undefined | unit 5 fuels and combustion

THERMO

UNIT 5

Fuels and Combustion

Q1) What is fuel? Give classification of fuels.

A1) Fuel is a substance that produces high amount of energy on controlled combustion. It may do so by reacting with some other substance. The reaction is highly exothermic in nature.

Classification of Fuels:

Based on occurrence:

Primary or natural fuels: These occur naturally in the nature. e.g. Coal, Wood, Crude Oil.
Secondary or derived fuels: These are derived from the naturally occurring fuels. E.g. Petroleum products, charcoal.

Based on Physical State of occurrence:

Liquid fuels: These are found as liquids in nature. E.g. Crude oil, Natural gasoline.

Solid fuels: These are found as solids in nature. E.g. Wood and Coal.

Gaseous fuels: These are found as gases in nature. E.g. Natural gas.

Q2) Explain proximate analysis of fuel.

A2) Proximate analysis:

This is an approximate analysis of fuel to find the basic composition of fuel. The results obtained are only approximate and proper values can be obtained by ultimate analysis.

Following are the important deductions of proximate analysis:

It is important in finding the combustion properties of fuel. The factors that are determined by this method are: moisture content, ash, volatile matter and fixed carbon.

Moisture increases weight of the fuel without increasing its calorific value. The evaporation of water reduces the calorific value of the fuel. Ash also increases weight without producing any heat during combustion.

Volatile matter are all those contents in the fuel that are converted to gas during combustion.

Fixed carbon is the carbon remaining on the surface of grates as soot. High proportion of fixed carbon reduces the combustibility of fuel.

Q3) Explain ultimate analysis of fuel.

A3) Ultimate analysis:

This is an analysis of fuel which gives exact composition of fuel. Percentage of different components can be found by completely combusting the fuel in the furnace.

Following are the important deductions of ultimate analysis:

It is useful for calculating mass balance for a chemical or thermal process.

Ultimate analysis of fuel is carried out to determine the proportions of C, H, O, N and S.

Presence of toxic metals like Cd, Cr, Hg, Ni, Pb, Sn and Zn in ash can be determined by ultimate analysis.

Presence of other non-toxic metals in the fuel like Fe, Mn, Ca, Mg, Na and more can be determined by ultimate analysis.

Q4) Derive the reaction for combustion of fuel.

A4) Coal is mainly Carbon. Its complete combustion in presence of sufficient air is given by following reaction:

C + O2 = CO2 + 33.94 kJ/gm of C

12 gm of C + 2 x 16 gm of O2 = 44 gm of CO2 + heat

∴ 1 gm of C + 2.67 gm of O2 = 3.67 gm of CO2 + 33.94 kJ

The O2 content of air mass-wise is 23.2%.

∴ to produce 2.67 gm of O2 required in the above reaction, air required is,

100/23.2 x 2.67 = 11.5 gm

As per combustion theory assumptions, after combustion, 1 gm of C produces 3.67 gm of CO2 and 8.83 gm of N2.

Incomplete combustion reaction of coal:

2C + O2 = 2CO + 10.12 kJ/gm

(2 x 12) C + (2 x 16) O2 = (2 x 28) CO + heat

∴ 1 gm of C + 1.33 gm of O2 = 2.33 gm of CO + 10.12 kJ

∴ to produce 1.33 gm of O2 required in the above reaction, air required is,

100/23.2 x 1.33 = 5.75 gm

As per combustion theory assumptions, after combustion, 1 gm of C produces 2.33 gm of CO and 4.42 gm of N2.

Q5) Explain the term Equivalence Ratio.

A5) Equivalence ratio

The ratio of air to fuel in AF mixture that would just completely combust the fuel without any excess air, is called as Equivalence ratio.

It is very important to obtain the quality and quantity of work produced by the fuel.

If, in AF mixture, A:F ratio is less than the Equivalence ratio, then the mixture is rich in fuel and there is no possibility of complete combustion.

If, in AF mixture, A:F ratio is more than the Equivalence ratio, then the mixture is lean in fuel and excess air is present which is unnecessary and causes heat losses in the process of combustion.

Q6) Explain HCV and LCV.

A6) Calorific value: The amount of energy produced by complete combustion of fuel is called as Calorific value of that particular fuel.

It is normally expressed in kJ/kg.

Higher Calorific Value (HCV)

Fuel consists of elements that produce energy on combustion.

But the total amount of heat produced by combustion of fuel is not completely available to perform external work.

Some of the heat is utilized to evaporate the moisture content present in the fuel itself.

HCV is the maximum amount of energy that is produced on complete combustion of fuel. It is also generally termed as gross calorific value or GCV.

Lower Calorific Value (LCV)

If the amount of heat or energy utilized in evaporation of moisture content is removed from the HCV, we get LCV for the fuel. It is generally termed as net calorific value or NCV.

Q7) Explain the construction and working of Bomb Calorimeter.

A7) Bomb Calorimeter

Calorimetry is the science of measuring quantities of heat, as distinct from “temperature”. The instruments used for such measurements are known as calorimeters.

Let's look at the Bomb Calorimeter in detail.

Four essential parts of bomb calorimeter:

A bomb or a vessel in which the combustible charges can be burned.

A bucket or container to hold the bomb in a measured quantity of water, with 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.

Selection of Calorimeter:

The same calorimeters are used for all oxygen- combustible samples; solids and liquids alike.

Specially constructed, extra high strength bombs are available for burning explosives and similar hazardous materials, and bombs with a platinum lining or made of special corrosion resistant materials are available for use with samples which liberate unusual amounts of fluorine, chlorine or other corrosive combustion products.

Adiabatic calorimeters were the preferred choice in most industrial laboratories. Modern adiabatic jackets with microprocessor controls can approach the theoretical goal of eliminating all heat leaks, providing excellent precision in calorimetric tests, but specialized jackets and control systems are required.

Operation of Calorimeter:

Before a material with an unknown heat of combustion can be tested in a bomb calorimeter, heat capacity of the calorimeter must first be 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 changing with use, energy equivalents are determined empirically at regular intervals by burning a sample of a standard material with a known heat of combustion under controlled and reproducible operating conditions.

Benzoic acid is used almost exclusively as a reference material for fuel calorimetry because it burns completely in oxygen; it is not hygroscopic and is readily available in very pure form.

For example: Consider a standardization test in which 1.651 grams of standard benzoic acid (heat of combustion 6318 cal/g) produced a temperature rise of 3.047°C.

The energy equivalent (W) of the calorimeter is then calculated as follows:

W = (1.651) (6318)/3.047 =2416 cal/°C

It is important to note that the energy equivalent for any calorimeter depends on a set of operating conditions, and these conditions must be reproduced when the fuel sample is tested if the energy equivalent is to remain valid.

Q8. Explain the construction and working of Boy’s Calorimeter.

A8) It was designed to get accurate values of the HCV and LCV of fuel.

Mainly, it was designed for use with gaseous fuels only.

The gas calorimeter is designed to make sure that the heat from the burner flows up through the calorimeter container and back down again inside the container and back up again before exhausting.

From engineering point of view, we have to design methods that can extract higher heat or energy from the burning process and follow the process, with minimal losses of heat energy.

After some calculations and empirical procedures, with the state of the water in the combustion products, energy obtained from the combustion can be different.

The values that represent higher heating value and lower heating value are respectively when water is in liquid form and in gaseous form.

Basic theory is energy absorbed by the water when evaporating results in reduced heat output from the process.

So, with the help of this experiment, we can measure the difference of the HCV and LCV of the liquid petroleum gas. Higher calorific value has higher energy, as by the name of it than the LCV.

Some of the process is being disturbed by the higher heating values because of the condensed water, also on the other hand some of the process is being disturbed because of the formation of the water vapour.

In addition to this, these values are calculated under standard conditions because the ease of the comparison. So that the state that we are going to choose is much more important in practical cases.

Basic practical application is the use of fuel oil, gasoline or petrol, coke, coal, combustion water, foodstuffs and building materials. Also, these calorimeters can be used to measure the energy balance of Nano- material and ceramics.

Theory

We consider about the heating value, amount of energy released when a fuel is burnt completely is steady flow process and the products are returned to the state of the reactants.

There are two calorific values defined according to the state of the water at the combustion products.

Higher calorific value, higher heating value – when H2O in the product is in the liquid form

Lower calorific value, lower heating value – when H2O in the product is in the gaseous form.

HV is equal to the absolute value of the enthalpy of combustion of the fuel at a specified state.

HV = |hc|

Basic relation of HCV and LCV is HCV = LCV + (Nhfg)H2O

With the results of the practical, we can calculate the LCV.

• Gas volume = volume flow rate × time (cm3 )

• Gauge pressure = value × 10 × ( 1000 / 13600) (cm of Hg)

• Absolute pressure = Gauge pressure + Atmospheric pressure (cm of Hg)

• Correction of gas volume (V) : 𝑃1𝑉1 / 𝑇1 = 𝑃2𝑉2 / 𝑇2

• Increased temperature = 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡u𝑟𝑒 𝑜𝑢𝑡 − 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑖𝑛 (𝐾)

• Latent heat (Q) = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 × 𝑠𝑝𝑒𝑐𝑡𝑓𝑖𝑐 𝑙𝑎𝑡𝑒𝑛𝑡 ℎ𝑒𝑎𝑡 (𝑘𝐽)

• HCV = (𝑀𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑤𝑎𝑡𝑒𝑟×𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑒𝑎𝑡 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦×𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑖𝑛𝑐𝑟𝑒𝑚𝑒𝑛𝑡) / V𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑓𝑢𝑒𝑙 𝑢𝑠𝑒𝑑 𝑎𝑡 𝑔𝑖𝑣𝑒𝑛 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜n

Procedure

The upper part of the calorimeter is removed, open the gas flow and light up the burner.

Constant pressure head is set to the apparatus and open the water flow.

The stop watch is set and at that moment water direction is changed to collect the water that is circulating through the apparatus.

Water is collected that is circulating through the calorimeter for 5 minutes.

Temperature readings are taken after 5 minutes of collecting the water.

Volume flow rate is taken by the flow meter and readings are calculated with the help of given chart.

All the values are noted down and the HCV can be calculated.

Q9) Explain the flue gas analysis by Orsat apparatus.

A9) Flue Gas Analysis by Orsat Apparatus

In the proximate and ultimate analysis, the analysis of fuel was done to find out the combustion properties of the fuel.

In Orsat apparatus, we can analyze the exhaust from combustion by weight. This weight analysis can be then converted to volume analysis by using Avogadro’s hypothesis.

In order to know whether the combustion occurring is complete or incomplete, analysis of flue gases is done.

CH4 + 2O2 ------> CO2 + 2H2O

2CH4 + 3O2 ----------> 2CO + 4H2O

If combustion of fuel is complete then CO2 is released.

If combustion of fuel is incomplete then CO if released.

Thus, it can be affirmed that

If the gases contain considerable amount of CO, it shows that the combustion is incomplete (i.e. considerable wastage of fuel is taking). It also indicates that the process is occurring under short supply of oxygen.

If the gases contain a considerable amount of O2, it shows that the oxygen supply is in excess. The combustion is complete but excess air being circulated means heat transfer losses would occur and the thermal efficiency of the system will get affected.

CONSTRUCTION

• It is made of a water-jacketed measuring burette which is connected in series to 3 absorption bulbs, each through an individual stop-cock.

• The opposite side is provided with a 3-way stop-cock, one end of which is connected to a U-tube stuffed with glass wool (to avoid entry of smoke particles, etc.)

• The burette is surrounded by a jacket filled with water. This helps to keep the temperature of the gas constant.

• The bottom end of the burette is connected to reservoir of water through a long tube made of rubber.

• Each absorption bulb is filled with different solution to help in absorption of different components from the flue gases.

• The first bulb has 250 gm of KOH in half a litre distilled water which will absorb only CO2.

• The second bulb has 25 gm of pyrogallic acid mixed with 200 gm of KOH in half a litre distilled water. This will absorb CO2 and O2.

• The third bulb has 100 gm of cuprous chloride with 125 ml of liquified ammonia and 375 ml of water. This is capable of absorbing CO, CO2 and O2.

WORKING

Flue gases are first passed through the hygroscopic Calcium chloride which would remove all the moisture present in the flue gases, if any.

When the 3-way stop-cock is opened, the burette is filled with flue gases.

This entire arrangement is surrounded by a jacket filled with water to maintain the temperature.

The stop-cock for KOH is opened and the flue gases enter the KOH bulb.

All the CO2 is absorbed here.

Same procedure is repeated for the other two bulbs.

The increase in volume of the bulb contents indicates the various components absorbed in each bulb.

Q10) Explain the terms Enthalpy of formation and adiabatic flame temperature.

A10) Enthalpy of formation

It is also referred to as Heat of Formation.

It is the enthalpy change that occurs when it is formed from its constituents.

The enthalpy of change measured under standard conditions is called Standard Enthalpy of Formation.

The standard state is taken as pressure of 1 atm at 25 degrees C temperature.

When 1 mole of a substance is formed under these conditions, the corresponding value of enthalpy of formation is called as Standard Enthalpy of Formation.

Adiabatic Flame Temperature

In an adiabatic process (Q = 0), the temperature attained by the products of combustion is highest. This maximum possible temperature is called Adiabatic Flame Temperature.

This value depends on

The reactants

The degree of reaction

The air quantity used.

For a particular fuel, at a particular state burned with air at a particular state, the adiabatic flame temperature attains its maximum value when complete combustion occurs with the theoretical amount of air. The actual temperature encountered inside the reaction chamber is always lower than the theoretical adiabatic flame temperature.

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