COMBUSTION GENERATED PARTICULATE EMISSIONS

Transcription

1 COMBUSTO GEERATED PARTCULATE EMSSOS KEET L. TUTTLE.s. aval Academy Annapolis, Maryland ABSTRACT This paper addresses the generation of particlate emissions dring combstion of wood waste fels in spreader-stoker boilers. The prpose is to bring the reslts of research in wood wastes to the attention of sers of all solid waste fels. The scope is limited to reporting reslts of research condcted on the Oregon State University Pilot spreader-stoker test facility sponsored by the.s. Department of Energy. This paper reports the experiments the athor has been involved in; it reviews the reslts and conclsions that have been reported; states applications of reslts to commercial wood fired boilers that have been made; finally, this paper reports the analysis of steady-state data which was beyond the scope of the original experiment. The applicability of this research on wood wastes to solid wastes is discssed in general. OMECLATURE g/m 3 (SO 1% CO) = standardized particlate (grains/sdcf) mass emisson rate; Mass emitted per standard, dry nit volme corrected to 1% CO kglh. m (lb/hr fe) = combstion rate is the fel mass flow per nit area of grate mass mean diameter = fel particle size, average size of second largest dimension of fel particles, measred in mm (in.) UF A/F = ratio of nderfire air mass flow rate to fel mass flow rate, does not inclde overfire air flow rate SOLD WASTE COMBUSTO Combstion of solid wastes will probably contine to grow in poplarity as a means of disposal and as a sorce of energy. Brning solid fels on grates is only one way, althogh an important way, to brn them. Boilers that have grates, sch as spreader-stoker boilers, are in widespread se and are an effective way to brn many different solid fels. One of the more difficlt fels to brn is solid wastes. Althogh mnicipal solid wastes are different in many ways from wood wastes, they may have enogh characteristics in common with wood wastes to allow research on wood wastes to be applicable. The validity of the research reslts to be presented has been dem-

2 onstrated only for wood wastes in spreader-stoker boilers. This paper addresses the generation of particlate emissions dring combstion rather than the collection of those particles. With wood wastes, improving combstion has been proven to be as effective or more effective than improving collection efficiency [3]. ntition may sggest that increasing fel particle size is the wrong way to go. That wold be correct for sspension brners. t was once commonly accepted that wood-fired spreader-stoker boilers shold be designed to bm a sbstantial fraction of the fel particles in sspension. Recent research has shown why that is not tre [3]. Grate fired boilers, as in Fig. 1, brnot wood fel particles better if combstion occrs on the grate [3]. ncreasing fel particle size is important in spreader-stoker boilers. Evidence of that will be presented in Fig.. The emissions from smaller fel particles are consistently higher. Some wood-fired spreader-stoker boilers spply all of the combstion air throgh the grate. owever, the need for overfire air to redce particlate emissions and improve combstion efficiency has been clearly demonstrated [1-4J. As more air is provided overfire, less air need be provided nderfire, throgh the grate. With less air passing throgh the grate and throgh the fel bed, the fel particles are less likely to be blown off the grate, entrained by the gases and carried ot of the combstion chamber [3]. The effect that redcing nderfire air has on particlate emissions can be seen in Fig.. The emissions when higher nderfire air ratios are sed are consistently higher. Combstion rate, the rate at which fel is brned per nit area of grate, is almost niversally accepted as having an effect on particlate emissions. This is becase combstion rate is the one variable that is normally varied throgh a wide range by boiler operators. An increase in emissions at higher combstion rates has been observed by many. This effect has been reported consistently [1-4], and Fig. consistently shows an increase in emissions as combstion rate increases in all for experiments. RESEARC EFFORTS The research to be presented was initiated in 1975 by Jnge [1]. Fnding was provided by the U. S. Department of Energy to develop a pilot plant test facility at Oregon State University. A small spreader-stoker boiler was sed to test varios waste wood fels. A schematic of the facility is presented in Fig. 1. Experiments were condcted at the facility ntil 198. n addition to Jnge's work, early reslts were presented in a doctoral dissertation by Tttle [3]. That research reslted in Patent o. 4,41,67 being awarded for a method of brning wood to control particlate emissions. The early reslts were reported in the Jornal of the Air Polltion Control Association [4]. The process was later sefl in a commercial application of biomass gasification [5]. The reslts of this research have been applied (Tttle) to commercial boilers to pass emissions compliance tests. Three hypotheses were tested dring the doctoral research and their effects shown to be statistically significant [3]. (a) The original hypothesis was that the velocity of the air passing throgh the fel bed wold have a sbstantial effect on particlate emissions. (b) t was also hypothesized that redcing amont of nderfire air proportional to fel flow rate wold develop a deeper fel bed. (c) A third hypothesis was that a deeper fel bed wold tend to trap easily entrained particles. n 1978 more comprehensive research was condcted in the Oregon State University test facility []. The main objective was to determine whether load swings increased emissions. The effects of slgs of wet fel and of changing fel particle size was also of interest. ine independent variables and ten dependent variables were tested. Three large fractional factorial experiments were designed sing compterized statistical programs. The largest experiment, designed to test load swings is shown in detail in Tables 1 and. Only the nsteady-state data points are shown. Data taken at the steady-state condition on either side of each nsteady-state test rn are still being analyzed. Some of the steady-state data were analyzed by Kester [6]. TEST DESG The tests were planned in two phases: Phase -to determine the levels of factors sed in Phase ; and Phase -to answer the stated test objectives. Phase was sed to provide information on the facility operation, to establish baseline data, and to determine the levels of factors to be sed in Phase. Dring the test, the project team became familiar with the boiler's operation and test eqipment. The data logger and data redction program ere operated and verified. Phase experiments tested for measrable effects of step changes in fel size, moistre, and feed rate on completeness of combstion and stack opacity.

3 r (1 16 o Description of Components (1) Screw Conveyor With Fel Feed opper () Vibrating Fel Conveyor (3) Fel Spreader Roll (4) Water Wall Lined, Spreader Stoker Combstion Chamber (lxlx 3 m) (5) igh-temperatre Cmbstion Gas Dct (6) nlet Air Dct - Under fire Air (7) nlet Air Dct - Over fire Air (8) Air Preheater - Triple Pass, Plate Type (9) Stack Gas Dct (1) Stack Gas Sample Port (11) Cyclone Separator (Dia=m) (1) Stack Gas to Atmosphere (13) Forced Draft Fan (14) Mass Emission Monitor (15) Particlate Metering (16) Lear-Siegler Transmissometer w Wood Fel Ellters Uere, _t FG. 1 SCEMATC LLUSTRATO OF TE MODFED OREGO STATE UVERSTY WOOD COMBUSTO TEST FACLTY (Ref. [])

4 . 6, Z i! Q. \.es, t. 1- C UJ \, 1: C c \. 'M.,." c 3,. M S.,." E-< <; bo t-l ;:l U 1 E-< E-< E-< Z z ;:l <; ;:l p air ::r: derfire lower n 1 f 1 particles, t-l e larger z,-. kg/h.m lb/hr ft COMBUSTO RATE (mass flow rate of fel per nit area of grate) FG. EFFECTS OF COMBUSTO RATE, UDERFRE AR, AD FUEL PARTCLE SZE O PARTCULATE EMSSOS GEERATED BY COMBUSTO OF WOOD WASTES (Crves by Linear Regression, Least Sqares Fit) A statistically designed experiment based on the variables to be tested was sed to gather a broad range of data on the experimental boiler. Since this was the first pilot boiler test of its kind, the test provided the preliminary experimental reslts on which more indepth experiments can be based. n order to cover the range of variables initially of interest, experiments were statistically designed. The tests assred 99% confidence that any effects of a magnitde as large as one standard deviation of the experimental error will be detected. t is possible to detect smaller effects with less certainty. n addition, the compter analysis will spply interpolated data within the range of experiments. The experiments as designed represent the most efficient method of acqiring the information soght. Key experiments cold be sed to acqire the similar information over a mch longer period of time and at increased expense. One experiment was statistically designed to analyze the effects of step changes in controllable fel parameters. The experiment reqired 3 trials. The largest experiment measred the effects of step changes in combstion rate. The experiment took 5 trials as shown in Table 1. The factors and levels of f actors are explained in Table. EXPERMET RESULTS The data presented are the data taken at steady-state conditions to spport the nsteady-state data analyses in 1978 []. Althogh sbstantial valable steady-state data were acqired, its analysis was beyond the scope of the original project. Althogh the original experiment showed that load swings, rapid changes in moistre and rapid changes in fel size did not affect average particlate emissions in the manally controlled pilot plant, the steady-state data were no analysed and reported. t shold be noted that the effects of fel particlate size, nder air/fel ratio and combstion rate were dramatic enogh to be noticed dring the data collection. 4

5 TABLE 1 STEP-CAGE COMBUSTO RATE EXPERMET, A STATSTCALLY DESGED X 3 X X 3 X X 3 X 3 FRACTOAL FACTORAL EXPERMET (Ref. []),CTOR Level 1 Level Level J Factor Factor Filctor ) 1 J (,) 1. eo.. (,) >. eo.. eo... (,) > Factor F Factor F Factor F Factor F Factor F Factor F Factor F Factor F Factor' l' ) ; ) J 8 Z ) t )7. - eo. 1.. U eo. ) 45 5).. (,) >.. eo. eo. (,) ) 19 ) <C.. eo. ) 8 9. (,) > eo. 57 U. eo., 66. " (,) > 1 4) eo. 1. eo. ) <.: > eo. 17 n 74 : mbered spaces in matrix signify the actal nsteady-state test rn. Steady-state test rns are not shown. Replicated test points contain two rn nmbers. Factors and levels of factors are defined in Table. The three variables which significantly affect particlate emissions are combstion rate, fel particle size, and the amont of nderfire air. The combstion rate is shown along the abscissa in Fig.. The fel particle size and nderfire air effects are shown by the crves from least sqares fit throgh the represented data points. Fel particle size can be represented by the mass mean diameters, which means that half the fel by mass is smaller than the diameter stated. Fel particle size can also be represented by the size of the screen openings throgh which the fel will or will not pass. For example, the coarse fel, mm ( - X + % in.) passed throgh the larger screen opening and stayed on the smaller screen. The mass mean diameter of the larger sized fel particles was mm (.8 in.); the finer fel was 1.5 mm (.6 in). The finer fel is poplarly called 8 mm mins (% in. mins), which means that all of the small sized fel wold pass throgh an 8 mm (% in.) sqare hole. Underfire air is represented as the ratio of the mass flow rate of the nderfire air to the mass flow rate of the fel. With respect to its effect on particlate emissions, the nderfire air is most precisely represented 5

6 TABLE DEFTO OF FACTORS AD LEVELS OF FACTORS TE STEP CAGE COMBUSTO RATE EXPERMET (Ref. []) Factor A. Leve 1 1. Level. Factor B. Leve 1 1. Level. Level 3. Factor C. Levell. Level. Factor D. Levell. Level. Level 3. Factor E. Direction of Combstion Rate Change ncrease in Rate Decrease in Rate Size of Combstion Rate Step Change Step change of 4.5 kilograms (1 lb) of dry fel per hor, eqals 4.9 kilograms of fel per hor per sqare meter of grate (1 lb/hr ft ) Step change of.7 kilograms (5 lb) of dry fel per hor, eqals 4.4 kilograms of fel per hor per sqare meter of grate (5 lb/hr ft ) Step change of 45 kilograms (1 lb) of dry fel per hor, eqals 4.4 kilograms of fel per hor per sqare meter (1 lb.hr ft ) Moistre Content of Fel 4 percent by weight, 66.7 percent by dry basis 5 percent by weight, 1 percent by dry basis Combstion Rate 7 kilograms of dry fel per hor, eqals 44 kilograms per hor per sqare meter of grate, eqals 4.7 GJ/h. m, eq9ls.95 to 1.5 GJ/h. m 3 (6 lb/hr ft,.41 million Bt/hr ft, to 3 thosand Bt/hr ft3 ) 73 kilograms of dry fel per hor, eqals 94 kilograms per hor per sqare meter of grate, eqals 6.6 GJ/h. m, eq9ls.95 to 1.5 GJ/h. m 3 (6 lb/hr ft,.49 million Bt/hr ft, 5 to 4 thosand Bt/hr ft 3 ) 318 kilograms of dry fel per hor, eqals 34 kilograms per hor per sqare meter of grate, eqals 6.5 GJ/h. m eqals 1.1 to 1.7 GJ/h. m 3 (7 lb/hr ft,.57 million Bt/hr ftl, 3 to 45 thosand Bt/hr ft3) Size Distribtion of Fel Levell. Fines, Eight millimeter mins, -8 mm Doglas fir bark (3/8 inch mins) Level. Coarse, Thirty-two millimeter mins, eight millimeter pls, -3+8 rrm Doglas fir bark (11/4 inch mins, 3/8 inch pls) Factor F. Level 1. Level. Level 3. Factor G. Level 1. Level. Level 3. Underfire Air/Fel Ratio For mass nits of Underfire Air per nit of dry fel Six mass nits of Underfire Air per nit of dry fel Eight mass nits of Underfire Air per nit of dry fel Excess Air Percent 3 percent in excess of chemically correct amont for complete combstion 55 percent in excess of chemically correct amont for complete combstion 8 percent in excess of chemically correct amont for complete CQ/l1bstion 6

7 in this ratio [-5]. Another way to express air is as the overfire air/nderfire air ratio [1]. The effect that increasing the combstion rate has on particlate emissions can be seen on every crve in Fig, as well as in the data points taken as a whole. n these data the effect of nderfire air is comparable to that of comblltion rate. Figre shows that dobling the nderfire/air to fel ratio has abot the same tendency to increase emissions as does dobling the combstion rate. The slopes of the crves imply a possible interaction between nderfire air and combstion rate. The effect of nderfire air on emissions clearly exists in Fig. for both fine fel and coarse as well as across the entire range of combstion rates. The emissions from finer fel are consistently higher in Fig. than the emissions from the coarser fel. The size of the fel particles shows a mch greater effect on the generation of particlate emissions than either nderfire air or combstion rate at the levels these variables were tested. At nderfire air/fel ratios of less than 4: 1, particlate emissions have been fond to be even more strongly affected [3, 4]. Three other independent variables exist in these data. They are moistre content, excess air, and load swings. They accont for some of the scatter in the data. n addition, experimental error is a large contribtor to the scatter. Sbstantial experimental error may have been introdced by combining the steady state data with the nsteady state data. Statistical significance of the effects of flctating fel conditions is based mainly on mltiple regression analysis and the nll hypothesis sing the Stdent's T test []. Two way analysis of variance was sed for statistical significance of main effects and interactions for combstion rate and nderfire air in earlier tests [3, 4]. t shold be noted that fel moistre content indirectly affects particlate emissions. To generate eqal steam flow rates in a boiler, wetter fel mst be brned at a higher combstion rate to release the same net energy as drier fel. Brning the wetter fel at a higher combstion rate wold reslt in greater emissions. The effect of combstion rate on particlate emissions has long been recognized by boiler operators. These research efforts merely qantified that effect [1-4]. There are several reasons why application of these research findings to other solid wastes can be expected to improve efficiency and redce particlate emissions. Most importantly, spreader-stoker boiler operation is one of the main reslts of the research. Any solid fel brned in a spreader-stoker is expected to respond to the combstion rate variable and the need for overfire air. Althogh the actal ratios of air to fel are fel dependent, the effect of high air flow rates throgh the fel bed and the trapping effect [3] of deeper fel beds are expected to affect all fel beds the same way. Those solid wastes which are celllose are similar chemically to wood wastes. Physically solid waste fel particles will differ in size and shape from wood fel particles. owever, the more easily a fel becomes entrained in the combstion gases the more important it is to keep the fel particles on the grate. Both larger fel particle size and lower nderfire air/fel ratios help to keep fel particles on the grate. Fel particle size and nderfire air effects may be important only if a solid fel prodces solid carbon dring pyrolysis. Fels which prodce little or no charcoal may bm well in sspension regardless of size. owever, carbon particles which are prodced do not have time to bm ot in sspension nless they are similar in size to plverized coal particles. COCLUSOS Three variables have been shown to have a sbstantial effect on the rate at which particlate emissions are prodced by combstion on grates: (a) Combstion rate has a significant effect on the rate of particlate emissions. igher heat release rates reslt in higher levels of emissions [1, 3]. This effect can also be seen in Fig.. (b) Fel partial size has a sbstantial effect, as seen in Fig., on the prodction of particlate emissions. Fel smaller that 8 mm (3/8 in.) has greater emissions that fel screened to larger than 8 mm (3/8 in.). (c) The amont of nderfire air has a significant effect on particlate emissions. As the ratio of nderfire air to fel flow rate increases, the rate of particlate emissions increases [1, 3]. This effect can also be seen in Fig.. A nmber of parameters have been shown to not have a significant effect on particlate emissions []. ncreases in emissions cased by pward load swings were nllified by decreases in emission cased by downward load swings []. Based on the reslts of these tests [], the following statements can be made: (a) Fel moistre step changes in the range tested, 5-1% dry basis, did not directly effect the rate of total particlate emission []. (b) Load swings in the range tested did not have a significant effect on particlate emissions. Load swings were simlated by step changes in the fel feed rate and air flow rate []. There are three variables which can be controlled 7

8 to redce the generation of particlate emissions generated by combstion of wood wastes. Combstion of most solid wastes in spreader-stokers are expected to respond to these variables. (a) Redcing the combstion rate, i.e., decreasing the nit's capacity is known to be effective. The cost may prove to be exorbitant in many cases. (b) ncreasing fel particle size is effective and may be economical if efforts are limited to keeping larger fel from being plverized. (c) Redcing nderfire air may reqire operator training or capital expenditre for retrofits. t has been the athor's experience on wood fired boilers that implementation of redced nderfire air technology to existing boilers has reqired training of the operators and engineers involved. Capital expenditres cold atomate the process and redce or eliminate training reqirements. REFERECES [1] Jnge, David C. "nvestigation of the Rate of Combstion of Wood Reside Fels." Corvallis, 1977, 15 pp. (Oregon State University Technical Progress Report o. on U. S. Energy Research and Development Administration Contract o. EY -76-C-6-7, Task Agreement o. ). [] Tttle, Kenneth L. "nterim Test Report on OSU/ Weyerhaeser Experiment, 1978." Oregon State University Wood Combstion Test Facility, Corvallis, pp. (in Technical Progress Report o. 16, D. C. Jnge). [3] Tttle, Kennth L. "Combstion Mechanisms in Wood Fired Boilers." Doctoral dissertation, Corvallis, Oregon State University, 1978, 19 pp. [4] Tttle, Kenneth L., and Jnge, David C. "Combstion Mechanisms in Wood Fired Boilers." JAPCA 8 (o.7, Jly 1978): [5] Tttle, Kenneth L. "Review of Biomass Gasification Technology." n Progress in Biomass Conversion, edited by D. A. Tillman and E. C. Jahn, Vol. 5 Orlando, Florida: Academic Press, 1984, [6] Kester, Richard A. "itrogen Oxide Emissions from a Pilot Plant Spreader Stoker Bark Fired Boiler." Doctoral dissertation, Seattle, University of Washington, 198, 17 pp. 8