Download It help u to study best and scoring pdf. and more Lecture notes Fossil Fuels in PDF only on Docsity!
Analysis of coal
Coal is a fossil fuel which occurs in layers in the earths crust. It is formed by the partial decay of plant materials accumulated millions of years of ago and further altered by action of heat and pressure. The process of conversion of wood into coal can be represented as Wood Peat Lignite Bituminous Anthracite
- Peat: Peat is brown-fibrous jelly like mass.
- Lignite: these are soft, brown colored, lowest rank coals
- Bituminous coals: These are pitch black to dark grey coal
- Anthracite: It is a class of highest rank coal Fuel Percentage of carbon Calorific value (k.cal/kg) Applications Wood 50 4000 - 5000 Domestic fuel Peat 50 - 60 4125 - 5400 Used if deficiency of high rank coal Lignite 60 - 70 6500 - 7100 For steam generation in thermal power plants Bituminous 80 - 90 In making coal gas and Metallurgical coke Anthracite 90 - 98 8650 - 8700 In households and for steam raising
Analysis of Coal
The analysis of coal is helpful in its ranking. The assessment of the quality of coal is carried out by these two types of analyses. A) Proximate analysis B) Ultimate analysis
Proximate analysis: In this analysis, the percentage of carbon is indirectly determined. It is a quantitative analysis of the following parameters.
- Moisture content 2. Volatile matter 3. Ash 4. Fixed carbon Moisture Content: About 1 gram of finely powdered air-dried coal sample is weighed in a crucible. The crucible is placed inside an electric hot air-oven, maintained at 105 to 110 0 C for one hour. The crucible is allowed to remain in oven for 1 hour and then taken out, cooled in desiccators and weighed. The process of heating, cooling and weighing of crucible is repeated until we get a constant weight. Loss in weight is reported as moisture. ๐ท๐๐๐๐๐๐๐๐๐ ๐๐ ๐ด๐๐๐๐๐๐๐ =
ร ๐๐๐
Volatile Matter : The volatile matter present in the coal may be combustible gases (CH 4 , CO, H 2 etc.) or non-combustible gases (CO 2 , N 2 ). The dried sample (moisture free coal) taken in a crucible in and then covered with a lid and placed in an electric furnace or muffle furnace, maintained at 925 ยฑ 20 oC. The crucible is taken out of the oven after 7 minutes of heating. The crucible is cooled first in air, then inside desiccators and weighed again. The process of heating, cooling and weighing of crucible is repeated until we get a constant weight. Loss in weight is reported as volatile matter on percentage-basis. ๐ท๐๐๐๐๐๐๐๐๐ ๐๐ ๐ฝ๐๐๐๐๐๐๐ ๐ด๐๐๐๐๐ =
ร ๐๐๐
Ash: Ash is the non combustible, useless matter which is left behind after the combustion of coal. Ash content reduces the calorific value of coal. The weighed amount of coal sample is taken in a crucible and then heated without lid in a muffle furnace at 700 ยฑ 50 oC for ยฝ hour. The crucible is then taken out, cooled first in air, then in desiccators and weighed. Hearing, cooling and weighing are repeated, till a constant weight is obtained. The residue is reported as ash on percentage-basis. ๐ท๐๐๐๐๐๐๐๐๐ ๐๐ ๐จ๐๐ =
ร ๐๐๐
Ultimate analysis: This is the elemental analysis and often called as qualitative analysis of coal. This analysis involves the determination of carbon and hydrogen, nitrogen, sulphur and oxygen. Determination of Carbon and Hydrogen: About 1 to 2 gram of accurately weighed coal sample is burnt in a current of oxygen in a combustion apparatus. C and H of the coal are converted into CO 2 and H 2 O respectively. The gaseous products of combustion are absorbed respectively in KOH and CaCl 2 tubes of known weights. CaCl 2 tube absorbs moisture: CaCl 2 + 7 H 2 O CaCl 2 .7H 2 O While KOH bulb absorbs CO2: 2KOH + CO 2 K 2 CO 3 + H 2 O The increase in weight of CaCl 2 tube represents the weight of water formed while the increase in weight of KOH bulb represents the weight of CO 2 formed. ๐ท๐๐๐๐๐๐๐๐๐ ๐๐ ๐ช๐๐๐๐๐ =
๐ฐ๐๐๐๐๐๐๐ ๐๐ ๐๐๐๐๐๐ ๐๐ ๐ฒ๐ถ๐ฏ ๐๐๐๐ ร ๐๐
๐พ๐๐๐๐๐ ๐๐ ๐๐๐๐ ๐๐๐๐๐๐ ๐๐๐๐๐ ร ๐๐
ร ๐๐๐
๐ฐ๐๐๐๐๐๐๐ ๐๐ ๐๐๐๐๐๐ ๐๐ ๐ช๐๐ช๐๐ ๐๐๐๐ ร ๐
๐พ๐๐๐๐๐ ๐๐ ๐๐๐๐ ๐๐๐๐๐๐ ๐๐๐๐๐ ร ๐๐
ร ๐๐๐
Determination of Nitrogen: A good coal should have very little nitrogen content. Kjeldahalโs Method: About 1 gram of accurately weighed powdered coal is heated with concentrated H 2 SO 4 along with K 2 SO 4 and CuSO 4 (catalyst) in a long-necked Kjeldahlโs flask. After the solution becomes clear (when whole nitrogen is converted into ammonium sulphate) , it is treated with excess of KOH and the liberated ammonia is distilled over and absorbed in a known volume (V 1 ) of standard H 2 SO 4 solution (0.1 N). The unused acid is then determined by back titration with standard NaOH solution (0.1 N). N 2 + H 2 SO 4 (NH 4 ) 2 SO 4 (NH 4 ) 2 SO 4 + 2 NaOH 2Na 2 SO 4 + 2NH 3 + 2H 2 O 2NH 3 + H 2 SO 4 (NH 4 ) 2 SO 4 Let V 2 ml of 0.1 N NaOH was required to neutralize excess acid, the percentage of N in coal is calculated as follows: ๐ท๐๐๐๐๐๐๐๐๐ ๐๐ ๐ต๐๐๐๐๐๐๐ =
ร ๐. ๐
Ash: Ash is a useless, non-combustible matter, which reduces the calorific value of coal. Moreover, ash causes the hindrance to the flow of air and heat, thereby lowering the temperature. Hence, lower the ash content, better the quality of coal. The presence of ash also increases transporting, handling and storage costs. It also involves additional cost in ash disposal. The presence of ash also causes early wear of furnace walls, burning of apparatus and feeding mechanism. Oxygen: Oxygen content decreases the calorific value of coal. High oxygen-content coals are characterized by high inherent moisture, low calorific value, and low coking power. Moreover, oxygen is a combined form with hydrogen in coal and thus, hydrogen available for combustion is lesser than actual one. An increase in 1% oxygen content decreases the calorific value by about 1.7% and hence, oxygen is undesirable. Thus, a good quality coal should have low percentage of oxygen.
