Carryover load Your task is to use the obtained experimental result from the probe measurement in combination with the information given in this handout to calculate the carryover load (g/nm 3 dry flue gas) in the Metsä-Fibre, Rauma boiler. Reporting: one report / group. The results of the carryover load calculations will also be reported to Metsä Fibre, Rauma. The steps for determining the carryover load are summarized below: Measure Probe deposit thickness at leading edge Assume that The particles and flue gas flow are evenly distributed over the cross section Impaction efficiency is 1 (all particles hitting probe stick) Other assumptions? Estimate Flue gas flow 1
Calculation of the carryover load The carryover load is given as grams of carryover per normal cubic meter dry flue gas (g/nm 3 dry flue gas). In order to calculate the carryover load, information is needed on two items: the amount of carryover and on the amount of flue gas. The calculation of the amount of carryover is based on the results from the probe measurement and the amount of flue gas is estimated based on a flue gas calculation. Guidance for calculating the amount of carryover and the amount of flue gas is given below. Calculation of the amount of carryover Before beginning with the calculation of the amount of carryover, it is worthwhile to consider first in what format should the amount of carryover be expressed, i.e., what should the treatment of the results of the probe measurement result in. The combustion process in the recovery boiler is continuous and the flue gas calculation will result in a flow of flue gas, i.e., the unit will be normal cubic meters of dry flue gas per second (Nm 3 /s dry flue gas). The rate of deposit build-up from the probe measurement (mm/min) should be expressed as grams of carryover per second (g/s) in order to arrive at the carryover load (g/nm 3 dry flue gas). The carryover load can be then calculated as: CarryOverFlow FlueGasFlow g / s Nm 3 / s g Nm 3, where both the carryover flow and the flue gas flow are assumed to be evenly distributed over the recovery boiler cross-section at the probe measurement location. Calculation of the flue gas flow For calculating the carryover load, the flue gas flow should be expressed as normal cubic meters of dry flue gas per second (Nm 3 /s dry flue gas). The flue gas can be assumed to follow the ideal gas law. Flue gas is formed by combustion of black liquor. In estimating the flue gas flow it is assumed that the combustible elements carbon and hydrogen form gaseous components according to the following: C CO 2 H H 2 O Additionally, water from black liquor is vaporized, and the flue gas also contains excess oxygen from the combustion air, and nitrogen. 2
Nitrogen and sulfur from black liquor are found in the flue gas of a real recovery boiler, but can be assumed not to be part of the flue gas in this calculation. The components of the flue gas can be summarized as: CO 2 (from C CO 2) H 2 O (from H H 2 O + vaporized water from black liquor) O 2 (the excess amount of oxygen from combustion air) (from combustion air) N 2 For C and H to form the components CO 2 and H 2 O, respectively, oxygen is needed. The black liquor contains some oxygen, but not in an amount sufficient to convert all C and H to CO 2 and H 2 O, respectively. Additional oxygen is provided by introducing combustion air. The additional oxygen needed for complete combustion of C and H to CO 2 and H 2 O, respectively, is the stoichiometric amount of oxygen. As a rule, in practical combustion processes the mixing of reactants is not perfect and oxygen is introduced in excess in order to ensure complete combustion. For many fuels all oxygen in the fuel can be assumed to be available for combustion. For black liquor, part of the oxygen in the fuel will be bound in the smelt. In addition, part of the carbon in the fuel is bound in the smelt. In this exercise the oxygen and carbon in smelt can be assumed to be found as Na 2 CO 3. This is based on the assumption that smelt consist of Na 2 S and Na 2 CO 3 only. (An alternative approach for estimating the smelt composition would be to consider for example K, Cl and the sulfur reduction degree. Then the smelt would contain the compounds Na 2 S, Na 2 SO 4, Na 2 CO 3, NaCl, K 2 S, K 2 SO 4, K 2 CO 3, KCl.) How to estimate the amount of flue gas is illustrated and summarized below. 3
Combustion air (O 2, N 2 ) Element C H O N S Na K Cl Rest Black liquor Water Combustibles (C, H and O) Smelt(ash) Flue gas CO 2 H 2 O O 2 N 2 Smelt Na 2 S, Na 2 CO 3, N, K, Cl, rest The steps for estimating the amount of flue gas can be summarized as: 1) From the black liquor flow, establish how much of each element is in smelt and how much is in combustibles. Start by calculating the flow of smelt, the remaining part is combustibles. 2) Assume all combustible C and H form CO 2 and H 2 O, respectively. 3) Calculate how much combustion air is required for complete combustion of C and H, and to give the correct excess of oxygen in the dry flue gas. The dry flue gas consists of CO 2, O 2 and N 2. The exact value for the amount of excess oxygen will be obtained from Rauma mill (usually 2-3 vol-% in dry flue gas). 4) Calculate the volumetric flow of dry flue gas in Nm 3 /s. The flow of combustion air (point three in above list), can be determined either by calculating it analytically or by carrying out a flue gas calculation based on a guessed flow of combustion air. The procedure for calculating the flow of flue gas based on a guessed flow of combustion air is repeated until the correct excess of oxygen in the dry flue gas is reached (instead of repeating the calculation by hand, consider carrying out the calculation, for example, in a spreadsheet program such as Excel). 4
Process data from Metsä-Botnia recovery boiler operation The values given in brackets indicate the magnitude; the actual values are obtained from the Rauma mill. - As fired black liquor feed (liters/s): (40) - Black liquor dry solids content (wt-%): (75) - As fired black liquor temperature ( C): (130) - Flue gas O 2 content (vol-% in dry flue gas): (3) Boiler geometry - Cross sectional area of boiler floor (m 2 ): 12,5 * 12,5 - Distance to the tip of the bull nose from front wall (m): 6,5 Other data 1) Black liquor elemental composition (dry basis) Element Wt-% C 31.9 H 3.4 O 34.6 N 0.07 S 6.7 Na 20.5 K 2.5 Cl 0,2 Rest 0,12 2) Assume smelt composition based on: - All Na, S, N, K, Cl and rest in smelt - Na and S form Na 2 S and Na 2 CO 3 3) Black liquor density vs. solids content at 25 C 5
rho_t / rho_25 4) Black liquor density as function of temperature T 25 Where: 1 3,69 10 4 T 25 1,94 6T 25 10 T = temperature ( C) ρ T = black liquor density at T (kg/m 3 ) ρ T = black liquor density at T=25 (kg/m 3 ) 2 Effect of temperature on black liquor density 1.01 1 0.99 0.98 0.97 0.96 0.95 0.94 0.93 20 40 60 80 100 120 140 temperature (C) The above equation is valid for a temperature range of 20-100 C and dry solids content to 65%. You may use the equation to estimate the black liquor density outside the equations range of validity, or find an alternative estimate for the density. 5) Deposit bulk density = 2600 kg/m 3 This number is based on Assuming deposit is 50% Na 2 SO 4 and 50% Na 2 CO 3 Density of pure substances: Na 2 SO 4 = 2700 kg/m 3 Na 2 CO 3 = 2500 kg/m 3 6) NTP conditions: T=273,15K and P=101325 Pa 7) Universal gas constant: R= 8,31451 J/mol/K = 0,0831451 bar dm 3 / (mol K) 6
8) At NTP V gas =22,4 l/mol 9) Periodic table of the elements 7