Anna University Long Questions and Answers

Unit - IV

Chapter 5

Fuels

Anna University Long Questions & Answers

 

Part - B

1. What is meant by proximate analysis of coal? What are the quantities estimated in this analysis and their significance? (A.U May 2015)

(or)

What is proximate analysis? Write its significance. (Chen A.U. May 2009)

(or)

What is meant by proximate analysis. (A.U July 2016, Dec 2015)

(i) Proximate analysis

Proximate analysis is the analysis involving the determination of the following physical constituents.

(ii) Determination

(a) Moisture content

About 1 gm of powdered air-dried coal sample is taken in a crusible, and is heated at 100 – 105°C in an electric hot-air oven for 1 hour. The loss in weight of the sample is found out and the % of moisture is calculated as

% of moisture in coal =  loss in weight of the coal / weight of air-dried coal × 100

(b) Volatile matter

After the analysis of moisture content the crusible with residual coal sample is covered with a lid, and is heated at 950 – 20°C for 7 minutes in a muffle furnace. The loss in weight of the sample is found out and the % of volatile matter is calculated as

% of volatile matter in coal = loss in weight of the coal / weight of air-dried coal × 100

(c) Ash content

After the analysis of volatile matter, the crusible with residual coal sample is heated without lid at 700 + 50°C for 1/2 an hour in a muffle furnace. The loss in weight of the sample is found out and the % of ash content is calculated as

% of ash content in coal = weight of ash formed / weight of air-dried coal × 100

(d) Fixed carbon

It is determined by subtracting the sum total of moisture, volatile and ash contents from 100.

% of fixed carbon in coal = 100 – % of (moisture content + volatile matter + ash content)

Significance (or) Importance of Proximate Analysis

(a) Moisture content

High percentage of moisture is undesirable because it reduces the calorific value of coal,

(b) Volatile matter

High percentage of volatile matter is undesirable because it reduces the calorific value of coal,

(c) Ash content

High percentage of ash content is undesirable because it reduces the calorific value of coal,

(d) Fixed carbon

High percentage of fixed carbon produces higher calorific value.

 

2. Discuss the ultimate analysis of coal. (TCY A.U. Dec 2009) (A.U. Dec 2009, May 2017)

(or)

Describe the ultimate analysis of coal. (A.U. June 2014)

It involves the determination of the followings

(a) Carbon and Hydrogen contents

A known amount of the coal sample is burnt in a current of O₂ in a combustion apparatus. The carbon and hydrogen, present in the coal sample, are converted into CO₂ and H₂O respectively according to the following equations.

C + O₂  → CO₂ ↑

H₂ + 1/2O₂ → H₂O↑

The liberated CO₂ and H₂O vapours are absorbed respectively in KOH and anhydrous CaCl₂ tubes of known weights.

(i) % of carbon

(ii) % of hydrogen

(b) Nitrogen content

..The determination of nitrogen content is carried out by Kjeldahl's method. A known amount of powdered coal sample is heated with con. H₂SO₄ in presence of K₂SO₄ (catalyst) in a long necked flask (called Kjeldahl's flask). Nitrogen in the coal is converted into ammonium sulphate and a clear solution is obtained.

The clear solution is then heated with excess of NaOH and the liberated ammonia is absorbed in a known volume of standard N/10 HCI.

The volume of unused N/10 HCl is then determined by titrating it against standard N/10 NaOH. Thus the amount of acid neutralised by liberated ammonia from coal is determined. From this the percentage of nitrogen is calculated as follows. % of N₂ in coal

= 14 × Volume of acid consumed × Normality / 1000 × weight of coal sample × 100

(c) Sulphur content

 A known amount of coal sample is burnt completely in a bomb calorimeter. During this process sulphur is converted into sulphate, which is extracted with water. The extract is then treated with Bal₂ solution so that sulphates are precipitated as BaSO₄. The precipitate is filtered, dried and weighed. From the weight of BaSO₄ obtained, the sulphur present in the coal is calculated as follows.

 (d) Ash content

Determination of ash content is carried out as in proximate analysis

(e) Oxygen content

The percentage of oxygen is calculated as follows.

% of oxygen in coal = 100 – % of (C + H + N + S + ash)

 

3. With neat diagram, explain the manufacturing of metallurgical coke by Otto-Hoffman method. (A.U Dec 2014, June 2016)

(or)

Describe the Otto-Hoffman method of coke manufacture and the recovery of various by product.  (Coim A.U. July 2009)(Dec 2009)(TNV A.U. May 2009) (May 2008, Dec 2012, Dec 2015)

The oven consists of a number of silica chambers. Each chamber is provided with a charging hole at the top.

Coal is introduced into the silica chamber and the chambers are closed. The chambers are heated to 1200°C by burning the preheated air and the producer gas mixture in the interspaces between the chambers.

Fig. Otto - Hoffmann's by product oven

The air and gas are preheated by sending them through 2nd and 3rd hot regenerators. Hot flue gases produced during combustion are allowed to pass through 1st and 4th regenerators until the temperature has been raised to 1000°C. While 1st and 4th regenerators are heated by hot flue gases, the 2nd and 3rd regenerators are used for heating the incoming air and gas mixture.

. For economical heating, the direction of inlet gases and flue gases are changed frequently. The above system of recycling the flue gases to produce heat energy is known as the regenerative system of heat economy. When the process is complete, the coke is removed and quenched with water.

Time taken for complete carbonisation is about 12-20 hours. The yield of coke is about 70%.

The valuable by products like coal gas, tar, ammonia, H2S and benzol, etc. can be recovered from flue gas.

Recovery of by-products

 (i) Tar: The coal gases are first passed through a tower in which liquor ammonia is sprayed. Tar and dust get dissolved and collected in a tank below, which is heated by steam coils to recover back the ammonia sprayed.

(ii) Ammonia: The gases are then passed through another tower in which water is sprayed. Here ammonia gets converted to NH OH.

(iii) Naphthalene: The gases are again passed through a tower, in which cooled water is sprayed. Here naphthalene gets condensed.

(iv) Benzene: The gases are passed through another tower, where petroleum is sprayed. Here benzene gets condensed to liquid.

(v) Hydrogen Sulphide: The remaining gases are then passed through a purifier packed with moist Fe₂O₃. Here H₂S is retained.

The final gas left out is called pure coal gas which is used as a gaseous fuel.

 

4. What are the characteristics of good metallurgical coke? (A.U May 2007) (i)

Purity: The moisture, ash, sulphur and phosphorus contents in metallurgical coke should be low, because moisture and ash reduce the calorific value. Sulphur and phosphorus may contaminate the metal.

(ii) Porosity: Coke should be highly porous so that oxygen will have intimate contact with carbon and combustion will be complete and uniform.

(iii) Strength: The coke should have very high mechanical strength inorder to withstand high pressure of the overlying material in the furnace.

(iv) Calorific value: The calorific value of coke should be very high.

(v) Combustibility: The coke should burn easily.

(vi) Reactivity: The reactivity of the coke should be low because low reactive cokes produce high temperature on combustion.

(vii) Cost: It should be cheap and readily available.

 

5. Explain the petroleum refinary process in detail with neat sketches. What are the properties and applications of its various fractions. (AU June 2013)

The process of refining involves the following steps.

Step 1: Separation of water (Cottrell's process)

The crude oil from oil well is an extremely stable emulsion of oil and- salt water. The crude oil is allowed to flow between two highly charged electrodes, where colloidal water droplets combine to form large drops, which is then separated out from the oil.

Step 2: Removal of harmful sulphur compounds

Sulphur compounds are removed by treating the crude oil with copper oxide. The copper sulphide formed is separated out by filtration.

Step 3: Fractional distillation

The purified crude oil is then heated to about 400°C in an iron retort, where the oil gets vapourised. The hot vapours are then passed into the bottom of a “fractionating column" (Fig :). The fractionating column is a tall cylindrical tower containing a number of horizontal stainless steel trays at short distances. Each tray is provided with small chimney covered with a loose cap.

When the vapours of the oil go up in the fractionating column, they become cooler and get condensed at different trays. The fractions having higher boiling points condense at lower trays whereas the fractions having lower boiling points condense at higher trays.

Fig Fractional distillation of crude petroleum

The gasoline obtained by this fractional distillation is called straight-run gasoline. Various fractions obtained at different trays are given in table.

Table : Various fractions, compositions and their uses

Heavy oils on refractionation gives

Name of the Fraction : Uses

1. Lubricating oils : As lubricants.

2. Petroleum jelly or vaseline : Used in medicines and cosmetics.

3. Grease : Used as lubricant.

4. Paraffin wax : Used in candles, boot polishes etc.

Pitch at above 400°C : Used for making roads, water proof roofing etc.

6. What is synthetic petrol? How is it manufactured by Bergius process? (TCY A.U. Dec 2009) (TNV A.U May 2009)(Chen A.U. May 2009)

(or)

Describe the manufacture of petrol by Bergius process. (A.U. June 2014)

If coal is heated with hydrogen to high temperature under high pressure, it is converted to gasoline. The preparation of liquid fuels from solid coal is called hydrogenation of coal (or) synthetic petrol.

Bergius process (or) (direct method)

In this process, (fig.) the finely powdered coal is made into a paste with heavy oil and a catalyst powder (tin or nickel oleate) is mixed with it. The paste is pumped along with hydrogen gas into the converter, where the paste is heated to 400 – 450°C under a pressure of 200 – 250 atm.

During this process hydrogen combines with coal to form saturated higher hydrocarbons, which undergo further decomposition at higher temperature to yield mixture of lower hydrocarbons. The mixture is led to a condenser, where the crude oil is obtained.

The crude oil is then fractionated to yield (i) Gasoline (ii) Middle oil (iii) Heavy oil.

Fig. Bergius process

The middle oil is further hydrogenated in vapour phase to yield more gasoline. The heavy oil is recycled for making paste with fresh coal dust. The yield of gasoline is about 60% of the coal used.

 

7. What is power alcohol? Explain its manufacture and properties. (A.U Dec 2015)

(Or)

Write notes on power alcohol (A.U. Dec 2014)

When ethyl alcohol is blended with petrol at. concentration of 5-10%, it is called power alcohol.

Manufacture

Manufacture of power alcohol involves the following two steps

Step 1: Manufacture of Ethyl alcohol

Ethyl alcohol can be synthesised by fermentation of carbohydrates in presence of yeast. This fermentation yields only about 20% alcohol.

Concentration of alcohol can be increased up to 97.6% by fractional distillation yields rectified spirit.

Step II: Conversion of ethyl alcohol into power alcohol

But, for the use in IC engines, 100% alcohol (absolute alcohol) is prepared by removing last traces of water from rectified spirit. It can be done by the following method.

Alcohol is distilled in the presence of dehydrating agent, which holds the water.

Finally absolute alcohol is mixed with petrol at concentration of 5-10% to get power alcohols.

Properties

1. Power alcohol has a lower calorific values (7000 k.cal/kg).

2. It has high octane number (90).

3. Its anti-knocking properties are good.

4. It generates 10% more power than the gasoline of same quantity.

5. Its compression ratio is also higher.

 

8. What is bio-diesel? Explain trans-esterification and advantages of bio-diesel. (A.U. May 2017)

Fuel that is made from natural elements such as plants, vegetables is called bio-diesel.

(Or)

Bio-diesel is a vegetable oil based diesel fuel, consisting of long-chain alkylester.

Manufacture : Trans-esterification (or) alcoholysis

It involves treatment of vegetable oil (sunflower oil, palm oil, soyabean oil, mustard oil, etc) with excess of methanol in presence of catalyst to give mono ethyl esters of long chain fatty acid and glycerine. It is allowed to stand for some time and glycerine is separated.

Alcoholysis reaction is represented as

Methyl esters of fatty acids, thus formed, are called “Bio-diesel”. Bio-diesel is a pure fuel before blending with conventional diesel fuel. Bio-diesel can be blended with petroleum diesel.

Advantages

1. Bio-diesel is bio-degradable.

2. It is prepared from renewable resources.

3. The gaseous pollutants are lesser as compared to the conventional diesel fuel.

4. Bio-diesel can be produced from different types of vegetable oils.

5. Best engine performance and less smoke emission are achieved.

 

9. What is meant by knocking in petrol engines? How is knocking prevented? (A.U May 2015)

Definition

Knocking is a kind of explosion due to rapid pressure rise occurring in an IC engine.

In a petrol engine, a mixture of gasoline vapour and air at 1:17 ratio is used as fuel.

This mixture is compressed and ignited by an electric spark. The products of oxidation reaction (combustion) increases the pressure and pushes the piston down the cylinder.

If the combustion proceeds in a regular way, there is no problem in knocking. But in some cases, the rate of combustion (oxidation) will not be uniform due to unwanted chemical constituents of gasoline. The rate of ignition of the fuel gradually increases and the final portion of the fuel-air mixture gets ignited instantaneously producing an explosive sound known as “Knocking”. Knocking property of the fuel reduces the efficiency of engine. So a good gasoline should resist knocking

Prevention of Knocking

1. By using petrol of high octane value.

2. Blending petrol of high octane number with petrol of low octane number, so that the octane number of the latter can be improved,

3. Addition of anti-knock agents like Tetra-Ethyl Lead (TEL).

 

10. What is metallurgical coke? How is it superior than coal? Describe any one method of manufacturing metallurgical coke. (A.U May 2015)

(i) Metallurgical

When bituminous coal is heated strongly in the absence of air, the volatile matter escapes out and the mass becomes hard, strong, porous and coherent which is called metallurgical Coke.

(or)

The coke used in the metallurgical purpose is called metallurgical coke.

(ii) Coke is superior to coal

Coke is superior to coal because

(i) Percentage of fixed carbon and hence the calorific value of coke is high.

(ii) Percentage of moisture, volatile and ash contents are higher in coal, where as they are low in coke.

(iii) Manfacture of metallurgical coke

Refer Annexure Q.No.3; Page E4 - E6.

The oven consists of a number of silica chambers. Each chamber is provided with a charging hole at the top.

Coal is introduced into the silica chamber and the chambers are closed. The chambers are heated to 1200°C by burning the preheated air and the producer gas mixture in the interspaces between the chambers.

Fig. Otto - Hoffmann's by product oven

The air and gas are preheated by sending them through 2nd and 3rd hot regenerators. Hot flue gases produced during combustion are allowed to pass through 1st and 4th regenerators until the temperature has been raised to 1000°C. While 1st and 4th regenerators are heated by hot flue gases, the 2nd and 3rd regenerators are used for heating the incoming air and gas mixture.

. For economical heating, the direction of inlet gases and flue gases are changed frequently. The above system of recycling the flue gases to produce heat energy is known as the regenerative system of heat economy. When the process is complete, the coke is removed and quenched with water.

Time taken for complete carbonisation is about 12-20 hours. The yield of coke is about 70%.

The valuable by products like coal gas, tar, ammonia, H2S and benzol, etc. can be recovered from flue gas.

Recovery of by-products

 (i) Tar: The coal gases are first passed through a tower in which liquor ammonia is sprayed. Tar and dust get dissolved and collected in a tank below, which is heated by steam coils to recover back the ammonia sprayed.

(ii) Ammonia: The gases are then passed through another tower in which water is sprayed. Here ammonia gets converted to NH OH.

(iii) Naphthalene: The gases are again passed through a tower, in which cooled water is sprayed. Here naphthalene gets condensed.

(iv) Benzene: The gases are passed through another tower, where petroleum is sprayed. Here benzene gets condensed to liquid.

(v) Hydrogen Sulphide: The remaining gases are then passed through a purifier packed with moist Fe₂O₃. Here H₂S is retained.

The final gas left out is called pure coal gas which is used as a gaseous fuel.