Exercise 2 Text Coal combustion furnace

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1 Department of Energy Politecnico di Milano Via Lambruschini 4 - 20156 MILANO Exercises of “ Fundamentals of Chemical Processes” Prof. Gianpiero Groppi Exercise 2 Analysis of a pulverized coal combustion furnace In a power plant, a suspensionNburning furnace burns pulverized coal particles. The coal has the following composition, in terms of mass fractions on a dry and ashNfree basis (ultimate analysis): C 85.5% H 5.5% N 1% S 1% O 7% The coal also contains fractions of humidity and ashes, which amount respectively to 8% and 6% on a total basis (proximate analysis). The pulverized coal enters the furnace at 25°C and 1 bar and is burned with air, at 50°C and with 30% excess, at 1 atm and relative humidity of 25%. The exhaust gases exit the furnace at 150°C. In the steam production section, the incoming liquid water is pumped at 28 bar and 90°C (the effect of pressure on the enthalpy content of water can be neglected) and is converted to saturated steam at 28 bar and 238°C. Assuming that the heat losses amount to 3% of the LHV of the coal, calculate the amount of steam produced per kg of coal and the efficiency of the furnace. 2 DATA Coal: T IN = 25°C Dulong equation for the LHV of coal ( ) kg kJHOHHHVLHV kg kJS OHCHHV 90 .25 3 .94814457 .338 2 ⋅+⋅−= ⋅+     −⋅+⋅= H 2O = humidity fraction Air: T IN = 50°C, P = 1atm, R.H. = 25% a 50°C, P SAT H2O (50°C) = 12.349 kPa Water/Steam: h IN H2O (90°C) = 376.92 kJ/kg H2O hOUT H2O (238°C) = 2804 kJ/kg H2O Exhaust Gases: T OUT = 150°C, P = 1atm Specific Heat at constant pressure [J/mol/K] CP (50°C) Air 29.10 H2O 33.73 CP (150°C) N2 29.24 O2 30.06 CO 2 39.54 H2O 34.13 3 Solution It is convenient to assume 100 kg of total coal as the basis for the material balances. In this way, it is possible to estimate the coal composition…