Energy in Agriculture, 6 (1987) 195-213
195
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Development and Evaluation of a Biomass-Fuelled Furnace P.W. CLAAR II l, M.F. WAHBY2 and W.F. BUCHELE 1
~Department o[ Agricultural Engineering, Iowa State University, Ames, IA 50111 (U.S.A.) 2Department of Agricultural Engineering, University of Cairo, Cairo (Egypt) ( Accepted 27 March 1987)
ABSTRACT Claar, P.W., II, Wahby, M.F. and Buchele, W.F., 1987. Development and evaluation of a biomassfuelled furnace. Energy Agric., 6: 195-213. The paper describes the development and testing of concentric-vortex type, agricultural residuefired furnace which can be used for heating the air to a grain dryer. Measurements were made of furnace combustion efficiency at various fuel feedrates and air flow rates. The highest furnace efficiency obtained was 85% at a feed rate of 82 kg/h and an air flow rate of 1810 kg/h. The temperature of the flue gas varied from 420 to 870 °C at various feed rates and air flow rates. The maximum heat release occurred when the furnace was supplied with 310% excess air over that needed for complete combustion. The turndown ratio for the furnace varied from 3.1 to 1, on a wet basis moisture content.
INTRODUCTION
Energy plays an important role in the agricultural crop production system. Current cultural practices are heavily dependent upon fossil fuels as the energy source. These energy requirements that are needed by American agriculture have been reported by researchers such as Nelson et al. (1975) and by Cast (1984). Biomass is one of the alternatives readily available for many energy applications. Although this type of energy will not solve the energy problem, it will at least be the alternative energy source for farming practices and maintaining and expanding grain production. Burning of agricultural residues is not a new idea, but there hve been problems associated with the burning of residues in grate-type furnaces. Agricultural residues are burned in two ways. First, by gasification; the gasification Journal Paper No. J-11914 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA, Project No. 2535.
0167-5826/87/$03.50
© 1987 Elsevier Science Publishers B.V.
196 process is a two-stage process: incomplete combustion takes place in the gasitier chamber, then ignition and burning of the gases in the combustion chamber. The second is by direct or complete combustion of the fuel in the combustion chamber, then introducing the flue gases of combustion directly into the heat-related application. Vortex-type furnaces are classified under the direct combustion processes. The vortex principle creates two-vortex spirals: cold air travels downward along the inside wall of the combustion chamber and the flame and hot products of combustion travel upward in the inner spiral at the center of the combustion chamber, and it causes thorough mixing of the air with the volatile gases and the entrainment {entrapment) of unburned particulate matter in the flame spiral to increase the combustion rate and completeness of fuel combustion. In addition, the vortex sustains the temperature of the combustion chamber, creates a turbulence in the movement of fuel particles, and allows enough time for the complete burning of the fuel and larger particulate matter. These are the elements which control the combustion rate of solid fuels. Each furnace designed to burn residues is unique; its performance is independent of other types of furnaces. Each size and design of biomass furnace has a characteristic trend which is a function of volume, combustion chamber geometry, gas velocity, turbulence, and other factors. BIOMASS FURNACES Studies were conducted at Iowa State University on the use of direct combustion system for converting agricultural crop residue to thermal energy. Kajawski et al. (1977) tested an incinerator-type furnace, built by a senior design class, for burning cornstalks to provide heat for drying high-moisture grain. The results of the test indicated that crop residues are a reliable, low-cost source of energy for grain drying. They suggested additional quality testing of the grain to evaluate the acceptability of biomass dried corn in the market. Dairo (1979) developed and further tested the above cornstalk-fired furnace and found that the effect of feed rate on the exhaust temperature and efficiency of the furnace was significant. He suggested additional work to be done to improve the overall performance of the furnace. Claar et al. (1980) designed, developed and tested a concentric-vortex, agricultural residue furnace. The first furnace was designed to provide sufficient time, temperature, and turbulence of the combustion gases to permit the complete combustion of the fuel. The furnace was tested by burning a variety of agricultural crop residues. The development program consisted of developing a uniform and continuous fuel feeding system and an easy ash removal. Easy fabrication methods and minimal maintenance and operator attention were to be features of the second furnace prototype. Anderson et al. (1981) used a blend of ambient air and the exhaust gases
197 from the above furnace to dry high moisture corn, using corn cobs as fuel. The dried corn was chemically analyzed for harmful deposits and analysis of a 93g corn sample indicated no hazardous residues. Also, there were only minor corrosion and discoloration on the surface of the dryer blower from the products of combustion. Dahlberg (1977) reported on a direct-fired combustion chamber-type incinerator furnace that burned corn cobs for drying whole ear seed corn. This dryer was located in the Pioneer Hi-Bred seed corn plant, Belle Plain, IA. Several problems were found during the operation of the incinerator type furnace: slag formation on the grate, corrosion occurring on the metallic parts of the dryer, and particulate material being deposited on the corn located in the drying floor. Singh et al. (1980) developed and tested a cyclone-type husk-fired furnace for either heating the drying air or generating steam in a boiler for parboiling of paddy in the rice mill. The furnace was capable of generating a temperature in the order of 1000°C and a maximum furnace efficiency of 80% occurred at the husk feed rate of 20 kg/h and an air flow rate of 168 m3/h (100 cfm). They reported some problems with the disposal of ash in continuous operation of the furnace. The second type of biomass furnace requires the gasification of the fuel as a first step, then ignition of the gases from the gasifier in the second step. The products of combustion from the second step might have the same characteristics as those of direct combustion in terms of utilization. Payne et al. (1980), in their work on a gasifier combustion process for converting corn cobs into thermal energy, reported that the quality of the exhaust appeared sufficiently clean for grain drying application. Corn cobs up to 46% moisture content were successfully used and the thermal efficiency of the gassifter was 70-80%. COMBUSTIONPRINCIPLES FOR THE CONCENTRIC-VORTEX,CELL FURNACE Combustion is defined as the rapid chemical combination of oxygen with the combustible elements of the fuel. The objective of good combustion is the release of heat from the fuel while minimizing the losses from combustion imperfections and excess combustion air. The rate of combustion of a solid fuel is dependent upon: (1) temperature of the combustion chamber to sustain the ignition of the fuel and the chemical reactions; (2) turbulence in the movement of fuel particles relative to the molecules of oxygen; (3) ratio of the air and fuel; (4) chemical and physical properties of the fuel itself (Hurley, 1931 ). The combustion principles for the concentric-vortex, cell furnace (Fig. 1) are as follows: Secondary air enters the combustion chamber through four tuyeres located at the top of the upper furnace plenum. The air is preheated in its downward
198
~ SLAG
/--- G A S I F I C A T I O N
&
~
~lu=
FEED A U G E R
DEHYDRATION
Fig. 1. Diagram of the concentric-vortex, cell furnace.
vortex spiral by convective and radiative heat transfer from the inner flame spiral as the air approaches the fuel bed. Also, the downward vortex spiral partly mixes with the rising gases to proportion oxygen gradually until the air reaches the fuel bed. The downward vortex spiral prevents the slagging fly ash from impinging on the furnace shell. The fly ash in the downward spiral will be below its softening temperature of 790-815°C and will fall onto the grate for removal. Also, the unburned particles, which are centrifugally separated from the inner flame spiral into the downward vortex airstream, will be reignited at the top of the fuel bed. Further, the concentric-vortex action minimizes problems with clinkers, refractory erosion, and corrosive deposits by condensing the slagging fly ash when burning fuels with highly fusible ash. The upward flame spiral has variable volume and velocity. This flat, tangential flame increases the residence time for the entrained particles and combustion gases to burn and permits the particles to follow a longer path in the combustion zone. A longer particle retention time is provided by a balance between the radial forces, which are a function of the mass flow, and the centrifugal forces, which are a function of the tuyere air-inlet area.
199 Within the flame spiral, convective and radiative heat transfer takes place between the entrained particles and the combustion gases. Also, the base of the inner flame spiral tends to be oxygen starved, hence gasification of the fuel occurs. Additional secondary air enters into the combustion zone through four tuyeres located at the bottom of the upper furnace plenum. This unheated air is mixed with the combustion gases and entrained particles in the upward flame spiral to increase the burning rate. Hence, higher burning rates are obtained with a turbulent, tight flame spiral. In general, the degree of mixing the air with the volatile gases and the entrained particles and the burning rate are dependent upon the following: (1) air inlet volume; (2) air inlet velocity; (3) air penetration of the flame spiral; and (4) number and height of the air entry tuyeres. The basic physical law for the concentric-vortex principle is the conservation of angular momentum. The centrifugal force developed by the flame spiral tends to make the entrained particles move outward, but it is balanced by an inward drag force developed by the downward flow of secondary combustion air. Hurley (1931) and Perry et al. (1950) developed the mathematical relationships to describe the principle. As the biomass fuel is slowly pushed along over the grate of the furnace, combustion occurs in three consecutive, overlapping phases: (1) evaporation of the moisture in the fuel; (2) distillation of the volatile gases from the fuel and their combustion; and (3) combustion of the fixed carbon. The combustion process in the fuel bed is illustrated in Fig. 2. A model of the furnace could be used to calculate the mass and energy balances and to predict the flight times and mass reductions of the combustible particles entrained in the furnace exhaust gases as well as the influence of fuel and operating parameters on particulte emissions. For the development of the furnace, it is desirable to know the characteristics of combustion in the fuel bed. The type of fuel bed, as defined by Nicholls and Eilers (1933), is fixed by the absolute direction of the flow of the fuel and its flow relative to the combustion air. The type of fuel bed burning in the furnace follows the 'unrestricted-sidefeed' principle. 'Unrestricted-ignition-sidefeed' burning imposes no physical restrictions on the rate at which fresh fuel may be ignited with a fixed air supply rate. The fuel bed moves in a direction perpendicular to the movement of the overfire combustion air. Mayers (1937) developed the mathematical relations to calculate the temperature and combustion rates at the ignition in the fuel bed. With 'unrestricted-ignition-sidefeed' burning, the vortex airstream, which is preheated in the upper plenum, increases the rate of reaction in the fuel bed and, consequently, the burning rate for the same supply of combustion air.
200
OFF-BED GAS COblPOSIT ION (H20, TAR, CH4, ~2, CO, CO2, 02, & N2 )
GAS P}~SE EQUILIBRIUM
(CO2, CO, N2, 02)
MIXING DF
MIXING OF
GASIFICATION
WATER VAPOR
PRODUCTS ~ND
GASIFICATION
AGRIC. RESIDUE
ZONE
PYROLYSIS
(OFF-GASES)
PRODUCTS (H20)
(H20, Tar, CH 4, H 2, . . . .
c_o,__co_:)
_
ASSL~IED PYROLYSIS PRODUCT DISTRIBUTION OVERGRATE AIR
_
WET AGRIC. RESIDUE
DRY AGRIC. RESIDUE
CHAR
CHAR COMBUSTION ZONE
EVAPORATION OF LIQUID WATER
<
GASIFICATION & PYROLYSIS ZONE
DRYING ZONE
Fig. 2. Diagram of the combustion process in the concentric-vortex, cell furnace. DISCUSSION AND RESULTS-PERFORMANCE EVALUATION OF THE CONCENTRICVORTEX, CELL FURNACE
Experimental techniques The furnace is equipped with a side-feeding auger that automatically feeds the biomass fuel into the bottom of the combustion chamber. Biomass fed into the furnace by the side auger is pushed across the grate by the auger. The feed rate is controlled by a timer connected to the auger driving motor. Corncobs are used as the fuel to evaluate the furnace performance. The moisture content of the fuel varies between 12% and 26%. Figure 3 is a frequency chart of the size distribution of the corn cobs. The furnace is operated with no active supply of primary combustion air. A small, indeterminate amount of primary air, however, flows into the furnace through the feeding auger. The amount of secondary air is controlled by a sliding air damper located between the fan and furnace. The test procedure for the evaluation program is carried out as follows ( Wahby, 1982) : First, the corn cobs used for the tests are manually dumped into the hopper
201 FREQUEN~T ~50
140
120
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100
.xx. 80
,YY~ 60
Lx~, ,xxx
40
,xx~
20
>000
38
64
89 SIZE
114
140
165
(LENGTH). mm
Fig. 3. Frequency chart of corn cob size distribution.
of the auger before starting each run. About 18 kg of cobs are dropped directly into the combustion chamber of the furnace for starting the combustion process. Next, a pint of diesel fuel is poured over the pile of corn cobs in the combustion chamber, and the pile is ignited. The furnace fan and feeding auger are immediately started after the ignition of the diesel fuel and cobs. The furnace requires 50 min to reach the steady-state operating condition. Temperature readings are recorded 15 min after ignition. Ambient air temperature and stack temperature are recorded every 5 min during the test. The total run time of each test is 2.5 h. The performance of the furnace is experimentally evaluated by conducting tests of 13 combinations. Two independent test variables are parameters are chosen for this experiment: (1) corn cob feeding rate, which varies from 43.1 to 120.2 kg/h; and (2) air damper opening, which varies from 7.4 to 43.4 mm. Figure 4 shows a diagram of the experimental statistical, randomized design along with the values of the independent variables chosen for the test.
202
43.4
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I 43.1 54.4 (95.1) (120) Feed
rate
81.6 (180) (F),
kg/h
t 108.9 120.9 (240) (264.9)
(lb/h)
Fig. 4. Experimentaldesigndiagram.
Stack temperature The stack temperature is significantly affected by the feed rate of corn cobs and the air damper opening. The maximum stack temperature achieved is 871°C at a feed rate of 120.2 kg/h of corn cob, and an air damper opening of 25.4 mm, while the minimum stack temperature was 420°C at a feed rate of 43.1 kg/h and the same air damper opening. The statistical analysis of the results shows a partial correlation between the stack temperature and moisture content of the corn cobs. This effect seems true during the start-up period of the burn because the water in wet corn cobs would cool the fire. This made it difficult to maintain a fire inside the combustion chamber during that period. When the furnace reached the steady-state condition, the radiation from fire bricks and refractory materials inside the furnace back to the fuel pile helped maintain the combustion temperature regardless of the corn cob moisture content. At the steady-state condition, the moisture content of corn cobs has little or no effect on the stack temperature. The best fitted model for predicting the stack temperature is a quadratic model, through which a contour plot for the stack temperature was drawn as Fig. 5. Figure 5 shows the stack temperature as it is affected by the corn cob feeding rate and air damper opening (the two independent variables). It is possible to read different stack temperatures at a constant feed rate by changing the air damper opening, or by increasing the feeding rate of corn cobs at
203
422
35.2
A 1 R {3 2 8 . 2 A PI P E R 0 E N 21.2
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68
77
86
CORN COB5 FEED R A ] E , LEGENO~
re2
• -----
325.503 579.316 833.]29
- .......
95
104
113
122
kg/h 410.108 563.921
~ -----
494.712 748.525
Fig. 5. Contour surface of the temperature, T 2 ( o C ) .
constant air damper opening. The stack temperature increases as rate of feeding of corn cobs increased. Figure 5 shows that each feed rate has a maximum stack temperature; this temperature will be reached at a certain air damper opening, and the amount of combustion air will make more fuel to be completely burned until the stack temperature reaches its maximum value. At this point, any increase in the air damper opening will cause a decreasing in the stack temperature due to the increase of the amount of combustion air; i.e., there will be an excess in the amount of combustion air. This suggests that a line which passes through the maximum stack temperature would separate the contour plot into two regions. The first region, below this line, would be during the incomplete combustion of the fuel where insufficient oxygen is supplied to the furnace, while the second region, above the line, would be during the complete combustion of the fuel. But as the amount of excess combustion air increases, the stack temperature would begin decreasing below that maximum temperature.
204
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-----329. 537
9'5
,kg/h
40.981 . . . . . . . 214. 114
I04
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271.
----e.--- 98.692 .... 826
Fig. 6. Contour surface of the percentage excess combustion air. The predictive model for stack temperature is as follows: 7'2 = Bo + BI F + B2 DAo + B3F DAO + B4F 2 + B 5 DAO 2 = -- 337.9 + 1 0 . 4 F + 30.2 DAO - - 0.114F DAO - - 0.016F 2 _ 0.42
DAO
2
where DAO is the air damping opening (ram), and F fuel feed rate ( kg/h ). The prediction of the stack temperature, when using the response surface method, is controlled by the average standard error (ASE). According to Box et al. (1978), If the A S E value is close to or less than 3.5, interpretations of the response surface should not be made. In the case of stack temperature, the value of A S E is calculated to be 15.8, which means that it is worthwhile to interpret the response surface. Excess air The amount of excess combustion air varied from a minimum of 34.6% to a maximum of 416%, as calculated from the equation:
205
excess air (XSA) ( % ) = (air supplied - air required) ><100 air required Obviously, the amount of excess combustion air is highly correlated to the amount of combustion air supplied to the furnace. Excess combustion air also affects the efficiency of the furnace. Figure 6 shows the contour surface of the TABLE 1 Corn cob data Test
F (kg/h)
DAO (mm)
T12 (°C)
T2 (°C)
PM (g/h)
A (kg/h)
XSA (%)
EFF (%)
MC (%)
1 2 3 4 5 6 7 8 9 10 11 12 13
81.65 108.86 81.65 81.65 120.16 108.86 81.65 81.65 54.43 43.14 81.65 54.43 81.65
25.40 38.10 25.40 25.40 25.40 12.70 7.37 43.43 38.10 25.40 25.40 12.70 25.40
22.2 25.0 21.1 26.6 27.7 27.7 17.2 25.5 23,8 24.4 20.5 26.1 22.2
629.4 619.4 697.7 688.3 871.1 746.1 515.5 551.6 488.8 420.0 650.5 457.7 653.3
126 88 66 27 86 80 48 104 43 35 60 56 61
1811.99 1446.10 1276.32 1064.41 869.79 1119.76 1087.77 1455.72 1144.07 1197.19 1267.20 1119.20 1480.84
312.70 147.02 190.70 142.43 34.61 91.27 147.76 231.56 290.86 416.10 188.63 282.36 237.29
84.5 52.0 67.3 57.5 34.0 50.6 41.4 60.0 60.2 66.7 66.2 55.0 72.0
23.61 25.36 13.51 12.08 13.10 26.19 19.86 13.52 22.41 21.57 19.62 19.62 20.38
T12, T2, stock temperatures; PM, particulate m a t t e r rate; A, combustion air flow rate; MC, moisture content. TABLE 2 Inlet air condition Test
T12 ( °C )
RH (%)
o) ratio
PE ( m m Hg)
MCWB (%)
1 2 3 4 5 6 7 8 9 10 11 12 13
22.2 25.0 21.1 26.7 27.8 27.8 17.2 25.6 23.9 24.4 20.6 26.1 22.2
68 55 63 58 86 89 74 66 75 90 51 87 60
0.0114 0.0111 0.0100 0.0129 0.0206 0.0211 0.0091 0.0137 0.0140 0.0174 0.0077 0.0186 0.0101
771 768 763 763 762 762 769 764 767 759 765 761 766
23.61 25.36 13.51 12.08 13.10 26.19 19.86 13.52 22.41 21.57 17.34 19.62 20.38
~o, h u m i d i t y ratio; PB, barometric pressure; MCWB, moisture c o n t e n t (wet basis).
206 TABLE 3 Orsat analysis data Test
COx (% )
02 (%)
CO (% )
N2 (% )
o~ fl y (kg mole per 100 kg df)
1 2 3 4 5 6 7 8 9 10 11 12 13
4.875 8.000 7.000 8.167 11.670 10.167 8.125 6.000 5.167 3.812 7.000 5.250 6.000
17.000 15.333 14.000 14.500 11.170 10.167 13.250 16.830 16.333 18.188 14.625 16.250 15.500
0.005 0.000 0.005 0.050 0.000 0.500 0.050 0.000 0.010 0.080 0.000 0.040 0.000
78.120 76.667 78.995 77.283 77.160 79.166 78.575 77.170 78.490 77.920 78.375 78.460 78.460
16.071 9.619 11.320 9.440 5.242 7.448 9.647 12.911 15.220 20.097 11.239 14.889 13.134
77.408 47.219 53.926 45.972 32.369 35.413 46.208 62.958 72.967 97.046 53.964 71.408 62.958
a , a m o u n t of O~ supplied; fl, a m o u n t of dry flue gases; ~, a m o u n t of water vapor in flue gas.
TABLE 4 Exhaust gas analysis (No. o f k g moles per 100 kg df) Test
Nco,
No~
Nco
NN~
NN~
NH~O
Nso2
1 2 3 4 5 6 7 8 9 10 11 12 13
3.774 3.778 3.775 3.755 3.778 3.600 3.754 3.778 3.770 3.700 3.778 3.749 3.778
13.159 7.240 7.550 6.666 3.616 3.600 6.123 10.596 11.918 17.638 7.892 11.604 9.759
0.00387 0.00000 0.00270 0.02299 0.00000 0.17710 0.02310 0.00000 0.00730 0.07760 0.00000 0.02860 0.00000
60.471 36.201 42.599 35.529 24.976 28.035 36.308 48.585 57.272 75.618 42.294 56.027 49.422
0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015
4.334 3.645 3.794 3.856 3.752 4.133 3.602 4.284 4.560 5.687 3.590 5.047 3.947
0.00563 0.00563 0.00563 0.00563 0.00563 0.00563 0.00563 0.00563 0.00563 0.00563 0.00563 0.00563 0.00563
207 TABLE5 Exhaustconditiondata a Test
DAR
DAS
XSA
T2 (°C)
Tmax (°C)
HpT H (kJ/kg)
HpACT (kJ/kg)
1 2 3 4 5 6 7 8 9 10 11 12 13
18.543 18.543 18.543 18.543 18.543 18.543 18.543 18.543 18.543 18.543 18.543 18.543 18.543
76.527 45.805 53.903 44.954 24.961 35.468 45.940 61.481 72.477 95.700 53.518 70.901 62.541
312.70 147.02 190.70 142.43 34.61 91.27 147.76 231.56 290.86 416.10 188.63 282.36 237.29
629.4 619.4 697.8 688.3 871.1 746.1 515.6 551.7 488.9 420.0 650.6 457.8 653.3
732.2 1110.0 993.3 1126.7 1471.1 1365.6 1132.2 871.1 771.1 604.4 993.3 782.2 876.7
18512 18709 18609 18665 18666 18641 18729 18633 18641 18704 18551 18710 18645
15637 9723 12525 10731 6350 9428 7755 11183 11217 12482 11546 10297 13419
aDAR,required dry air (kg mole dry air per 100 kg df) ; DAS supplied dry air (kg mole dry air per 100 kg df); T ..... m a x i m u m t e m p e r a t u r e ( ° C ) ; HpTH,theoretical enthalpy of products; HpACT, actual enthalpy of products.
excess combustion air as it is affected by the feed rate of corn cobs and air damper opening. The mount of excess combustion air decreases as the corn cob feed rate is increased and the air damper opening was decreased. The interpretation of excess air from the contour surface (Fig. 6) could safely be made, since the value of the ASE is 9.05. Also, the statistical analysis results showed a significant effect of the fitted model for the prediction of excess combustion air, with the feeding rate considered to have a highly significant effect on the determination of the excess combustion air.
Efficiency The calculation of the furnace efficiency is based on the ratio of the enthalpy of the products of combustion of corn cobs at measured temperature to the enthalpy of the products at calculated maximum temperature. An example of the calculation of the furnace efficiency is found in the Appendix and Tables 1 to 5. The maximum temperature would take place at adiabatic condition, i.e., no heat loss and complete combustion. The actual temperature will drop below the maximum temperature because of heat losses or excess amount of air. Figure 7 shows the contour plot of the response surface of the fitted model of efficiency. This figure shows that efficiency increased with the increase of air damper opening of by decreasing the feed rate. This indicates that the efficiency would increase as the amount of combustion air increased, where a po-
208 43,2
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95
CORN COBS FEED ff.lqTE , k£/h -18.574 - 26.854
x
43.414
----- 68 255
....... 51.694
104
113
122
---o-- 95.134 . . . . 59974
Fig. 7. Contour surface of the furnace efficiency ( % ).
sitive correlation is found between the efficiency and combustion air. It is also found that the efficiency had a positive correlation with the amount of excess air. The prediction of the efficiency from the response surface contour can be made since the A S E value is 4.49.
Example ofperformanceprediction An example of the furnace performance prediction is given to demonstrate the usage of the contour plots. Three dependent parameters were used in the prediction example since they were the only parameters which the statistical analysis showed were significant. These are: stack temperature (T2), furnace efficiency (EFF), and percent excess combustion air (XSA). Let us assume that a heat process application required a stack temperature in the range of 580-640 °C A (1076-1184 °F). Further, by placing the contour
209
CORN COBS FEED RATE , k g / h --Tz,=C
--
--XSA,%
....
EFF, %
Fig. 8. Performance prediction contour surfaces of the furnace (combined plot). plots of T2, EFF, and XSA on top of each other, a combined plot as in Fig. 8 would be created which would predict the magnitude of other variables. T h r e e points are chosen in Fig. 8 to d e m o n s t r a t e the prediction of other parameters; other points could be selected from the plot. T h e probable prediction of the furnace feeding rate, air dam pe r opeing, efficiency, and excess air would be as follows:
Corn cob feed rate (kg/h) Air damper opening (mm) Furnace efficiency ( %) Excess air (%)
Point 1
Point 2
Point 3
70.0 26.2 70.0 214-272
80.0 15.2 60.0 156-214
90.0 38.2 60.0 156-214
T h e above prediction values were evaluated on the basis of the a m o u n t of corn cobs used. Since point 2 has the same efficiency and excess air as in point
210
3, there is no reason to choose point 3 because the same results would be achieved with fewer corn cobs at point 2. Also, point 1 has the advantage of higher furnace effiency over point 2. Furthermore, the evaluation of choosing the operating point could depend on the operation condition of the furnace when operating with a lesser or a greater amount of excess combustion air. If the process required a large amount of excess air, point 1 would be better suited than point 2, but with the benefit of high furnace efficiency and with the benefit of lower quantity of corn cobs than point 2. Other alternatives could be found and evaluated by using Fig. 8. A practical prediction of the furnace performance can be made by using the line of maximum stack temperature shown in Fig. 8. Let us assume that a heat application process required a stack temperature of 748 ° C. Locate this temperature on the line of maximum temperature (Fig. 8). Draw a vertical line to determine the required feed rate of corn cobs, and a horizontal line to determine the air damper opening setting. The following furnace performance parameters were read from Fig. 8: (1) corn cobs feed rate: 99.5 kg/h; (2) air damper opening; 22 mm; (3) furnace effiency: more than 51.7%; (4) percent excess air: less than 156%. SUMMARY
A concentric-vortex, cell furnace is developed to convert agricultural biomass into thermal energy. The design of the furnace is a function of the volume, combustion chamber geometry, gas velocity, combustion air turbulence, and other factors. The performance of the furnace is determined by a series of 13 tests to convert corn cobs into thermal energy, under uncontrolled air inlet temperature conditions. Two independent variables are studied: (1) the corn cob feed rate and the air damper opening. The three dependent variables affected by the feed rate and the air damper opening are: (1) stack temperature; (2) furnace efficiency; and (3) percentage of excess combustion air. Stack temperatures up to 871°C are achieved with a feed rate of 120.1 kg/h and an air damper opening of 25.4 mm. The excess combustion air ranges from 34.6 to 416%. The combustion efficiency is calculated by using the enthalpy of the products of combustion at measured temperature to the enthalpy of products of combustion at maximum adiabatic temperature. The efficiency of the furnace varies from 34 to 85%.
REFERENCES Anderson, M.W., Claar, P.W., II and Bern, C.J., 1981. Corn drying evaluation utilizing a concentric-vortex biomass furnace system. ASAE Pap. 81-3015, American Society of Agricultural Engineers, St. Joseph, MI, 16 pp.
211 Box, G.E.P., Hunter W.G. and Hunter, J.S., 1978. Statistics for Experimenters. Wiley, New York, 653 pp. CAST, 1984. Energy uses and production in agriculture. Rep. 99, Council for Agriculture Science and Technology, Ames, IA, 145 pp. Claar, P.W., II, Buchele, W.F. and Marley, S.J., 1980. Development of a concentric-vortex, agricultural-residue furnace. In: ASAE Agricultural Energy-Biomass Energy, Crop Production, Vol. 2. American Society of Agricultural Engineers, St. Joseph, MI, pp. 349-356. Dahlberg, R.W., 1977. Corn cobs - - an energy source for drying seed. Paper 32nd Corn and Sorghum Research Conference, American Seed Trade Association, 7 December 1977. Dairo, J.A., 1979. Development of a cornstalk-fired furnace. M.S. Thesis, Iowa State University, Ames, IA, 86 pp. Hurley, T.F., 1931. Some factors affecting the design of a small combustion chamber for pulverized fuel. Inst. Fuel London, 4 (4) : 243-250. Kajewski, A.H., Marley, S.J. and Buchele, W.T., 1977. Drying corn with a crop residue fired furnace. ASAE Pap. 77-3525, American Society of Agricultural Engineers, St. Joseph, MI, 9 pp. Mayers, M.A., 1937. Temperature and combustion rates in fuel beds. Trans. ASME, 59: 279-288. Nelson, L.F., Stickler, F.C. and Burrows, W.C., 1975. Recognizing energy-efficient agriculture in the complex U.S. food system. ASAE Pap. 75-7505, American Society of Agricultural Engineers, St. Joseph, MI, 19 pp. Nichols, P. and Eilers, M.G., 1933. The principles of underfeed combustion and the effect of preheated air on overfeed and underfeed fuel beds. Trans. ASME 56: 321-335. Payne, F.A., Ross, I.J. and Walker, J.N., 1980. Gasification-combustion of corncobs and analysis of exhaust. ASAE Pap. 80-3025, American Society of Agricultural Engineers, St. Joseph, MI, 22 pp. Perry, H., Corey, R.C. and Elliot, M.A., '1950. Continuous gasification of pulverized coal with oxygen and steam by the vortex principle. Trans. ASME, 72 (9): 599-610. Singh, Rajvir, Maheshwari, R.C. and Ojha, T.P., 1980. Development of a husk fired furnace. J. Agric. Eng. Res., 25: 109-120. Wahby, M.F., 1982. Combustion analysis of vortex biomass furnace. Ph.D. Diss., Iowa State University, Ames. IA, 159 pp. APPENDIX
Thermal efficiency calculation T h e t h e r m a l efficiency of the f u r n a c e was calculated b a s e d on the a s s u m p tion t h a t the c o m b u s t i o n of c o r n cobs is complete. T h e low level of c a r b o n m o n o x i d e in the e x h a u s t s u p p o r t s this a s s u m p t i o n . T h e efficiency of the furnace is b a s e d u p o n the ratio of the e n t h a l p y of the p r o d u c t s of c o m b u s t i o n at actual m e a s u r e d t e m p e r a t u r e to t h a t value at m a x i m u m ( a d i a b a t i c ) temperature. T h e e n t h a l p y of the p r o d u c t s of c o m b u s t i o n of corn cobs at the m e a s u r e d t e m p e r a t u r e is the difference b e t w e e n the e n t h a l p i e s at the m e a s u r e d t e m p e r a t u r e a n d at the reference t e m p e r a t u r e of 25 ° C: h 2 - h o . T h e e n t h a l p y of the p r o d u c t s of c o m b u s t i o n of c o r n cobs at the m a x i m u m t e m p e r a t u r e is the difference b e t w e e n the e n t h a l p i e s of the p r o d u c t s at maxim u m t e m p e r a t u r e a n d at t h e reference t e m p e r a t u r e : h2max- ho.
212 The furnace efficiency was calculated by the following equation: EFF ( % ) --
enthalpy of products at actual temperature × 100 enthalpy of products at maximum temperature AHp ( actual temp ) × i00 AHp ( maximum temp )
Example o[ calculation Test3. At 25°C: hco~ = 9 271.89 kJ per kg mole COe ho2 = 8 688.63 kJ per kg mole 02 hco = 8 676.49 kJ per kg mole CO hN2 = 8 675.56 kJ per kg mole N2 hH~o = 9 910.02 kJ per kg mole H 2 0 hso, = 10 555.76 kJ per kg mole SOe Fuel:
Cp
=2.09 k J / ( k g °C)
- H c = 18 664.24 kJ per kg fuel dry For test 3: T12= 21°C~-294 K T2 = 7 1 5 ° C ~ 9 7 1 K Calculation of maximum temperature: Hp = H r or Z ni( hi2 - hio) = ni( hil -- h i o ) - H c Hp = 3.7748 ( hco~ - 9371.69 ) + 7.5496 ( ho2 - 8688.63 ) + 0.0027 (hco - 8676.49) + 42.6138 ( hN~ -- 8675.56) + 3.7935 (hH~o -- 9910.02) + 0.00563 ( hso~ -- 10555.76) kJ per 100 kg
(a)
213
H2 = 1 8 664.24(100) + [11.320 ( 8 7 5 4 . 3 - 8 6 8 8 . 6 ) + 42.5858 ( 8 5 6 2 . 2 - 8675.56) + 0.8685 (978.1 - 9910.02) ]
(b)
= 1 866 3 8 2 . 4 - 7 045.5 -- 1 859 336.9 kJ per 100 kg From (a) and (b) : 23 676 891 = 3.7748hco2 + 7.5496ho2 +0.0027hco + 42.6138hN2 + 3.7935hH2o + 0.00563hso2 ni
hi
Tm~x(K) 1389 C02 02 CO N2 H20 S02 Total n,h~=
3.7748 7.5496 0.0027 42.6138 3.7935 0.00563
64 45 43 36 52 66 2 628
1250 619.9 236.7 619.9 244.0 885.6 666.6 410.0
56 40 38 38 46 58 2 334
1267 667.9 231.2 810.2 469.9 613.5 574.0 252.9
57 40 39 39 47 59 2 369
614.8 828.7 383.6 036.1 354.3 545.1 281.5
Tmax =1267 K m ~ 0 1 1 ° C Calculation of the enthalpy of products: At Tz = 971 K: Hp = n i ( h971 - h 2 9 s )
= 3 7 748(41 2 0 1 . 0 - 9 371.69) +7.5496(30 3 8 0 . 8 6 - 8 688.63) +0.0027(29 4 0 1 . 9 0 - 8 676.49) +42.6138(29 1 9 0 . 3 7 - 8 675.56) +3.7935(34 7 1 7 . 1 8 - 9 910.02) +0.00563(43 4 1 1 . 1 1 - 1 0 555.76) Hp -- 1 252 474 kJ per 100 kg fuel By the same procedure, the enthalpy of the products at maximum temperature was calculated to be: Hpmax =
1 860 937 kJ per 100 kg fuel
The efficiency was calculated as: EFF =
1 252 474 X 100 = 67.3% 1 860 937