Tham khảo tài liệu 'materials handbook 15th ed - g. brady_ h. clauser_ j. vaccari (mcgraw-hill_ 2002) episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 240 COAL lard and pork products. The clove tree attains a height up to 40 ft m bearing in 7 or 8 years and continuing to bear for a century yielding 8 to 10 lb to kg of dried cloves annually. Clove stems are also aromatic but contain only 5 to 6 oil of interior value. Clove was one of the most valued spices of medieval times. It grew originally only on five small islands the Moluccas in a volcanic-ash soil and was carried by Chinese junks and Malayan outriggers to India from whence the Arabs controlled the trade bringing the tree also to Zanzibar. The Victoria of Magellan s fleet returned to San Lucar with 26 tons 24 metric tons of cloves enough to pay for the loss of the other four ships and the expenses of the voyage around the world. COAL. A general name for a black mineral formed of ancient vegetable matter and employed as a fuel and for destructive distillation to obtain gas coke oils and coal-tar chemicals. Coal is composed largely of carbon with smaller amounts of hydrogen nitrogen oxygen and sulfur. It was formed in various geological ages and under varying conditions and it occurs in several distinct forms. Peat is the first stage followed by lignite bituminous coal and anthracite with various intermediate grades. The mineral is widely distributed in many parts of the world. The value of coal for combustion purposes is judged by its fixed carbon content volatile matter and lack of ash. It is also graded by the size and percentage of lumps. The percentage of volatile matter declines from peat to anthracite and the fixed carbon increases. A good grade of coal for industrial powerplant use should contain 55 to 60 fixed carbon and not exceed 8 ash. The heating value should be 13 500 to 14 000 Btu lb 31 400 to 33 700 kJ kg . Finely ground coal or powdered coal is used for burning in an air blast like oil or it may be mixed with oil. Coal in its natural state absorbs large amounts of water and also because of impurities and irregular sizes is not so
