Dew Point of Flue Products from Manufactured-Gas Combustion JESSE S. YEAW
AND
LOUIS SHNIDM4N
Rochester Gas and Electric Corporation, Rochester, 4. Y.
gases. Maconachie (6) showed that the dew point of the flue products for one mixture containing 0.00041 per cent sulfur trioxide and 0.00146 per cent sulfur dioxide by volume was 158" F., whereas the calculated value was 122" F. Johnstone ( 3 ) showed in the case of a plant using high-sulfur coals, that a difference of 0.008 per cent of sulfur trioxide in the flue gases caused an increahe of 125" F. in the dew point. Very little sulfur acid causes a considerable increase in the dew point, and it becomes apparent that, where accurate information is desired, evperiinental determinations must be made. Experimental Procedure DRYQLJEXCHERWHERECOKE Is COOLEDWITHOUT
The general method by which the dew points were determined has been dewribed by various inve,tigators (3, 5 , 7 ) and is based on the increme in conductivity of a glass surface as a re,ult of the deposition of a trace of moidure a t the dew point. A design of the apparatus used in this work is shown in Figure 1.
THE
USE OF W.4TER
T
HE experimental work described is part of a general investigation being conducted a t the Chemical Laboratory of the Rochester Gas and Electric Corporation concerning condensation and corrosion in atmospheres containing the products of combustion of manufactured gas. Condensation problems are more frequently encountered in the use of manufactured gas than in the use of solid or liquid fuels, because of the larger proportion of hydrogen in the gas and the relatively lower flue temperatures in its products of combustion (6). The temperature a t which condensation in a gaseous mixture begins, as it is cooled from a higher temperature, is called the "dew point." When the composition of the combustible gas is known, it is comparatively simple to calculate the theoretical dew point of the flue products resulting from the conibustion of any mixture of this gas with air, by estimating the quantity of water present in these flue gases from the chemical equations involved in the oxidation. This has been done for rarious mixtures of a manufactured gas distributed in a large eastern city, and the data are presented in Table I, which also includes the analysis of the combustible gas (these data are indicated in Figure 2 by the solid line). It has been found, however, that the actual dew points of such flue gases do not correspond to the data calculated in this manner. The experimentally determined values are always higher than the calculated data. This deviation can be ascribed t o the presence of traces of sulfur trioxide in the flue
The experimental results for the dew points of the flue products resulting from the combustion of the manufactured gas show that the true dew points are higher than those calculated from the water estimated to be present, according to the chemical equations involved in the combustion, by an increasing amount as the excess air in these gases decreases. The difference is about 18' F. (10' C.) at the point of theoretical combustion with no excess air. A consideration of the vapor pressures of the various constituents of flue gases indicates that this deviation can be taken to represent the effect of the presence of a trace of sulfur trioxide in the flue products resulting from the combustion of 9 grains of sulfur per 100 cubic feet of the manufactured gas. 1476
Cooi.ixc. ( h 1 . s hT THE C O A L I .
4
(;AS
\n....,
CONDP>7-slNG I'LANr ' ~ F
increasiug amount as tlie execst. air in these gases decreases. The differelice is aliout 18" F. (loe C.) at the point of tlieo-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1478
TABLE I. EXCESS AIR Air in Dry Flue Products
Excess Airb (Dry Basis)
%
%
0 10 20 30 40 50 80 70 80 90
0.0
10.3 23.3 39.7 62.0 92.9 139.5 216.9 371.8 838.5 1766.0
95
HxO Carried H20 from by Air Total Dry Combination of (50% Satd. Flue Products Satd. Gas a t 60' F.) CU. ft. cu. ft. cu.ft. 1.07 0.04 4.35 4.83 1.07 0.05 1.07 0.05 5.44 1.07 0.06 6.21 1.07 0.07 7.25 1.07 0.08 8.70 1.07 0.10 10.88 1.07 0.13 14.50 21.75 1.07 0.19 43.50 1.07 0.38 87.00 1.07 0.73 None
..
...
....
100
AND C.4LCULATED
COC Illurninants
02
co H,
CH4
HnO
Na Av. total sulfur content
-
DEW POINTS Total Wet Flue Products cu. ft. 5.46 5.95 6.56 7.34 8.39 9.85 12.05 15.70 23.01 44.95 88.80
...
FOR
VOL. 27, NO. 12
MANUFACTURED GAP
Total H t O cu. ft. Grams 1.11 24.0 1.12 24.1 1.12 24.2 1.13 24.4 1.14 24.6 1.15 24.8 1.17 25.3 1.20 25.9 1.26 27.2 1.45 31.3 1.80 38.9
..
..
Analysis of Combustible Gas 3.14% by vol. 3.34 Gross B. t. u. per cu. f t . 0.29 Air required t o burn 1 cu. ft.. cu. ft. 9.34 Combustion products resulting, ou. ft.: 44.91 25.65 COa 1.73 Nl 11.60 Ha0 9 grains or 0.58 gram per 100 cu. ft. of gas.
Ha0 in Wet Flue Products
Dew Point ( 1 )
% 20.3 18.8 17.1 15.4 13.6 11.7 9.7 7.6 5.5 3.2 2.0 0.87
F. 141 138 135 131 126 12 1 114 106 95 78 64 41.4
640 4.88 0.54 3.81 1.07
Data given for the combustion of 1 cu. ft. of gas.; all volumes given a t 60' F. and 30 inches mercury pressure; 1 cu. f t . water vapor a t 60° F. pound ( I ) . b Percentage excess air based on theoretical dry air required for combustion-i. e., 4.68 cu. it. per cu. ft. of gas burned. a
TABLE11. DATASHEETOF TYPICAL DEW POINTDETERMINATIOS Water Temp. Hg ther- ThermoTime mometer couple Min. O F . O F . 0 119 122 121 4 118 121 7 118 11 117 121 16 117 120 20 117 120 25 117 120 30 116 118 38 116 119 48 118 122
Thermocouple 1 2
Cor. Condition of Surface 1 2
Galvanometer p-a
O F .
OF.
O F .
OF.
OF.
134 132 132 132 131 130 130 130 130 133
136 134 134 134 132 131 130 131 132 134
131 129 129 128 128 127 127 128 127 129
133 131 131 130
132 130 130 129 129 128 127 128 128 130
Av. room temp,., O ,F. Rate gaa combination, cu. ft./hr. Flue products, cu. ft./hr. Detd. dew point. a F.
77 4.5 12 129
129
128 127 129 129 130
0.0 0.0 1.5 1.5 1.5 1.5 4.5 4.5 3.0 0.0
Obsvd. .4ppearance Dry Cloudy Cloudy Cloudy Cloudy Damp Wet Wet Dry/ng Drying
--Flue Gas 1 2 Analysis, %- .I>,, COZ 5.5 5.4 5.4 5.4 0 2 11.1 11.3 11.2 11.2 yo air in flue products, 53.3
-
0.04758
retical combustion with no excess air. It is evident that the true dew points of these gases depend not only upon the presence of water vapor, but also upon the presence of one or more other constituents in the mixtures. The principal constituents of flue gas are nitrogen, water, carbon dioxide, and oxygen, when t h e c o m b u s t i o n is complete. Small quantities of sulfur dioxide and trioxide are also found, and traces of the oxides of n i t r o g e n have been detected in similar mixtures (4). All of these gases are soluble in water to some extent, but two of them unite with water in the vapor state and form two additional vapors which are soluble in water i n a l l p r o p o r tione. These new vapors a r e s u l f u r i c a n d
DECEMBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
nitric acids. Whereas all of the other gases may be found in minute traces in the droplets formed a t the dew point regardless of their amount in the vapor volume, only sulfuric and nitric acids will be found in accurate proportions depending definitely upon their concentration in the vapor phase, because they dissolve in all proportions in the water formed. For all practical purposes, therefore, the effect of all of the vapors in flue gas, except sulfuric and nitric acids, upon the dew point may be neglected, except in so far as they act as inert substances in diluting the vapor volume. EXPERIMENTAL RESULTSFOR DEW. P O I N T S O F V.4RIOUS MIXTURES OF M.4STJFACTURED GAS WITH AIR -Flue Gas dnalyais (Dry Basis)Con 02 Air Dew Point Teat No. % % % F. 1 11.0 1.2 5.7 158 10.6 2 2.0 10.5 152 3 10.1 3.1 14.8 150 4 9.2 4.4 20.9 151 5 8.7 5.4 25.7 143 7.0 33.3 136 6 7.8 8.5 40.4 137 7 6.7 8 6.3 9.8 46.6 130 9 5 4 11.2 53.3 129 10 4.8 12.2 58.1 120 11 4.4 13.4 63.8 119 13.8 65.7 116 12 4.1
TABLE111.
The dew point may be defined as the temperature a t which the vapor phase is in equilibrium with a minute quantity of the liquid phase in any system. When sulfuric or nitric acids or both are present in the vapor state, the vapor pressure at the dew point will be the vapor pressure of the solution formed. The droplets will contain each constituent in proportion to its partial pressure in the vapor phase if the solution is ideal. Unfortunately many solutions are not ideal, and necessary data must be determined experimentally. Some of these data are available ( 2 ) ,but the tables are not complete. The available vapor pressure data for solutions of water and nitric acid in all proportions (9) are relatively close to those for pure water a t similar temperatures. Since only traces of the nitrogen oxides have been found in flue gases (4, their effect upon the dew point may also be neglected. The available vapor pressure data for solutions of water and sulfuric acid (2) show that, as the acid increases, the vapor pressure of the solution decreases sharply. This indicated that the effect of sulfuric upon the dew points for a mixture
1479
containing this vapor would be large. As such a mixture is cooled, a temperature is reached where a droplet of sulfuric acid solution forms. This is the dew point and also the boiling point of this solution of sulfuric acid. If an appreciable quantity of this vapor is present in the vapor volume, the droplet will be a comparatively strong solution of sulfuric acid. More concentrated solutions with still lower vapor pressures result when the proportion of sulfuric acid in the vapor volume is increased, which in turn results in still higher dew points. A method for roughly estimating the dew points from the analysis of the vapor volume for sulfur trioxide, based on vapor pressure data for water-sulfuric acid solutions, has been worked out by Johnstone ( 3 ) . The reason for the phrase “roughly estimating” is twofold: (1) It has been found difficult to estimate accurately the concentration of sulfur trioxide in a flue gas mixture. (2). The vapor pressure data for the necessary sulfuric acid solutions are not complete, and the extrapolation of existing data introduces probable errors.
It appears from the foregoing considerations of vapor pressure data that the presence of sulfur trioxide in flue gases is the major factor responsible for the increase in dew point over that calculated for the mixture on the basis of the moisture content only. Experimental confirmation of this conclusion is scarce at the prejent time, but the data available agree in showing that an increase in the sulfur trioxide content of the vapor volume corresponds with an increase in the dew point elevation (3,6). There is no standard method so far developed for the accurate determination of sulfur trioxide in flue gas, and many of the methods used in the past are questionable indeed. Literature Cited (1) Am. Gas .kssoo., “Combustion,” 3rd ed., 1932. (2) International Critical Tables, Vol. 111, p. 303, New York, McGraw-Hill Book Co., 1928. (3) Johnstone, H.F.,Univ. Ill. Eng. Expt. Sta., Circ. 20 (1929). (4) Joint Research Comm., Inst. Gas Engrs. and Leeds Univ., 33rd and 34th Repts., Parte I and 11, “Corrosion from Products of Combustion of Gas,” 1933 and 1934. (5) Maoonachie, J. E., “Deterioration of Domestic Chimneys,” Toronto, Consumers Gas Co., 1932. (6) Morgan, W. R.,Univ. Ill. Eng. Expt. Sta., Circ. 22 (1934). (7) Tchang-Telon, Rev. universe2Ze mines, 74, 161-3 (1931). RECEXWD M a y 10, 1935.
