Manufacture of metallurgical coke

Manufacture of Metallurgical Coke Cost: Coke should be cheap and easily available Manufacture of Metallurgical Coke: (i) Beehive Oven (ii) Otto Hoffmann oven (i) Beehive oven: A beehive oven is a fire-brick chamber having a dome-shaped structure The dimensions of a typical oven are 4m and 2.5m high The roof is provided with a hole for charging the coal from the top Another hole, the discharging hole is provided in the circumference of the lower part of the wall A number of ovens are built in a row with common walls between neighbouring ovens Fig 5: Beehive coke oven Demerits of Beehive ovens: The demerits are • No recovery of byproducts, which are useful chemicals and are allowed to escape • Lower coke yield due to partial combustion • Lack of flexibility of operation (ii) Otto-Hoffmann’s oven or By-product Oven: The beehive ovens have been replaced by chamber ovens which works on regenerative principle of heat economy All the valuable products are recovered from the outgoing flue gases Construction: It consists of no of narrow rectangular chambers made of silica bricks Fig 6: A single chamber of Otto Hoffmann’s oven Working: Coal is charged into the chamber o The coke ovens are heated to 1200 C by burning gaseous fuels The process of carbonization takes place layer by layer in the coal charge As the coal adjacent to the oven walls gets heated, a plastic zone is formed which moves away from the walls towards the central zone As the coal is converted into coke, there is decrease in volume This is because of the o removal of volatile matter in the form of tar and gas at about 500 C At further high temperature, the plastic mass solidifies into hard and porous mass called coke Regenerative principle is employed to achieve as economical heating as possible Regenerators are built underneath the ovens.The flue gases pass their heat to the checker o brick work of regenerators until the temperature rises to 1000 C Regenerators work on the principle of alternate heating and cooling cycles This is achieved by periodically changing the direction of flow of gases through the vertical flues every 30 or so Carbonization of a charge of coal takes about 11-18 hours After the process is complete, red hot coke is pushed outside by means of a ram which is electrically driven The coke falls into a quenching car The yield is 75 % of coal Fig 7: Otto Hoffmann’s Byproduct coke oven Recovery of byproducts: The gases and vapours evolved on carbonization in coke ovens are not allowed to mix with the combustion and are collected separately.The coke oven gas is treated separately for the recovery of the valuable byproducts Fig 8: Coke-Oven gas treatment plant (i) Recovery of Tar: The gas from the coke ovens is passed through a tower in which liquor ammonia is sprayed.Tar and dust get collected in a tank The tank is provided with a heating coils to recover back ammonia (ii) Recovery of ammonia: The gases are then passed through a tower where water is sprayed to recover ammonia The ammonia can also be recovered by dissolving it in H2SO4 to form (NH4)2SO4, which is then used as a fertilizer (iii) Recovery of Naphthalene: The gases are passed through a cooling tower, where water at a low temperature is sprayed.The gas is scrubbed with water until its temp reduces Gaseous Fuels  Classification of gaseous fuels (A) Fuels naturally found in nature - Natural gas Methane from coal mines (B) Fuel gases made from solid fuel - Gases derived from coal Gases derived from waste and biomass From other industrial processes (C) Gases made from petroleum - Liquefied Petroleum gas (LPG) Refinery gases Gases from oil gasification (D) Gases from some fermentation Gaseous Fuels  Calorific value • Fuel should be compared based on the net calorific value (NCV), especially natural gas Typical physical and chemical properties of various gaseous fuels Fuel Gas Relative 3 Air/Fuel ratio m /m Density Higher Heating Value kCal/Nm Flame Temp o C Flame speed m/s Natural Gas 0.6 9350 10 1954 0.290 Propane 1.52 22200 25 1967 0.460 Butane 1.96 28500 32 1973 0.870 Type of Fuels Gaseous Fuels  Natural gas • Methane: 95% • Remaing 5%: ethane, propane, butane, pentane, nitrogen, hydrogen, CO, carbon dioxide, other gases • High calorific value fuel • Does not require storage facilities • No sulphur • Mixes readily with air without producing smoke or soot Application of Natural Gas  Natural gas is finding increasing use as a domestic, transportation and industrial fuel  It is used as a raw material for the synthesis of methanol, formaldehyde and other chemical compounds  It is used as a source of H2  Ammonia used for the process for extracting nickel from its ore is made by reacting N2 with natural gas LPG  Varieties of LPG bought and sold include mixes that are primarily propane (C3H8), primarily butane (C4H10) and, most commonly, mixes including both propane and butane and isobutane depending on the season — in winter more [ propane, in summer more butane Propylene and butylenes are usually also present in small concentration A powerful odorant, ethanethiol, is added so that leaks can be detected easily Application of LPG  Liquefied petroleum gas (also called LPG, GPL, LP Gas, or liquid propane gas ) is a flammable mixture of hydrocarbon gases used as a fuel in cooking, heating appliances and vehicles  It is used as a feedstock for the manufacture of various chemicals and olefins by pyrolysis  Other industrial application includes its use in portable blow lamps, soldering, welding, annealing hardening, brazing, steel cutting etc  It is increasingly used as an aerosol propellant and a refrigerant, replacing chlorofluorocarbons in an effort to reduce damage to the ozone layer  When specifically used as a vehicle fuel it is often referred to as autogas Compressed Natural Gas(CNG) CNG is made by compressing natural gas (which is mainly composed of methane [CH4]), to less than 1% of the volume it occupies at standard atmospheric pressure It is stored and distributed in hard containers at a pressure of 200–248 bar (2900–3600 psi), usually in cylindrical or spherical shapes  Compressed natural gas (CNG) is a fossil fuel substitute for gasoline (petrol), diesel, or propane/LPG Although its combustion does produce greenhouse gases, it is a more environmentally clean alternative to those fuels, and it is much safer than other fuels in the event of a spill (natural gas is lighter than air, and disperses quickly when released) CNG may also be mixed with biogas, produced from landfills or wastewater, which doesn't increase the concentration of carbon in the atmosphere  Cars and locomotives Type of Fuels Comparing Fuels Fuel Oil Coal Natural Gas Carbon 84 41.11 74 Hydrogen 12 2.76 25 Sulphur 0.41 - Oxygen 9.89 Trace Nitrogen Trace 1.22 0.75 Ash Trace 38.63 - Water Trace 5.98 - Numericals on Combustion of fuel  Calculate the weight and volume of air required for combustion of 3kg of carbon  Calculate the minimum weight of air required for complete combustion of 1kg of fuel containing C=90%, H=3.5%, O= 3.0%, S= 0.5%, water=1%, N=0.5% and rest is ash  A gaseous fuel has the following composition by volume: H=20%, CH4= 5%, CO= 20%, CO2= 5% and N= 45% If 50% excess of air is used find the weight of air acutally supplied per m of this gas Power Alcohol: Ethyl alcohol is an important liquid fuel and when it is used in an internal combustion engines, it is called power alcohol Generally gasoline and alcohol are present in the ratio of 4:1 Its octane no is 90 Manufacture of Power Alcohol: Ethyl alcohol can be manufactured from hydrocarbons, carbohydrates, starches and molasses, waste sulphite liquor from paper and pulp mills Ethyl alcohol is mainly manufactured from molasses The formation of alcohol involves the following steps: (i) Dilution: High concentration of sugars in molasses (ii) Addition of dilute sulphuric acid: For fermentation the pH of the solution should be kept between to (iii) Addition of ammonium salts: When molasses contain insufficient yeast food, some neutrient substanceslke ammonium sulphate and ammonium phosphate are added to overcome the deficiency of phosphorous and nitrogen (iv) Addition of yeast: This is the main step in the formation of alcohol from mollases To the o solution a selected strain of yeast is added The mixture s kept at a temperature of 30 C for 2-3 days C H O + H O Invertase     → C H O + C H O 12 6 12 12 22 11 C H O Zymase    → 2C H OH + 2CO 12 (v) Distillation: It is carried out in a specially designed plant called coffey still Distillation of wash/wort (vi) Azeotropic distillation: The alcohol obtained after distillation is mixed with benzene or carbontetrachloride and subjected to distillation The benzene or carbontetrachloride distills over taking away a portion of alcohol and water leaving behind absolute alcohol Advantages of Alcohol blended gasoline: The octane no of alcohol is quite high Hence on blending with gasoline, it increases the octane no of gasoline There is no decrease in power output or increase in specific fuel consumption on blending alcohol with gasoline The starting difficulties because of high boiling point of alcohol is overcome, when it is used in blended form with gasoline Alcohol-petrol blend has a tendency to absorb any traces of moisture present Disadvantages of Alcohol blended Gasoline: The calorific value of alcohol is low and hence the alcohol lowers the calorific value of petrol Since the air required for complete combustion of blend is less than the air required for petrol The ratio of flow of blend is to be increased to 1.56 times to that of pure petrol by altering the size of carburetor jet [...]... Processing of Crude Petroleum: Petroleum is found deep below the earth crust The oil is found floating over salt water or brine Generally, accumulation of natural gas occurs above the oil Fig 9: Pumping of oil Refining of Petroleum Crude oil reaching the surface, generally consists of a mixture of solid, liquid and gaseous hydrocarbons containing sand and water After the removal of dirt, water and much of. .. percentage of iso octane present in a mixture of iso-octane and n-heptane, which has the same knocking characteristics as that of fuel under examination, under same set of conditions Thus a gasoline with an octane no of 80, would give the same knocking as a mixture of iso octane and n-heptane containing 80% of iso octane by volume Greater the octane number, greater is the antiknock property of the fuel... about in the presence of a catalyst at much lower temperatures and pressures The catalyst used is mainly a mixture of silica and alumina Most recent catalyst used is zeolite The quality and yield of gasoline is greatly improved by this method Advantages of catalytic cracking over thermal cracking: • High temp and pressure are not required in the presence of a catalyst • The use of catalyst not only... the nature of the products • The yield of the gasoline is higher • No external fuel is required for cracking • The process can be better controlled so desired products can be obtained • The product contains a very little amount of undesirable sulphur because a major portion of it escapes out as H2S gas, during cracking • It yields less coke, less gas and more liquid products • The evolution of by-product... etc Sulphur is present in the form of derivatives of hydrocarbons such as alkylsulphides, aromatic sulphides etc Nitrogen is present in the form of pyridine, quinoline derivatives, pyrrole etc Comined oxygen is present as carboxylic acids, ketones and phenols The objectionable odour of crude petroleum is due to the presence of sulphur compounds in it Classification of Crude Petroleum Residue obtained... be graded on octane number scale The cetane number of a diesel oil is defined as the percentage of cetane in a mixture of cetane and a-methyl naphthalene which will have the same ignition characteristics as the fuel under test, under same set of conditions Cetane is n-hexadecane The cetane rating of a fuel depend upon the nature and composition of hydrocarbon The straight chain hydrocarbons ignite... importance of liquid fuels is the fact that almost all combustion engines run on them The largest source of liquid fuels is petroleum The calorific value of petroleum is about 40000 kJ/kg There are other supplements of liquid fuels such as coal tar, crude benzol, syntheic liquid fuel made from coal etc Petroleum: The term petroleum means rock oil It is also called mineral oil Petroleum is a complex mixture of. .. the associated natural gas, the crude oil is separated into a no of useful fractions by fractional distillation The resultant fractions are then subjected to purification known as refining of petroleum Steps involved in refining of petroleum: (i) Demulsification: The crude oil coming out from the well, is in the form of stable emulsion of oil and salt water, which is yellow to dark brown in colour The... decreases gradually on moving upwards As the vapours of the crude oil go up, they become gradually cooler and fractional condensation takes place at different heights of column The residue from the bottom of the fractionating tower is vacuum distilled to recover various fractions Fig 11: Vacuum distillation of residual oil There is yet another type of fractional distillation called Top-flashing Fig... distillation called Top-flashing Fig 11: Top Flashing In top flashing, there is better control of product composition, but requires more pumps and instruments and hence is an expensive process Cracking: Gasoline is the most imp fraction of crude petroleum The yield of this fraction is only 20% of the crude oil The yield of heavier petroleum fraction is quite high Therefore, heavier fractions are converted ... hot coke is pushed outside by means of a ram which is electrically driven The coke falls into a quenching car The yield is 75 % of coal Fig 7: Otto Hoffmann’s Byproduct coke oven Recovery of. .. Combustion of fuel  Calculate the weight and volume of air required for combustion of 3kg of carbon  Calculate the minimum weight of air required for complete combustion of 1kg of fuel containing...Cost: Coke should be cheap and easily available Manufacture of Metallurgical Coke: (i) Beehive Oven (ii) Otto Hoffmann oven (i) Beehive oven: A beehive oven