5 Calorific Value of Fuel

Prof. A.K Jain Jain

1. Introduction

The calorific value is the most important properties of a fuel. It determines the energy released by the combustion of fuel and helps in comparing the fuel efficiency.

2. Calorific Value

The calorific value of a fuel may be broadly defined as the heat liberated by a unit mass of a fuel sample when burned or oxidized under ideal combustion conditions i.e. in presence of sufficient oxygen in an enclosure of constant volume. Thus the term calorific value (or heat of combustion or heating value) denotes the heat liberated by the combustion of all carbon and hydrogen with oxygen to form carbon dioxide and water, including the heat liberated by the oxidation of other elements such as sulfur which may be present in the sample.

In this reaction the sample and the oxygen are initially at the same temperature and the products of combustion are cooled to within a few degrees of the initial temperature; also the water vapor formed by the combustion is condensed to the liquid state and the latent heat of condensation of water vapors is taken into consideration in the calculations.

3. Units of calorific value

•  Calories per kg of fuel (Cal/g), in CGS system of units
• British Thermal Units per pound of fuel (Btu/lb) in FPS system of units
• Joules per kg of fuel (J/g or MJ/kg) in SI system of units
• cal/g = 4.1868 J/g or kJ/kg
• One Btu is nearly equal to 252 cal and one pound is equal to 453 g 1 BTU = 252 cal = 0.252 kcal and 1 kcal = 3.968 BTU
• The instrument used for estimation of calorific value is known as adiabatic constant volume Bomb Calorimeter

3. Difference between Gross calorific value (GCV) and Net calorific value (NCV)

Gross calorific value (GCV) assumes all vapour produced during the combustion process is fully condensed. It is the total amount of heat produced, when unit mass/volume of the fuel has been burnt completely and the products of combustion have been cooled to room temperature (15° C or 60° F). It is the net heat produced, when unit mass/volume of the fuel is burnt completely and the products are permitted to escape.

Net calorific value (NCV) assumes the water leaves with the combustion products without fully being condensed. Fuels should be compared based on the net calorific value.

4. Factors affecting calorific value

Calorific value of coal varies from one coal sample to another. It depends on the ash, moisture content and the type of coal. But the calorific value of fuel oils is much more consistent. Biomass such as agricultural crop residues, fuel woods, agro industrial wastes are hygroscopic in nature and absorb moisture depending on the relative humidity of the atmosphere. So the calorific value is a function of the moisture content of the fuel. The Gross Calorific Values of common liquid fuel are given in table 1.

Table 1. Gross Calorific Values of common liquid fuels

5. Characteristics of Bomb Calorimeter

The measurement of calorific value is done by burning a representative sample in a high pressure oxygen atmosphere within a metal pressure vessel called “bomb”. The energy released by the fuel on combustion is absorbed by known quantity of water surrounding the bomb which is placed in a bucket. The resulting rise in temperature of the water or absorbing medium is a direct measure of the heat generated in the combustion process and is recorded. The heat of combustion of the sample is then calculated by multiplying the temperature rise in the calorimeter by a previously determined Energy Equivalent. Four essential parts in any bomb calorimeter are:

Bomb or vessel in which the fuel is burned,A bucket or container for holding the bomb in a measured quantity of water or any other absorbing media, together with a stirring mechanism,An insulating jacket to protect the bucket from transient thermal stresses during the combustion process, and A thermometer or other sensor for measuring temperature changes within the bucket.

The bomb is a strong, thick-walled metal vessel which can be opened for inserting the sample, for removing the products of combustion and for cleaning. Valves are be provided for filling the bomb with oxygen under pressure and for releasing residual gases at the conclusion of experiment. Electrodes to carry an ignition current to a fuse wire are also required. Since an internal pressure up to 100 kg/cm2 can be developed during combustion, most oxygen bombs are constructed to withstand pressures of at least 200 kg/cm2. A line diagram of the Bomb is shown in Figure No.1.

In the moist, high-pressure oxygen environment within a bomb, nitrogen present will be oxidized to nitric acid, sulfur present will be oxidized to sulfuric acid, and chlorine present will be released as a mixture of chlorine and hydrochloric acid. These acids combine with the residual high temperature oxygen also at high pressure to form a corrosive vapor which can corrode ordinary metals. Therefore the bomb must be made of materials which will not be attacked by these combustion products.

Figure 1. Bomb assembly

The calorimeter bucket must have adequate capacity to hold the bomb completely submerged in water, with a probe to read temperature and a stirrer to promote rapid thermal equilibrium without introducing excessive heat in the form of mechanical energy. Buckets are commonly made with a highly polished outer finish to minimize the absorption and emission of radiant heat.

6.Estimation of Calorific Value

Before a material with an unknown heat of combustion can be tested in a bomb calorimeter, the energy equivalent or water equivalent of the calorimeter is first determined. This value represents the sum of the heat capacities of the components in the calorimeter, notably the metal bomb, the bucket and the water in the bucket. Since the exact amount of each of the metals used in the bomb and bucket is difficult to determine and continually formed scaling, the heat transfer characteristics changes with use, energy equivalents are determined empirically at regular intervals of time. The Energy equivalent or reported as Water equivalent, is determined by burning a sample of a standard material with a known heat of combustion under controlled operating conditions.

Benzoic acid is used almost exclusively as a reference material because it burns completely in oxygen; it is not hygroscopic and is readily available in very pure form. The energy equivalent or water equivalent or the calorific value of the sample can be determined by writing a heat balance equation over the bomb. The equation No. 1 is heat balance over the bomb and may be used for computing water equivalent and also the calorific value of unknown samples.

The powdered material is converted into a pellet by a pelletizing machine generally provided along with the instrument. Along with the sample pallet, other components being used for placing the material in the bomb are 1) fuse wire for initiating the fire 2) paper for wrapping the pallet of the material 3) thread for tying the material in the paper. All these materials contribute to the heat released during combustion process and suitable corrections for fuse wire, paper and thread are included in the heat balance equation no. 1.

It is important to note that the energy equivalent for any calorimeter depends upon a set of operating conditions, and these conditions must be reproduced when the fuel sample is tested ifthe energy equivalent is to remain valid. A large numbers of experiments for determination of energy equivalent may be conducted and an average value may be used for further computation of calorific value (CV) of test sample. In case the quantities of fuse wire, thread and paper are kept same in all the experiments i.e. both energy equivalent and CV of test sample, the respective terms are taken in to account in the energy equivalent value. The Equation 1 then simplifies to:

Example: Consider a standardization test in which 1.00 gram of standard benzoic acid (heat of combustion 26.45 MJ/kg) produced a temperature rise of 2.58°C. Find out the energy equivalent of Bomb Calorimeter.

Solution;

Using equation No. 2, the energy equivalent (W) of the calorimeter can be estimated by substituting values of

Weight of Benzoic Acid = 1.00 g or 1.00*10-3 kg

CV of Benzoic Acid = 26.45 MJ/kg

Rise in temperature (Tf-Ti) = 2.58 °C

W = =10.25 KJ/°C

After the energy equivalent has been determined, the calorimeter is ready for testing fuel samples.Samples of known weight are burned and the resultant temperature rise is measured and recorded.The amount of heat obtained from each sample is then determined by multiplying the observedtemperature rise by the water equivalent or energy equivalent of the calorimeter. Then, by dividing this value by theweight of the sample we obtain the calorific value (heat of combustion) of the sample on a unitweight basis in accordance with equation No.2.

Example: A fuel sample weighing 1.150 gram produced a temperature rise of 2.352 °C in a bomb calorimeter with an energy equivalent of 10.25 MJ/°C. Determine the gross heat of combustion.

Solution;

Weight of sample Ws = 1.150g

Rise in temperature (Tf-Ti) = 2.352 oC

Water equivalent W = 10.25 KJ/°C

Using Equation No.2, the only unknown is the CVs while other parameters are known. Substituting the values of Ws, Tf, Ti, and W, in equation No. 2, we get;

1.150* CVs = 10.25*2.352

Solving the above relation for CVs

CVs = 20.96 KJ/g or 20.96 MJ/kg

The calorific value of biomass material is 20.96 MJ/kg

All biomass fuels such as agricultural crop residues, fuel woods, agro industrial wastes are hygroscopic in nature and absorb moisture depending on the relative humidity of the atmosphere. The calorific value is a function of the moisture content of the fuel. In literature the calorific value of fuels are generally reported on moisture free basis.

To obtain the calorific value on moisture free basis, take the fuel and keep it for the estimation of moisture using appropriate procedure such as standard oven method. Find the moisture and simultaneously use the fuel for estimation of calorific value (CV) using the bomb calorimeter. Using the moisture content of the fuel and calorific value determined from the bomb calorimeter, find out the CV on dry weight basis using the following method.

Where:

CVd = Calorific value of fuel dry weight basis

CVw = Calorific value of fuel wet basis as determined by Bomb Calorimeter

Mf = Moisture fraction in the fuel

Example: A biomass fuel has 12% moisture and it’s gross calorific value determined from bomb calorimeter is 16 MJ/kg. Determine its CV on dry weight basis.

Sol:

Moisture content of biomass is 12%

Therefore, the moisture fraction is 12/100 = 0.12

Calorific value of wet fuel = 16 MJ/kg

Using Equation No. 3 and substituting the values of CVw and Mf and solving for CVd

CVd = 16 [1/(1-0.12)]

CVd = 18.18 MJ/kg

Therefor the CV of biomass on dry weight basis is 18.18 MJ/kg

7. Effect of Moisture on Heating Value:

During any of the thermal conversion process, a considerable part of the heat is used to evaporate the moisture which is never recovered in any practical situations and the effective hating value of the biomass gets reduced. It may be noted that this heat loss represents only the heat of evaporation of inherent and surface moisture and not the heat loss caused by evaporation of decomposition moisture. Therefore, if we use the Higher Heating Value and make the necessary moisture correction, the resulting heating value is Net Heating Value

The net heating value and the moisture content of a biomass can be correlated by the following expression Equation No.4. In this equation, , Mf, CVn and CVd are latent heat of vaporization of water, moisture fraction of biomass, heating value of wet and dry biomass respectively.

CVn = (1-Mf) x CVdMf

Where:

CVn = net heating value (MJ/kg)

CVd = heating value on dry weight basis (MJ/kg)

Mf = moisture fraction of biomass

= latent heat of vaporization of water (2.26 MJ/kg)

The theoretical limit of moisture for cellulose at which the combustion is no longer self-sustaining is 88%, however, in practice, the moisture content at which the biomass combustion can be sustained is much lower i.e. 70%. For gasifier, the optimum moisture content of the biomass is 15%, and higher moisture in biomass leads to poor gasifier performance. Also high moisture lowers the effective heating value of the biomass and should be avoided while using as fuel in furnaces.

Example: A biomass material has its gross higher calorific value and Moisture content as 18 MJ/kg and 15% respectively. Estimate its net calorific value.

Solution:

Moisture content of biomass = 15%

Therefore, Moisture fraction Mf is = 15/100 = 0.15

Calorific value of fuel (wet weight basis) = 18 MJ/kg

Substituting values of all parameters in Equation No.4, we get

CVn = (1-0.15) * 18      2.26*0.15

Net calorific value = 14.96 MJ/kg

8. Lower Heating Value:

The values of calorific of a solid fuel is determined using standard bomb calorimeter, where known weight of biomass material is burnt in a constant volume bomb in presence of oxygen. It is a measure of heating value when combustion is taking place at constant volume and the water formed during combustion of the biomass is condensed. The latent heat of vaporization of water is also taken into account and this heating value is usually referred as the higher heating value (HCV).

In almost all the thermochemical conversion devices, such as furnaces, cook stoves, gasifiers’ etc. operation occurs at constant pressure and vapours of water formed during the combustion of the fuel and also present in the fuel, leave the deice with flue gases without getting condensed. In all such cases CV data obtained from Bomb Calorimeter do not represent the amount of energy available for a thermochemical conversion process. A significant amount of energy is needed to vaporize water and this energy is usually not recovered. Therefore, the energy that can be extracted from the fuel is less than most reported heating value data.

In order to avoid serious errors in the dimensioning of a thermochemical conversion devices and its economic assessment, heating value of the fuel at constant pressure and where latent heat of vaporization of water is not taken into account should be used.The heating value under these conditions is called lower heating value (LCV). It is therefore, more appropriate that the LCV should be used in preference to HCV for the energy and mass balance, and other design and performance evaluation calculations for a thermo-chemical conversion device.

8. Estimation of Lower heating value

Knowing the elemental analysis and higher heating value of the biomass, the lower heating value can be determined. It is usually 10 to 15% lower as compared to the higher heating value. The lower heating value can be linked with the higher heating value by the following expression. Where and Wf are the latent heat of vaporization of water and weight fraction of water formed during combustion process (weight of water per unit weight of biomass). The lower and higher heating values of selected biomass species are given in Table 1.

HCV = LCV +   Wf + expansion work (5)

Stoichiometric formula

Stoichiometric formula gives the atomic composition of carbon, hydrogen and oxygen in a biomass. Knowing the elemental analysis of a biomass, its stoichiometric formula can be determined. For any biomass if the stoichiometric formula is represented by C HxOy, where x and y are atomic ratios of hydrogen and oxygen, x and y can be determined using the following expressions.

The typical stoichiometric formula for biomass is CH1.4O0.6 and for coal CH0.9O0.1. Once we know the stoichiometric formula, the molecular weight, the water formed during combustion, stoichiometric air fuel ratio and Lower heating value for a fuel or biomass can be determined.

9. Water formed during the combustion process

Let the combustion of biomass is represented by the following generalized reaction:

Substituting the values of np, nrR and T in equation No. 11, W can be estimated volume expansion work(MJ/kg)

Now by substuting the values of Wf, expansion work, and higher heating value in equation No.

9 above the lower heating value of the biomass can be computed.

Example: The elemental composition of a biomass material by weight is: Carbon, 50%; Hydrogen, 5% and Oxygen 45% on moisture and ash free basis. If the calorific value of the material determined from Bomb Calorimeter is 18 MJ/kg, determine the following:

1. Stoichiometric formula

2. Weight of water formed during combustion of biomass (kg/kg)

3. Lower heating value on moisture and ash free basis