Page 478 - Boiler_Operators_Handbook,_Second_Edition

Appendices 463

To convert percentages to pound per pound (#/#) num- [5] to get the volume of dry flue gas [10]. Perform the
bers simply divide the percentage by 100. The values indicated calculation to determine what the percent of
in column B should add up to 1 or less, less if there’s carbon dioxide should be at the normal firing condition.
water and/or ash in the fuel. All the calculations on this
worksheet are based on one pound of fuel and all results The formula for calculating the actual volume of the dry
are per pound of fuel. flue gas is developed by adding 460 and the predicted
(or actual) stack temperature, dividing that result by 530
Multiply the pounds of combustible in the fuel (column then multiplying by the standard volume of dry product
B) by the factor in column C to determine the pounds [10]. Add the result
of oxygen required for each combustible and record it
in column D. Add up all the results in column D to To determine the total volume of flue gas we have to cal-
determine the pounds of oxygen required per pound of culate the volume of the water. This sheet approximates
fuel, entering it after the “pounds of O required.” Cal- it by using the formulas shown. Determine the pounds
2
culate the amount of nitrogen in the air by multiplying of water in the flue gas per pound of fuel by dividing
that result by 3.31 and entering it in the space provided. the percent of water in the fuel [1] by 100, adding the
water produced by the combustion of hydrogen in col-
The weight of the products of combustion for each com- umn E [3], and the moisture in the air which is equal
bustible is determined by adding the pounds per pound to the total air [8] multiplied by the fraction of water
of fuel to the pounds of oxygen required and recording that’s in the air. You can obtain that information from a
that result in column E. The pounds of nitrogen required psychometric chart or use 0.009 which is a typical value
in the air must be added to the weight of the nitrogen in for pounds of moisture per pound of dry air.
the fuel to get the total pounds of nitrogen per pound of
flue gas in column E. Combine the weight of water from If you’re using these calculations to get as precise a
the hydrogen in the fuel and it’s oxygen with the mois- value as possible for a given operating condition you
ture in the fuel [1] to get the total moisture from fuel [3]. should make it a point to get the moisture in air value
as precise as possible because that moisture can carry a
Determine the volume of the dry gas by multiplying lot of heat up the boiler stack. It can make a big differ-
the weights in column E by the factors in column F ence in calculating the boiler efficiency for two different
and enter the result in column G. Note that we only operating conditions like summer versus winter.
calculate the volume of carbon dioxide, sulfur dioxide,
and nitrogen because the oxygen from the theoretical air Add 0.62133 to the pounds of moisture then divide by
is consumed. The water volume isn’t calculated because the pounds of moisture to get the volumetric ratio of
we’re determining the volume of dry gas. moisture [13]. Divide the actual volume of dry gas [11]
by the percent of dry gas to wet gas (which is 100 mi-
Combine the dry gas volumes to get the total theoreti- nus the moisture ratio) to get the actual volume of wet
cal volume of dry gas [5]. Add the weight of oxygen and flue gas [14].
weight of nitrogen from the air to get the theoretical
weight of air required [6]. Divide the theoretical volume Formulas for the percent oxygen give results on a dry
of carbon dioxide [4] by the theoretical volume of dry gas basis [15] and a wet basis. Percent oxygen on a dry
[5] to determine the maximum possible percentage of car- basis is what you would get using a fyr-rite or an orsat
bon dioxide in the flue gas. Multiplying the theoretical air analyzer because the moisture is condensed from the
weight by 13.33 produces the volume of combustion air flue gas to get the measurements. The oxygen content
to burn one pound of fuel in standard cubic feet. indicated by an in-situ analyzer, such as a zirconium
oxide analyzer, measures the gases with the moisture as
The bottom box of the combustion calculation sheet is steam so it’s included in the volume of the flue gases.
set up for determining actual firing conditions. Multiply
the weight of air required [6] by the percent of excess air EFFICIENCY CALCULATION WORKSHEET
and divide by 100 to determine the weight of excess air
[7]. Add [6] and [7] to get total air required for normal The last worksheet in this appendix uses the information
combustion [8]. Calculate the volume of excess air [9] developed in the earlier ones to predict, or calculate,
by multiplying the excess air weight [7] by 13.33. Add the efficiency of a boiler burning the fuel having the
the volume of excess air [9] and the theoretical product ultimate analysis used for the combustion calculations.