Vol. XL No. 275. 



THE AGRICULTURAL NEWS. 



355 



THE FUEL VALUE OF MEGASS. 



The following information regarding the fuel 

 value of inegass is taken from Bulletin No. 40 of the 

 Agricultural and Chemical Series of the Experiment 

 Station, Hawaiian Sugar Planters' Association, issued 

 recently under the title Heat of Combustion of Bagasse 

 from Hawaiian Cane: — 



In the investigation of fuels for practical purposes there 

 is not much advantage in the direct comparison of thermal 

 values. The amount of heat that can be obtained from 

 a fuel is not dependent solely on its thermal value, though it 

 is necessary to know this in order to determine its value as 

 a fuel. The fuel value may be considered the net thermal 

 value, or the available heat per unit weight. The amount of 

 heat available depends on the thermal value, the moisture 

 content, and the total heat removed by all the products 

 resulting from the combustion in the furnace. 



The effect of the moisture in the megass on its fuel 

 value is, in the first place, to reduce the amount of combustible 

 matter per unit weight. In a pound of megass containing 

 50 per cent, moisture, there is only half a pound of fuel, so 

 that instead of there being 8,100 B.T.U.* per pound there 

 are only 4,050 B T.U. to start with. According to the mill 

 reports the moisture in megass in Hawaii averages about 45 

 per cent t and there would therefore, be 4,455 B T, U. per 

 pound of green megass. For other percent;iges of moisture 

 thelieat set free would be as follows:— 



Per cent, moisture. 



42 

 43 

 44 

 45 

 46 

 47 

 4S 

 49 

 50 

 51 



Heat contained per lb. of 

 megass, B.T.U. 



4,698 

 4,617 

 4,536 

 4,4.55 

 4,374 

 4,293 

 4,212 

 4,131 

 4,050 

 3,969 



The moisture in the megass reduces the amount of 

 available heat in another manner also — it carries away what- 

 ever heat is used in bringing it to the stack temperature. 

 According to Thurston this i.s usually 600°F. or over, but the 

 average in a number of sugar factories in Cuba, investigated 

 by Kerr, was only 434°F., and he believes that for the 

 highest efficiency it should not be over 500°F. Taking 

 500°F. as an average for the temperature in the stack, and 

 85' F. i as the temperature of the water in the megass, the 

 loss of heat from this source for each pound of average 

 megass would be as follows: 0-45 x (212 - 85) = 57 B.T.U, to 

 raise the water to the boiling point; 045 x 966^435 B.T.U 

 to convert it into steam, 966 being the latent heat of vapo- 

 rization of water; and 45 x (500 - 212) x 0-48 = 62 B.T.U. to 

 raise the temperature of the steam to 500 F., 048 being the 

 specific heat of steam. A total of 57 + 435 + 62 = 554 

 B.T.U. to be subtracted from the 4,455 B.T.U., in a pound 

 of green megass. 



There is a further loss also in the heat carried away by 

 the products of combustion, and excess ofi air. From the 

 «leiuentary analysis given previously we may assume a pound 

 of green megass to contain 055 x 0'48 = 0'264 pounds of 

 carbon and 0'55 x 0057 = 00314 pounds of hydrogen, which 

 on combustion would give 0204 x 3-67 = 969 pounds of 

 carbon dioxide and 0-0314 x 9=0'282 pounds qf water vapour, 



and would require 262x1159 = 306 pounds of air. 

 Taking the excess air as 100 per cent, we would then have 

 in addition to the carbon dioxide and water given above, 

 306 pounds of air and 3-06 x 0'77 = 235 pounds of nitrogen 

 leaving the furnace for every pound of megass burned. 

 The air would contam also, on an average, one and a half per 

 cent., or 6 12x0015 = 0092 pounds of water vapour. 

 From the following specific heats we can calculate the total 

 heat carried avvaj' by these products of combustion: — 



Carbon dioxide 

 Nitrogen 

 Air, dry 

 A\'ater vapour 



02163 

 0-2438 

 0-2375 

 048 



Assuming the temperature of the air entering the 

 furnace to be the same as that of the megass,*! we would 

 have. — 



Loss from combined water 282 x „".fi = 347 B.T.U. 



Loss,, water vapour 0092 x 048 x (500 - 85) = 18 B.T.U. 

 Loss „ carbon dioxide 969 x 02163 x (500 - 85) = 87 B T.U. 

 Loss ,, nitrogen 235 x 02438 x (500 - 85) = 238 B.T.U. 



Loss,, excess air 3 06 x 02375 x (500 - 85) = 302 B.T.U. 



Total loss from products of combustion 



992 B.T.U. 



The combined loss of heat from the products of com- 

 bustion and the moisture in the megass would then be 

 554 + 992=1,546 B.T.U., leaving available for generating 

 steam in a pound of the megass which we have taken as 

 typical, 4,455 - 1,546 = 2,909 B.T.U. For megass containing 

 different amounts of moisture the available heat, by the 

 same method of calculation, would be as follows: — 



Per cent, moisture. 



42 

 43 

 44 

 45 

 46 

 47 

 48 

 49 

 50 

 51 



Fuel value per tt>. of 

 megass, B.T.U. 

 3,129 

 3,057 

 2,982 

 2,909 

 2,835 

 2,762 

 2,687 

 2,614 

 2,540 

 2,468 



On the basis of these values not only different megasses 

 may be compared with one another, but megass with other 

 fuels through corresponding values for the latter. 



The amount of heat which these fuel values represent is 

 not actually obtained for the production of steam, because 

 there are other losses, such as radiation, but these are func- 

 tions of the furnace rather than the fuel. There is a slight 

 loss through the heat carried away by the ashes, which 

 directly affects the fuel value, but it is so small as to be 

 practically negligible. 



* In accordance with the usual custom the British Ther. 

 mal Unit and Fahrenheit scale of temperature will be used in 

 di.'ccus.sing fuel value. 



t The moi.stuiu in the megass as it enters the furnace is 

 j>robably somewhat less than this owing- to evaporation after 

 it leave.s the mills. 



t There is probably h variation of as much as 16°F. in this' 

 temperature in ditf'erent factories, depending on whether hot or 

 cold water is u.sed for maceration, the distance the niegas.s is 

 carried to the furnace, and the temperature of the air surround- 

 ing it. 



H It is probably .soniewliat less, as most of the air comei> 

 directly from the out.side through the grates.' 



