Module 1: History of Fuels. Lecture 6: Fundamental definitions, properties and various measurements

Transcription

1 1 P age Module 1: History of Fuels Lecture 6: Fundamental definitions, properties and various measurements

2 2 P age Keywords: Characterisation, analytical methods, standards 1.3 Fundamental definitions, properties and various measurements Various measurement techniques Characterisation of coal and petroleum fractions, as well as gaseous fuels is done by some specific test methods and apparatus. Some major characterization techniques are mentioned in this section. Some standards, such as ASTM (American Society for Testing and Materials), BS (British Standard), IS (Indian standard) etc. are followed to determine the properties. Measurement techniques for coal and coke Proximate analysis of coal is an important analytical technique to determine moisture, volatile matter, ash and fixed carbon content in terms of weight percentage. In this method, a specific amount of finely powdered coal (1 or 2 g) is heated in a petri-dish for one hour at 105 to C, sample is cooled and weighed. The weight loss is the external moisture content. The coal sample is then taken in a silica crucible and heated at C for 7 min, the sample is quickly cooled to avoid the oxidation and then weighed. The weight loss is expressed as volatile matter content. About 1 g of powdered coal is taken in a silica dish and heated in a muffle furnace at 400 to C for 30 mins, and then one hr at 725±25 0 C. The weight of the incinerated residue is the ash content. The fixed carbon is obtained by subtracting the weights of moisture, volatile matter and ash content from 100. Caking index for coal is determined by taking sample of the uniform mixtures of sand and coal of total 25 g in different weight ratios. The samples taken in crucibles are kept inside a muffle furnace at approximately C for 7 min. Then the crucibles are removed and allowed to cool. The lump or cake is observed to form from the mixtures. The cakes are taken out carefully and

3 3 P age 500 g weight is placed on the cakes. The percentage of loose particles comes out from the cakes are determined separately for each cake which gives a measure of the binding property of the coal sample. The maximum whole number ratio of sand to coal in the 25 g mixture which produces a cake after heating that can withstand a weight of 500 g without producing 5% loose grain is the caking index. Swelling index is one of the important properties of coal. Coal is heated under specified condition on flame in a crucible. The swollen mass is cooled and compared with a standard chart where profiles with numbers (which indicates swelling index) are given. The numbers are in increasing order from 0 to 9 with a 0.5 increment. The higher the number better is the caking and swelling characteristics. Calorific value of coal is determined by Bomb calorimeter. It is a thick walled steel cylindrical vessel with lid which is called Bomb. Two electrodes are inserted through the lid which are in contact with fuse and fuel sample of known weight. An oxygen inlet valve is provided with the lid through which high pressure oxygen gas (at about 25 to 30 atm) is supplied. Entire arrangement is held in a calorimeter containing known weight of water and a mechanical stirrer is provided to stir the water for uniform heating. A Beckmann thermometer is also provided to measure the change in temperature of water due to combustion of fuel. Fig 1. shows the schematic of the apparatus. The gross calorific value is calculated by the following formula C. V m m T T T /m Where, m 1 and m 2 are mass of water in calorimeter and water equivalent of bomb calorimeter respectively. m f is mass of fuel sample whose calorific value is to be determined. T 1 and T 2 are final and initial temperature of water sample. Tc is temperature correction for radiation losses. C w is specific heat of water.

4 4 P age Fig. 1 Bomb calorimeter The major property of coke is its strength and hardness. Among many tests to determine strength and hardness of coke, Micum test is important. In mecum test, 50 kg of coke samples are taken in a metallic cylindrical drum (Micum trommel) of 1 m length and 1 m diameter, and the drum is rotated about its horizontal axis at 25 rpm speed for 4 min. Iron angles are provided inside the drum wall. This rotation gives a combined effect of abrasion and shatter to coke samples. The particle size distribution after the test is determined by sieve analysis and this gives the account of brittleness of coke during its handling and use. Micum index or M80 denotes the percentage weight of coke lumps greater in size of 80 mm. Measurement techniques for petroleum fractions Distillation or vaporisation characteristics of a petroleum cut is determined by following ASTM method (ASTM D-86). This is the method of non-fractionating atmospheric distillation of light cuts such as, gasoline, kerosene etc. A specific amount of liquid fuel is taken in a standard round bottom flask with the arrangement of condenser and thermometer device. Fig 1. shows the standard set-up, consisting of a flask, condenser, collector and thermometer. The fuel is heated

5 5 P age by an electric heater at a specified rate and some part of vapour flows to the condenser through the side limb of the flask. Other part of the vapour gets condensed and flows down as liquid to the flask which acts as reflux. The condensed vapour from condenser is collected in the collector. The volume percent distilled is plotted against corresponding temperature which provides the vaporization characteristics of the particular fuel. A= Flask, B= Condenser, C= Collector, D= Thermometer, E= Controller switch Fig 2. ASTM distillation apparatus Flash point and fire point of a definite petroleum cut are measured in a standard apparatus. Different fractions may be classified based on their flash point, such as, fractions below 23 0 C flash point are highly inflammable, dangerous, between 23 to 66 0 C are moderately inflammable and above 66 0 C are termed as safe. Gasoline lies in the first category, kerosene in the second and higher oils, such as fuel oil and gas oils fall in the third. Abel apparatus is used for the oils having flash point below 50 0 C and Pensky-Martens apparatus is used for the oils having flash point above that temperature. Both these apparata are of closed cup type. A specified amount of oil is heated in a definite rate in a closed cup. A provision is made to remove a shutter to escape the fuel vapor and air mixture and an external flame is introduced to test the flash. The

6 6 P age temperature of the oil is determined by a thermometer at the time of flash. That is the flash point of the oil. Flash point of heavy oils such as lubricating oil, crude oil or residue can be obtained by Pensky-Marten open cup or Cleveland open cup apparatus. Pour point and cloud point of oil is determined by putting the oil in a specified test tube, where thermometer is inserted. The oil is cooled at specified rate and at short interval of time the oil is tested by tilting the test tube to see whether the oil is totally freezed. The maximum temperature where oil does not show any flow, that temperature is recorded and the reporting of pour point is done by adding C or 5 0 F to that. Before that freezing temperature comes, haziness on the top of the surface of the oil appears and that temperature is the cloud point of the oil. Smoke point of kerosene is determined in a standard apparatus (Fig. 3) where kerosene is poured in a specified holder (B) with wick on its top. The wick is lit and the height of its flame is so adjusted that it gives a flame with round top, without any shoot tail. The maximum flame height is measured by a mirror scale (C) at the back of the flame in the instrument. An exhaust (D) is provided at the top of the apparatus. A lid with glass cover (A) is used to cover the flame from wind. Fig.3 Smoke point apparatus

7 7 P age Burning quality of kerosene is determined by its char value. A standard lamp with glass chimney is used. Definite amount of kerosene is poured in the lamp and wick is lit. The lamp is allowed to keep lit for continuously 24 hours. The flame heights, initially and finally, are measured by the graduation on the chimney. The char deposit on the wick is scraped and weighed. The mg of char per kilogram of kerosene burned is expressed as char value of that kerosene. The decrease in flame height is also a measure of the quality of kerosene. Measurement techniques for gaseous fuel Compositional analysis of a gas mixture is done by Orsat apparatus as shown in Fig 4. It is a standard apparatus in which normally carbon dioxide, carbon monoxide, oxygen and nitrogen gas composition in volume percentages are reported. Sometimes, hydrogen, methane and unsaturated hydrocarbons can also be quantified in this apparatus. The apparatus consists of a graduated burette upto 100 divisions from bottom to top, an aspirator bottle, a series of pipettes having necessary fittings and a three way stop-cock connecting the top of the burette to the gas stream. Each pipette contains an adsorbent for removing one gas. The adsorbents used are caustic potash solution, freshly prepared alkaline pyrogallol solution and cuprous chloride in ammonia solution for removal of CO 2, O 2 and CO respectively. Unsaturated hydrocarbons can be removed by using bromine solution. The measured gas sample is passed to the pipettes one by one, by opening the stopcock and raising and lowering the aspirator bottle several times for ensure proper adsorption of gases in the respective adsorbents. The decrease in volume from the initial volume after passage through every pipette gives the account of volume percent of the respective gas. Fig.4 shows the typical figure of Orsat apparatus, where A is the graduated glass tube and E, D, C and B are the pipettes containing KOH, alkaline pyrogallol, bromine solution and

8 8 P age ammoniacal cuprous chloride solutions respectively, F is the stopcock, G is aspirator bottle, H is rubber tube. Fig. 4. Orsat Apparatus The gross calorific value at constant pressure of a gaseous fuel is determined by Junkers calorimeter. The method is based on heating a particular amount of water by the heat evolved by burning of the gaseous fuel. The calorific value is calculated by the following formula.,. Where, w= weight of hot water collected in kg, V= observed gas volume in m 3, T= temperature rise of water in 0 C, C p, water = Specific heat of water in kcal/kg.k, and K=[(P a +P g -P w ) in mm Hg/760 mm Hg] 273/(273+t), where, P a and P g are the atmospheric and gauge pressure of gas, P w is saturated vapour pressure of water at the temperature of the system, t ( 0 C).

9 9 P age Reference 1. McGraw Hill Encylopedia of Science & Technology, no. 4, 9 th edition, McGraw Hill, Fuels and combustion, S. Sarkar, 2 nd edition, Orient Longman Ltd., Petroleum Refinery Engineering, W. L. Nelson, 4 th edition, Mc-Graw Hill Book Company, 1987.