Department of Chemical Engineering

University of San Carlos – Technological Center

Nasipit, Talamban, Cebu City

ChE 322

Chemical Calculations 2

Solid Fuels

A report submitted to

Engr. Ramelito Agapay

Instructor, ChE 322

By

Chan, Stacy Mae

Embernate, Lloyd

Ursal, Khristine Rose

Table of Contents

I. Composition and Types of Solid Fuels

II. Coal

2.1 Natural occurrence

2.2 Classifications and Ranking

2.3 Advantages and Disadvantages

III. Solid Fuel Analysis

3.1 Proximate Analysis

3.2 Ultimate Analysis

3.3 Modified Analysis

IV. Heating Value Estimation

4.1 Dulong’s Formula

4.2 Calderwood Equation

4.3 Other HV Estimation Formulas

V. Combustion Calculations

5.1 Coal Combustion without Combustibles in Refuse5.2 Coal Combustion with Unburnt Combustibles in Refuse

I. Composition and Types of Solid Fuels

Solid fuels refer to the various types of solid material that are used as fuel to produce energy through combustion unlike gaseous and liquid fuels. Most solid fuels (with the exception of petroleum cokes) contain appreciable percentages of mineral compounds. It is also true that the moisture content is often considerably higher than is characteristic of gases and liquids. In the latter, in the absence of emulsified water, the moisture is limited to a relatively low value by the solubility of water in oils; while in the case of solid fuels, moisture can be bound to the surface by adsorptive forces or held mechanically in the pores and crevices. Otherwise, the chemical elements present in solid fuels are the same as those in gaseous and liquid. [Lewis, 1954] Solid organic fuels are classified into natural and artificial fuels.

a) Natural Solid Fuels1) Wood – may be burned directly as a fuel or may be converted

into charcoal or producer gas. Sawdust is sometimes burnt at sawmills supplemented with oil to reduce fuel cost.

2) Peat – brown fibrous mass of partially decayed plant material that has accumulated under water logged conditions.

3) Lignites – immature coals that are intermediate in composition between peat and bituminous coals and are about 1 to 100 million years old.

4) Coals – compact stratified mass of mummified plant debris that has accumulated during past geological ages about 100 – 300 million years old and has been altered by processes involving biochemical action, submersion in water and action of heat and pressure.

b) Artificial Solid Fuels1) Wood charcoal – solid residue from the carbonization of wood

which involves heating wood strongly in the absence of oxygen.

2) Peat charcoal – made by carbonizing peat at low temperature. 3) Lignite briquettes – air dried lignites.4) Lignite coke – air dried and carbonized lignite.5) Coke – carbonized coal. [Laurito, 1994]

II. Coal

Coal is a black or brownish black, solid combustible rock containing less than 40% non-combustible inorganic components formed by the accumulation, decomposition and compaction of plant materials. [doe.gov.ph]

II.1 Natural Occurrence The standard, uniformitarian process of coal formation begins in a swamp. In this water-saturated environment, dead mosses, leaves, twigs,

and other parts of trees do not decompose completely. Instead, this plant matter becomes a layer of peat. At various intervals, the swamp may be covered by sand and mud when a river floods or when ocean levels rise. Under the weight of these sediments, the peat may lose some of its water and gases, eventually turning into a soft brown coal called lignite. With increasing pressures or temperatures, more water and gases are driven off, forming the common bituminous family of coals. Finally, high temperatures and pressures may cause bituminous coal to turn into a hard black coal called anthracite. An increase in rank represents an increase in the proportion of carbon within the coal.

Figure 1. A figure illustrating the process of coal formation

The type or rank of coal is thought to depend more on depth of burial than time. For example, it is possible to find lignite and bituminous coals that seem to have formed at the same time, but in different places. In other words, rank is not a good predictor of age. [Major, 1996]

II.2 Classifications and Ranking

Coal classification is the grouping of different coals according to

certain qualities or properties, such as coal type, rank, carbon–hydrogen

ratio, and volatile matter.

In the United States, coal is classified according to the degree of

metamorphism, or progressive alteration, in the series from lignite (low

rank) to anthracite (high rank). The basis for the classification is according

to yield of fixed carbon and calorific value, both calculated on a mineral-

matter-free basis. Higher-rank coals are classified according to fixed

carbon on a dry, mineral-matter-free basis. Lower-rank coals are classed

according to their calorific values on a moist, mineral-matter-free basis.

The agglomerating character or its ability t o form or collect into a rounded

mass, is also used to differentiate certain classes of coals. Thus, to

classify coal, the calorific value and a proximate analysis (moisture, ash,

volatile matter, and fixed carbon by difference) are needed. For lower-

rank coals, the equilibrium moisture must also be determined. To

calculate these values to a mineral-matter-free basis, the Parr formulas

are used. [Speight, 2005]

The agglomerating character is used to differentiate between adjacent

groups. Coals are considered agglomerating if the coke button remaining

from the test for volatile matter will support a weight of 500 g or if the

button swells or has a porous cell structure.

Figure 2. A figure illustrating the classification of coals by rank

The Parr Formulas:

F '=100 (F−0.15S )

100−(M+1.08 A+0.55S )

V '=100−F

Q'=100 (Q−50 S )

100−(M+1.08 A+0.55 S )

These three formulas are approximation formulas.

F '= 100 F100−(M+1.1 A+0.1S )

Q'= 100Q100−(1.1 A+0.1S)

These two formulas are for classifying coals according to rank.

M, F, A, and S are weight percentages, on a moist basis, of moisture,

fixed carbon, ash, and sulfur, respectively; F′ and V′ are weight

percentages, on a dry mmf (mineral-matter-free) basis, of fixed carbon

and volatile matter, respectively; Q and Q′ are calorific values (Btu/lb), on

a moist non-mmf basis and a moist mmf basis, respectively. [Perry, 1997]

II.3 Advantages and Disadvantages

Advantages:

Coal is one of the most abundant sources of energy, more so than

oil and natural gas.

Coal is inexpensive when compared to other fossil fuels (or

alternative energy sources).

Coal is versatile enough to be used for recreational activities.

Burning coal can produce useful by-products that can be used for

other industries or products.

Electricity produced from coal is reliable.

Coal can be safely stored and can be drawn upon to create

energy in time of emergency.

Coal based power is not dependent on weather which cannot be

said for alternative forms of renewable energy such as wind or

solar power.

Transporting coal does not require the upkeep of high-pressure

pipelines and there is no requirement for extra security when

transporting coal.

Using coal reduces the dependence on using oil.

Disadvantages:

Coal combustion emits almost twice as much carbon dioxide per

unit of energy as does the combustion of natural gas, whereas the

amount from crude oil combustion falls between coal and natural

gas.

The burning of coal by large-scale factories to power industry has

led to acid rain in some regions.

Coal can be cleaned and/or turned into a liquid of gas but this

technology has yet to be fully developed and adds to the expense

of creating fuel via coal.

Coal mining can scar the landscape and the equipment used for

mining is large and noisy which may affect local wildlife.

Transporting coal can be problematic because it requires an

extensive transportation system and can also cause additional

pollution in the form of emissions from transportation vehicles.

There are limited stocks of coal remaining – they will be entirely

depleted this millennium if we continue to burn coal in the future at

the same rate we are today coal can be considered as a non-

renewable energy source.

The mining industry can cause health difficulties for miners and

fatalities due to the potentially dangerous nature of the work.

Burning dirty coal can create significant pollution problems. [fossil-

fuels.co.uk]

III. Solid Fuel Analysis

Types of bases:

AR (as-received) basis – represents the weight percentage of

each constituent in the sample as received in the laboratory. The

sample itself may be coal as fired, as mined, or as prepared for a

particular use.

AD (air-dried) basis – neglect presence of moistures other than

inherent moisture.

DB (dry-basis) – moisture-free (dry) basis is generally the most

useful basis because performance calculations can be easily

corrected for the actual moisture content at the point of use.

DAF (dry, ash-free) basis – is frequently used to approximate the

rank and source of a coal. For example, the heating value of coals

of a given source and rank is remarkably constant when

calculated on this basis.

DMMF (dry, mineral matter-free) basis – leaves out all moisture

and mineral matter constituents. [Perry, 1997]

III.1 Proximate Analysis

The proximate analysis of coal is an assay of the moisture, ash,

volatile matter, and fixed carbon as determined by series of prescribed or

standard test methods. It was developed as a simple means of

determining the distribution of products obtained when the coal sample is

heated under specified conditions. This analysis is in contrast to the

ultimate analysis of coal, which provides information about the elemental

composition. By definition, the proximate analysis of coal separates the

products into four groups: (1) moisture; (2) volatile matter, consisting of

gases and vapors driven off during pyrolysis; (3) fixed carbon, the

nonvolatile fraction of coal; and (4) ash, the inorganic residue remaining

after combustion.

Moisture: there are several sources of the water found in coal. The

vegetation from which coal was formed had a high percentage of water

that was both physically and chemically bound, and varying amounts of

this water were still present at various stages of the coalification process.

But the overall result of the continuation of the coalification process was

to eliminate much of the water, particularly in the later stages of the

process, as is evident from a comparison of the moisture contents of

different ranks of coal, from lignite to anthracite (Table 3.1). Water is

present in most mines and circulates through most coal seams. After

mining, many coals are washed with water during preparation for market

and are then subject to rain and snow during transportation and storage.

All of these sources contribute to the moisture in coal and to the problems

associated with measurement of this moisture. The role of water in coal

and the quantitative measurement of water are complicated because

water is present within the coal matrix in more than one form. Thus, the

total moisture includes both the surface moisture and the residual

moisture remaining in the sample after determining the air-dry loss. Thus,

M=R(100−A DL)

100+ADL

where M is the total moisture (% by weight), R the residual moisture (%

by weight), and ADL the air-drying loss (% by weight).

Volatile matter: volatile matter is the portion of a coal sample which

when heated in the absence of air at prescribed conditions, is released as

gases. It includes CO2, volatile organic and inorganic gases containing

sulfur and nitrogen. It is the percentage of volatile products, exclusive of

moisture vapor, released during the heating of coal or coke under rigidly

controlled conditions.

Fixed carbon: fixed carbon is the material remaining after the

determination of moisture, volatile matter, and ash. It is, in fact, a

measure of the solid combustible material in coal after the expulsion of

volatile matter, and like determination of the carbon residue of petroleum

and petroleum products represents the approximate yield of thermal coke

from coal. The fixed-carbon value is one of the values used in

determining the efficiency of coal-burning equipment. It is a measure of

the solid combustible material that remains after the volatile matter in coal

has been removed. For this reason, it is also used as an indication of the

yield of coke in a coking process. Fixed carbon plus ash essentially

represents the yield of coke. Fixed-carbon values, corrected to a dry,

mineral-matter-free basis, are used as parameters in the coal

classification system.

Ash: ash is the residue remaining after the combustion of coal under

specified conditions and is composed primarily of oxides and sulfates. It

should not be confused with mineral matter, which is composed of the

unaltered inorganic minerals in coal. Thus, ash is formed as the result of

chemical changes that take place in the mineral matter during the ashing

process. The quantity of ash can be more than, equal to, or less than the

quantity of mineral matter in coal, depending on the nature of the mineral

matter and the chemical changes that take place in ashing. The various

changes that occur include (1) loss of water from silicate minerals, (2)

loss of carbon dioxide from carbonate minerals, (3) oxidation of iron pyrite

to iron oxide, and (4) fixation of oxides of sulfur by bases such as calcium

and magnesium. [Speight, 2005]

Figure 3. A figure illustrating the composition and property ranges for various ranks of coal

III.2 Ultimate Analysis

The ultimate analysis of coal involves determination of the weight

percent carbon as well as sulfur, nitrogen, and oxygen (usually estimated

by difference). Trace elements that occur in coal are often included as a

part of the ultimate analysis.

The carbon determination includes carbon present as organic carbon

occurring in the coal substance and any carbon present as mineral

carbonate. The hydrogen determination includes hydrogen present in the

organic materials as well as hydrogen in all of the water associated with

the coal.

In the absence of evidence to the contrary, all of the nitrogen is

assumed to occur within the organic matrix of coal. On the other hand,

sulfur occurs in three forms in coal: (1) as organic sulfur compounds; (2)

as inorganic sulfides that are, for the most part, primarily the iron sulfides

pyrite and marcasite (FeS2); and (3) as inorganic sulfates (e.g., Na2SO4,

CaSO4). The sulfur value presented for ultimate analysis may include,

depending on the coal and any prior methods of coal cleaning, inorganic

sulfur and organic sulfur.

Moisture and ash are not determined as a part of the data presented

for ultimate analysis but must be determined so that the analytical values

obtained can be converted to comparable bases other than that of the

analysis sample. In other words, analytical values may need to be

converted to an as-received basis, a dry basis, or a dry, ash-free basis.

When suitable corrections are made for any carbon, hydrogen, and sulfur

derived from the inorganic material, and for conversion of ash to mineral

matter, the ultimate analysis represents the elemental composition of the

organic material in coal in terms of carbon, hydrogen, nitrogen, sulfur, and

oxygen.

The standard method for the ultimate analysis of coal and coke

includes the determination of elemental carbon, hydrogen, sulfur, and

nitrogen, together with the ash in the material as a whole. Oxygen is

usually calculated by difference. The test methods recommended for

elemental analysis include the determination of carbon and hydrogen,

nitrogen, and sulfur, with associated determination of moisture and ash to

convert the data to a moisture-ash-free basis. [Speight, 2005]

III.3 Modified Analysis

Modifications of the ultimate analysis for combustion calculations. It

includes:

C, N, S, ash

Moisture

Combined water (CW) – the oxygen in the coal (not present in

moisture) is treated as though it were already combined with

hydrogen.

Net hydrogen (NH) – hydrogen which requires O2 from air for

combustion. [Laurito, 1994]

IV. Heating Value Estimation

The thermal properties of coal are important in determining the

applicability of coal to a variety of conversion processes. For example, the

heat content (also called the heating value or calorific value) is often

considered to be the most important thermal property.

The calorific value is the heat produced by the combustion of a unit

quantity of coal in a bomb calorimeter with oxygen and under a specified

set of conditions. The unit is calories per gram, which may be converted

to the alternate units (1.0 kcal/kg = 1.8 Btu/lb = 4.187 kJ/kg).

The calorific value of coal is an important property. For example, the

gross calorific value can be used to compute the total calorific content of

the quantity of coal or coke represented by the sample for payment

purposes. It can also be used to compute the calorific value versus sulfur

content to determine whether the coal meets regulatory requirements for

industrial fuels. The gross calorific value can be used to evaluate the

effectiveness of beneficiation processes. Finally, the gross calorific value

can be required to classify coal.

If a coal does not have a measured calorific value, it is possible to

make a close estimation of the calorific value (CV) by means of various

formulas.[Speight, 2005]

IV.1 Dulong’s Formula

This equation may be used in conjunction with the flue gas analysis for

the calculation of the percent carbon in the coal, the flue gas analysis giving

the ratio of net hydrogen to carbon. Its coefficients assume negligible heat of

formation of the organic matter from the elements. Their values are based on

the heats of combustion of the elements involved. [Lewis, 1954]

CV=0.338C+1.44(H−O8 )+0.094 S

Where:

CV = calorific value in MJ/kg

C, H, O, S = % by weight of C, H, O and S

Net H = weight fraction of net H = total H - 18

O

Assumptions in Dulong’s Formula:

1) CV of the fuel is the algebraic sum of the heating values of

elemental components.

2) Oxygen is combined with hydrogen as in combined water and

moisture, so that surplus moisture available for combustion (net H)

is H – O/8.

3) That the heat of formation of coal is zero. [Laurito, 1994]

IV.2 Calderwood Equation

This equation is useful in finding the total carbon content of the coal if

the proximate analysis and the GCV are known.

%C=5.88+2.206 (CV−0.094 S )+0.0053 [80−(100VCMFC )]

1.55

Where:

S = sulfur as weight percent of the coal

B = HHV in Btu per pound of coal

VCM = volatile combustible matter in weight percent of coal

FC = fixed carbon as weight percent of coal

C = carbon as weight percent of coal

The sign of the last term is taken negative of 100 (VCM/FC) is greater

than 80. The percentages are expressed on an air-dried basis, but the

error introduced by using a basis with any reasonable moisture content or

even by using a moisture and ash free basis is usually negligible.

The Calderwood equation is particularly applicable to bituminous

coals, for which it was originally intended. For other grades of coal it has

been found to apply with very few exceptions with an error of not more

than 2%.

IV.3 Other HV Estimation Formulas

As early as 1940, of some 9 different formulas for calculating

heating value from the ultimate analysis and 11 formulas for

calculating it from the proximate analysis (an example is Dulong’s

formula). Three additional ultimate analysis formulas have been

proposed within the last three years. Namely: Boie, Grummel and

Davis, and Mott and Spooner.

Boie formula:

Q=151.2C+499.77H+45.0S−47.7 (O )+27.0N

Grummel and Davis formula:

Q=[ 654.3H(100−A )

+424.62] [C3 +H−(O )8

+ S8]

Mott and Spooner formula:

a) Q=144.54C+610.2H+4.5 S−62.46 (O ) if (O )≤15 %

b) Q=144.54C+610.2H+4.5 S−[65.88−30.96 (O )(100−A ) ] (O ) if (O )>15 %

In the above, Q is the gross heating value in Btu/lb on the dry

basis and C , H, S, (O), N, and A are the respective contents

of carbon, hydrogen, sulfur, oxygen, nitrogen, and ash in weight

percent, also on the dry basis. [anl.gov]

Figure 4. A figure illustrating a test of formulas for calculation of heating value

V. Combustion Calculations

The combustion of coal may be carried out by spreading it over a grate

and firing or by introducing it as pulverized fuel. Coal and air are brought

together at a temperature sufficient to decompose the coal into fixed carbon

and volatile matter and to cause the fixed carbon to ignite. Combustion then

becomes self supporting; i.e. sufficient heat is evolved to maintain these

conditions. However, coal contains non combustible matter (ash) which

separates from the materials that can be gasified and is removed from the

furnace as refuse. This refuse may or may not contain unburnt combustible.

For combustion calculations the combines % of N and S is neglected if the

total is less than or equal to 3%.

V.1Coal combustion with no combustibles in the refuse.

A furnace is fired with sub-bituminous coal containing 10.3%M, 34% VCM

and 7.7% Ash. It is also known to contain 1.2%N and 1.57%S. Its calorific

value is 22MJ/kg. Calculate the proximate, ultimate and modified analysis.

Solution:

Ultimate (%wt) Proximate (%wt) Modified (%wt)

C FC C

H M 10.3 M 10.3

O VCM 34 CW

S 1.57 Ash 7.7 Net H

N 1.2 N + S 2.77

Ash 7.7 Ash 7.7

Basis: 100 kg coalFC = 100 - (10.3 - 34 -7.7) FC = 48

a.) Proximate Analysis

Proximate Analysis (%wt)

FC 48

M 10.3

VCM 34

Ash 7.7

Total 100

Solution:

Using Calderwood’s Equation:

C = 5.88 +2.206 (CV -0.094S) + 0.0053[80-100 VCM/FC]1.55

= 5.88 + 48.37

C = 54.25%

Using Dulong’s Formula:

CV = 0.338C + 1.44(H – O/8) + 0.094S

22 = 0.338(54.25) + 1.44 (net H) + 0.094(1.57)

Net H = 2.44%

b.) Modified Analysis

CW = 100 – (54.25 + 1.2 + 1.57 +2.44 + 7.7)

CW = 32.84%

Modified Analysis (% wt)

C 54.25

Net H 2.44

CW 32.84

N + S 2.77

Ash 7.7

Total 100

c.) Ultimate Analysis

H balance between methods of analysis:

O balance between methods of analysis:

Ultimate Analysis (%wt)

C 54.25

H 6.09

O 29.19

N 1.2

S 1.57

Ash 7.7

total 100

H total = netH +H 2O18 (21 )

H total = 2. 44 +32.8418 (21 )

H total = 6 . 09 kg H2O

O total =H 2O18 (16

1 )O total =32 .84

18 (161 )

O total = 29 .19 kg O

6.2 Coal Combustion with Unburnt Combustibles

A furnace is fired with coal containing 6% moisture, 18% VCM, 67% FC and 9% ash. The refuse analysis shows 5% VCM, 23% FC and 62% ash. The HHV of the coal “as-fired” is 14,300 Btu/lb. Calculate the percentage of heating value that is lost in the refuse. The moisture in the coal is due to wetting down to prevent dusting. It is not moisture from the original coal.

Basis: 100 lb of coal as fired

To find HV of VCM:Basis: 1 lb of coal as fired

(9 lb ash)(100 lb refuse62 lb ash )=14 .51 lb ash

(14 .51 lb refuse)(0 .5 lb VCMlb refuse )=0 .725 lb VCM

(14 .51 lb refuse)(0 .23 lb FClb refuse )=3 .34 lb FC

(0 . 67 lb FC )(14,550 Btulb C )=9750 Btu generated by FC

14,300 - 9750 = 4550 Btu = HH/v of 0 . 18 lb VCM4550 Btu0 .18 lb VCM

=25,300Btulb VCM

Basis: 100 lb of coal as fired

VI. References

Lewis, W., Radasch, A. and Lewis, H. (1954) IndustrialStoichiometry: Chemical Calculations of Manufacturing Processes, 2nd edition,

Merriam & Webster Inc., Sampaloc, Manila. Laurito, E. (1994) Stoichiometry of Fuel Combustion and Related

Precess Industries, Maxie Lithographic Arts & Printing Press, San Jose, Quezon City.

Major, T. (1996)  GENESIS AND THE ORIGIN OF COAL AND OIL, Apologetics Press, Inc., Montgomery, AL, U.S.A. 

Speight, J. (2005) Handbook of Coal Analysis, John Wiley & Sons, Inc., Hoboken, New Jersey.

Perry, R. (1997) Perry's Chemical Engineer's Handbook, 7th edition, The McGraw-Hill Companies, Inc., United States of America.

http://www.anl.gov/PCS/acsfuel/preprint%20archive/Files/25_3_SAN %20FRANCISCO_08-80_0235.pdf

http://fossil-fuel.co.uk/coal/advantages-of-coal http://fossil-fuel.co.uk/coal/the-disadvantages-of-coal http://www.doe.gov.ph/ER/Coal.htm

(0 . 67 lb FC )(14,550 Btulb C )=9750 Btu generated by FC

14,300 - 9750 = 4550 Btu = HH/v of 0 . 18 lb VCM4550 Btu0 .18 lb VCM

=25,300Btulb VCM

(0 .725 lb VCM)(25,300 Btulb VCM )=18,320 Btu lost due to VCM loss

(3 .34 lb FC )(14,550 Btulb FC )=48,600 Btu lost due to FC loss

(48 ,600+18 ,320(14 ,300)(100 ) ) (100 )=4 .68 % heating value lost in refuse