviour 

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Macerals are divided into three main groups of liptinite, vitrinite and inertinite. Liptinite has the highest hydrogen content and volatile matter. Its volatile matter is roughly nice that of the associated vitrinite and as a result, liptinite has been linked with ignitability and flame stability. The liptinite is significant only in the pyrolysis stage of combustion, and affects the igniting process. General influence of macerals on combustion behaviour can be described as follows: The high volatile liptinite burns out rapidly. Vitrinite burns out at a rate that depends on its reflectance. Inertinite is generally, but not always, difficult to burn. Other factors being equal, the burnout depends on the heat release of the coal/blends. 28

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Combustion Behaviour

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Burnout increases with vitrinite reflectance, and decreased with inertinite content. These factors were combined into the MI. The mean vitrinite reflectance and the fuel ratio can qualitatively predict the burnout of coal an blends, The maceral index, MI, correlates the burnout, and has potential for correlating the ignitability and the flame stability. There should be no burnout problems for the coals and blends with the MI > 3, however, burnout problems are expected for the coals and blends with the MI < 1.

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Combustion behaviour 

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Minimum burner load for stable combustion rises as the fuel ratio increases The conversion ratio of fuel nitrogen to NOx and unburned carbon fraction increase almost linearly with fixed carbon, fuel nitrogen and the fuel ratio Vitrinite-rich coals essentially produced highly porous chars, the inertinite-rich coals produced large amounts of medium and low-porous chars. Burnout of coal/blends depends on the amount of volatile matter, which is quickly released from the coal

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Combustion behaviour

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Characteristics of coal particle trajectories, devolatilization, ignition time, ignition distance and burnout of pulverized coal involves (i) turbulent flow and turbulent transfer process; (ii) coal particles motion and turbulent diffusions; (iii) turbulent combustion; (iv) evaporation, devolatilizaton and carbon combustion; (v) heat transfer between gaseous particles and the walls of furnaces 31

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Combustion behaviour 

Inherent mineral in coal had no evident effect on the reactivity and kinetics of coal pyrolysis. Some inorganic material, such as CaO, K2CO3 and Al2O3, all had a catalytic effect on the reactivity of coal pyrolysis, their effects were closely related to temperature region and coal types. Addition of inorganic matter the activation energy decreased and the characteristic temperature of coal changed 32

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Induction Heating 

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The use of induction-heating in the ignition of pulverized coal has many advantages over conventional technologies in several aspects. It is easy to extend the capacity of inductionheating, from several kilowatts to several hundred kilowatts and being advanced technology, the induction-heating equipments have the stable operation performance. The induction-heating burner is consisted of two regions: ignition and combustion region. In the ignition region, the temperature is lower, compared with the temperature in the combustion region. This ensures the stable operation of the induction-heating system, especially for alloy tube which would be eroded at high temperature above 1200 0C. 33

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Coal Blending 

Pulverized coal fired power plants were originally designed to burn a fairly narrow range of fuels. Therefore, more care must be taken to maintain the quality of the coal going to the power plant. Consequently, utilities use fuel specifications based on experience (i.e. based on the performance of coals from a single source). The coal blending leads to the following advantages: - Reducing fuel costs - Controlling emission limits - Enhancing fuel flexibility and extending the range of acceptable coals. - Providing a uniform product from coal of varying quality. - Solving existing problems such as poor carbon burnout, slagging and fouling and also improve boiler performance. 34

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Reduction in C & NOx in FG 

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Designed burner to promote mixing near the burner by means of recirculation flow produced by the straight motion of primary air and the strong swirling motion of the secondary and tertiary air. This recirculation flow lengthens the residence time to pulverized coal particles in the high-temperature field near the burner outlet and accelerates the evolution of volatile matte and the progress of char reaction. Therefore, the amount of unburned carbon is effectively reduced, but the NOx concentration increases in this region. Recirculation flow is formed in the upstream high-gas-temperature region near the burner outlet, and this lengthens the residence time of coal particles in the high-temperature region, promotes the evolution of volatile matter and the progress of char reaction, and produces an extremely low-O2 zone for effective NO reduction.

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Thank you 17th May, 2008