Bioresource Technology 36 ( 1991 ) 215-221

Co-feeding and Co-firing Biomass with Non-Hazardous Waste and Natural Gas A. Green, H. van Ravenswaay, J. Wagner, B. Green, T. Cherry & D. Clauson

Clean Combustion Technology Laboratory, Department of Mechanical Engineering, University of Florida, Gainesville, Florida 32611, USA

Abstract

Previously we developed the concept of co-firing non-hazardous waste (NHW) with cellulosic bio- mass (CB) and natural gas (NG) in a modular waste to energy system as a source of energy which is economically competitive with natural gas even during periods of low oil prices. The practical pursuit of this NHW-CB-NG co-combustion con- cept in a region which can provide a variety of low- cost wood waste or cultivated biomass requires a versatile biomass feeding system, which needs min- imal CB preprocessing. In this report we describe a multichannel biomass feeding system developed for co-feeding a variety of CB types with NHW and NG. We also tabulate the properties of natural gas and cultivated biomass types used in our co-firing experiments. Finally, we briefly describe how NHW-CB-NG co-firing can help reduce our bal- ance of payments deficit and serve as a greenhouse mitigation measure.

Key words: Co-feeding, co-firing, cellulosic bio- mass, non-hazardous waste, greenhouse mitiga- tion.

INTRODUCTION

The discovery and control of fire fueled by wood many tens of thousands of years ago ranks as a civilizing force comparable to the discovery of speech and agriculture. The warmth afforded by controlled fire permitted man to migrate to colder and more productive agricultural lands and to develop technologies which led eventually to the industrial revolution. Today the combustion of fossil fuels (coal, petroleum, and natural gas) is the major source of energy in the generation of electricity, in supplying transportation energy and

215 Bioresource Technology 0960-8524/91/S03.50 © 1991 Great Britain

in producing heat. On the other hand, wood or, more generally, cellulosic biomass has in the United States become a relatively minor source of energy (about 3 exajoules (EJ)) despite the fact that it is potentially capable of supplying a much larger share of the energy consumed nationally (about 84 E J).

The combustion of community waste has in recent years become an important way of reduc- ing the volume and mass of the waste to be dis- posed of in landfills and is now becoming an additional source of energy. In previous studies (Green, 1986, 1989a, b,c; Green et al., 1985, 1987, 1988, 1989) we developed the concept of co-firing non-hazardous waste (NHW) with cellu- lose biomass (CB) and natural gas (NG) in a modular incinerator as a potential environ- mentally sound source of energy which is economically competitive with natural gas. The CB includes 'dirty wood chips' from forest residues, land clearing and line clearing opera- tions, tree surgeon residues, Christmas trees, pine cones, etc., as well as cultivated energy crops including leucaena, eucalyptus, elephant grass, energy cane and agricultural residues. The NHW which we use consists of institutional-household waste with large plastics and food-related paper components. These are representative of fast food restaurant waste or hospital ward waste, although toxic material and halogenated plastics are excluded by our collection rules at each house- hold unit. The NHW-CB-NG concept essentially takes advantage of the negative cost of NHW fuel (i.e. the tipping fee) and the low positive cost of CB fuel in many parts of the country, together with environmental and energy efficiency benefits of NG co-firing to overcome the higher capital and operating cost of a solid fuel boiler as com- pared to a NG boiler. At current low oil and gas prices, co-combustion of non-hazardous waste

Elsevier Science Publishers Ltd, England. Printed in

216 A. Green, H. van Ravenswaay, J. Wagner, B. Green, T. Cherry, D. Clauson

with biomass and with natural gas may be one of the few ways of matching the economics of natural gas and distilled oil use as a source of energy. The Clean Combustion Technology Laboratory (CCTL) has been engaged in research, develop- ment, and demonstration of co-combustion of fuels. The program, initiated in 1980, originally focused on co-combustion of gas and coal or gas- and coal-water slurries as alternatives to burning oil (Green, 1981; Green & Pamidimukkala, 1984; Green et al., 1986). With the decline in oil prices in 1986 and subsequent loss of interest in oil alternatives, our program has emphasized work on co-combustion of non-hazardous waste, cellu- losic biomass and natural gas in modular waste to energy systems. The use of NHW-CB-NG might be applicable to institutions, residential com- munities, industrial parks, shopping centers, military bases and small and possibly middle sized communities.

BIOMASS FEEDING

The facilities, instrumentation and modus operandi of our work on NHW-CB-NG co-firing have been described previously (Green et al., 1989). Having established the technical feasibility of NHW-CB-NG co-combustion in the first year's operational experience, the second year has largely been devoted to the development and installation of a versatile biomass feeding system, improved instrumentation and other refinements, and to extensive measurements.

Our NHW-CB-NG concept attempts to improve its economic competitiveness with NG by serving a double purpose -- disposal of NHW and providing energy with the help of CB. Furthermore, the biomass feeding system we have assembled appears to be capable of working with minimal biomass processing costs. Figure 1 illu- strates two views of the biomass feeder, which we have adapted from a gunnite machine manu- factured by Air Placement Equipment Co. Inc., of Cincinnati, Ohio (Airplaco Model 620). As modified, the system now has three hoppers. The left hopper with its 15-cm auger works quite well at 7.6 r.p.m, for hard wood chips. The right hopper with its 15-cm auger at 4"8 r.p.m, works well for pea coal and other dense material. A central hopper with its large 30-cm auger at 6.8 r.p.m, handles rather stringy line clearing chips and leaves. With these three different combina- tions, various feed rates can be obtained without changing motor speed. In the NHW-CB-NG co- firing mode, the biomass feeder is timed to feed between ram (NHW) feeds. Continuous biomass feeding could be used for CB-NG co-firing with the NHW bag feeding system inactivated.

The injection system (see Fig. 2) for our bio- mass feeding system uses a 20 cm × 25 cm open- ing cut in the top of the primary combustion chamber. An almost vertical chute interfaces the biomass feeder and the combustion chamber. A wind box and external fan provide overpressure so that the wood chips are educed into the primary chamber at a location opposite the gas burner. The fan speed has been adjusted

Center auger

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Side view and back view of biornass feeder.

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Co-feeding and co-firing biomass 217

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Fig. 2. Biomass feeder injection system.

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,Top sprocket _1 inch shaft

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Fig. 3. Drag chain assembly installed in left hopper.

empirically so as not to create general over- pressure in the primary chamber which might cause smoke leakage during ram feed injections of NHW.

In addition to the versatility afforded by our biomass feeding system, we have additional bio- mass feeding capability through our NHW bag feeding system. At certain times of the year a wooded community can be an extensive source of yard trimmings. Bagged in plastic, these trimmings can be fed through the regular ram feeding system. We have also found that pine straw, line clearing chips, and other stringy biomass are fre- quently available, and these can be fed loose (unbagged) on the conveyor belt, interspersed with NHW bags and fed directly through the ram feeder. Thus between the biomass feeder and injection system illustrated in Figs 1 and 2 and the ram feeder system previously described (Green et al., 1985) we have a multiplicity of choices for feeding many forms of biomass available at nominal or low costs in a low population density region.

Bridging of the biomass inside the hoppers has been the major biomass feeding problem encoun- tered thus far. The problems with the left hopper were twofold. First, the original slanted front wall, which was used to give the hopper a large capacity, provided initiation points for bridging. Furthermore, the small metering auger puts restrictions on the hopper geometry. To overcome these problems, the front wall has recently been given a negative slope. The rear wall has been covered with smooth, thin sheet metal. A drag chain has been installed to break up any bridges that are formed. Figure 3 shows the drag chain assembly inside the left hopper. A steel frame provides rigidity for a 2.5 cm hardened steel shaft

supported by four bearings above the walls of the hoppers.

The center hopper is more forgiving due to its steeper walls (three of its walls are vertical) and larger auger, which in effect acts as a live bottom. However, the drive shaft of the right auger, which runs through the center hopper, causes occasional bridging. This could be avoided in a production model designed specifically for biomass feeding.

Finally, some bridging problems which occurred in the injection chute have been cor- rected by enlarging the cones and by closely simulating free-fall conditions. Table 1 fists the types of biomass and feeding mechanisms used so far.

As a result of our own experiences and guidance from the rather sparse literature (Screw Conveyor Catalog and Engineering Manual, 1979; Kellyman, 1981; Miles, 1982) on biomass feed- ing, the following steps are recommended when designing biomass feeders for small modular com- bustors:

(1) Design hoppers with walls as close to vertical as possible. Wherever possible use a negative slope. Avoid tight constrictions or conversion points.

(2) Make hopper walls as smooth as possible, using a low friction coating if necessary.

(3) Whenever auger sizes less than 30 cm are required, oversize the auger or use five bottom feeders.

(4) Use vibrators on walls with sticking tend- encies.

(5) Use drag chains when the above pro- cedures fail.

(6) Use chutes with free fall for injection.

218 A. Green, H. van Ravenswaay, J. Wagner, B. Green, T. Cherry, D. Clauson

Table 1. Summary of the types of biomass and the feed rates obtained with the different feeding systems

Burn Type of biomass Feeding Continuous Moisture date system ~ feed rate content

kg h -1 %, wet

Bulk density k g m -3

16 Feb. 1989 Dirty pine chips bf-lh 238 18.1 11 Mar. 1989 Hard wood chips bf-ch 240 32"9 21 Mar. 1989 Dirty wood chips bf-eh 489 42.8 30 Mar. 1989 Old leucaena bf-lh 107 19.8

8 April 1989 Eucalyptus bf-ch 312 25"9 16 April 1989 Line clearing chips rf-ub n/a 19"5 18 May 1989 Old leucaena bf-lh 93 33.1 2 June 1989 Yard rakings rf-bg n/a 9"6

23 June 1989 Line clearing chips bf-eh 243 26.0 6 July 1989 Line clearing chips bf-ch 259 16"9

27 July 1989 Line clearing chips bf-eh 432 45"7 3 Aug. 1989 Mixed paper bf-ch 211 7'5

31 Aug. 1989 Line clearing chips bf-ch 426 42.4 14 Sept. 1989 Line clearing chips bf-eh 637 48.3 5 Oct. 1989 Mixed paper rm-bg n/a 8'8

26 Oct. 1989 Mixed biomass bf-ch 799 65.1 21 Nov. 1989 Mixed biomass bf-lh 175 50"0

n/r n/r n/r n/r 170 131 168

29 125 n/r n/r n/r n/r n/r n/r n/r n/r

"bf = Biomass feeder, lh = left hopper, ch = center hopper, rf = ram feeder, ub = n/r Not recorded. n/a This system is not used for continuous feeding of biomass.

unbagged, bg = bagged.

Most of these steps are known to specialists in biomass feeding. The unique aspect of our work is the multiplicity of channels to handle different fuel qualities. This minimizes the need for costly fuel processing which would adversely affect economic competitiveness.

BIOMASS HEATING VALUES AND MOISTURE CONTENTS

During every co-burn experiment performed by the CCTL a representative sample of the biomass was taken for further analysis. To account for the non-homogeneous moisture content distribution of the stored biomass, several samples were taken at different locations in the pile. These samples were then mixed together and stored in sealed polyethylene bags to prevent moisture loss. Sample sizes ranged from 2 to 5 kg. These samples were analyzed in the laboratory for mois- ture content and heating value.

To determine the moisture content, we devel- oped a rapid procedure using a microwave oven. Prior to analysis, about 200 g of the sample is taken out of the sealed bag and thoroughly mixed in the blender. This produces a fine homogeneous mixture. From this mixture two samples of about 10-20 g are placed in a crucible and heated in the

microwave. By measuring the weight reduction upon heating, the moisture content is determined.

To determine the dry higher heating value (HHV) of the biomass, a model 1241 automatic Parr bomb calorimeter was used. The dried bio- mass samples are compacted into 1-g pellets. For each type of biomass, three of these pellets are burned in the calorimeter to determine the HHV of the biomass. Table 2 shows the results of measurements for a variety of biomass types.

All biomass types analyzed thus far have a dry HHV ranging from 16.7 kJ g-i to 20.2 kJ g-l. Hardwood chips had the lowest value (16.9 kJ g-l), while yard rakings (leaves and pine needles) had the highest (20.1 kJ g-~). The heating value results which are compatible with those in the literature also indicate the effectiveness of the mixing and drying procedures used during pre- paration. Differences between two comparable runs ranged from 0"6% to 7.6%, with an average of 2.7%.

One of the most interesting observations drawn from our HHV measurements is that aging of the biomass does not lead to a significant drop in dry HHV. For example, 1-year-old leucaena stored outdoors had a dry HHV of 17.3 kJ g-I (30 March 1989), while fresh leucaena analyzed in 1988 measured at 17.0 kJ g-1 (27 July 1988). Similar results were obtained with tall grasses,

Co-feeding and co-firing biomass

Table 2. Moisture content andhigher heating values (HHV ) for various types of biomass used at UF-STC-CCTL

219

Burn Type of Moisture date a biomass content

(%, wet)

Higher heating value, kJ kg- i

(dry (wet)

21 July 1988 Wet Christmas chips 25'4 -- -- 21 July 1988 Dried Christmas chips 19.9 -- -- 27 July 1988 + Christmas chips -- 12 659 -- 27 July 1988 + Leucaena chips -- 11 379 -- 27 July 1988 + Tall grasses -- 11 346 -- 26 July 1988 Dried leucaena 8"0 -- -- 28 July 1989 Wet leucaena 15.8 -- --

4 Aug. 1988 Wet tall grasses 22"5 -- -- 30 Aug. 1988 Dried tall grasses 8"6 -- -- 17 Sept. 1988 Dried eucalyptus 25-9 -- -- 17 Sept. 1988 Dried dirty pine 42"5 -- -- 20 Sept. 1988 Sundried dirty pine 40"3 -- --

5 Oct. 1989 Sundried Christmas -- -- -- 16 Feb. 1989 Dirty pine chips 18'1 11 739 9 614 11 Mar. 1989 Hard wood chips 32'9 11 339 7 609 21 Mar. 1989 Dirty wood chips 42'8 1 l 719 6 703 30 Mar. 1989 Dry old leucaena 19"8 1 l 560 9 271 * 2-year old tall grass 74.5 11 421 2 912

8 April 1989 Fresh eucalyptus 25"9 11 660 8 640 16 April 1989 Dirty landcl, chips 19"5 12 237 9 851 18 May 1989 Old leucaena 33.1 -- -- * Pine needles and raking 13.4 12 181 10 548

2 June 1989 Yard rakings 9"6 13 244 11 972 23 June 1989 Line clearing chips 26.2 17 676 13 054

6 July 1989 Line clearing chips 16.9 17 425 14 489 27 July 1989 Line clearing chips 45.7 18 761 10 187

4 Aug. 1989 Mixed paper 7.5 18 269 16 899 31 Aug. 1989 Line clearing chips 42.4 19 577 11 276 14 Sept. 1989 Line clearing chips 48.3 17 514 9 063 5 Oct. 1989 Mixed paper 8'8 18 232 16 627

26 Oct. 1989 Mixed biomass 64"6 16 462 5 836 21 Nov. 1989 Mixed biomass 50"0 l 7 702 8 851 ** Old leucaena 33-0 19 101 12 798 ** Old Christmas trees 56-9 21 126 9 105 ** Old tall grass 31"2 18 147 12 485

a + Date biomass was analyzed for HHV. For burns prior to 16 Feb. 1989 these HHVs were adjusted for moisture content and used in calculations. No individual HHV analysis was performed for biomass types used prior to this date. *Not yet burned. **Not yet burned; analyzed 3 Dec. 1989.

which m e a s u r e d 17.0 kJ g -1 in 1988 , whi le a sample f r o m the s ame ba le had a H H V o f 17.1 kJ g - l in 1989 . It mus t be m e n t i o n e d that a c c o r d i n g to Mis l evy ( 1 9 8 7 unpub l i shed ) , aging o u t d o o r s can lead to d r y b iomass loss and , o f cour se , m o i s t u r e up take . H o w e v e r , it is e n c o u r a g i n g tha t a b iomass pi le po ten t i a l l y c an be u sed as a va luab le fuel f o r an e x t e n d e d pe r iod .

N H W - C B - N G CO-FIRING AS PART OF A N A T I O N A L S T R A T E G Y

Inc reas ing a t m o s p h e r i c c o n c e n t r a t i o n s o f gaseous spec ies wh ich s t rong ly a b s o r b the ea r th ' s long-

wave in f r a red r ad ia t ion are e x p e c t e d to e n h a n c e the ear th ' s na tu ra l g r e e n h o u s e effect . A s imple rad ia t ive hea t b a l a n c e ca lcula t ion , wh ich equa te s the s h o r t w a v e so lar inpu t to the ear th ' s l ongwave o u t p u t a f te r a l lowing fo r the ea r th ' s s h o r t w a v e reflectivity, ind ica tes tha t the ea r th ' s su r face is

33 K w a r m e r b y v i r tue of na tu ra l g r e e n h o u s e s gases ( H 2 0 , CO2, an d 0 3 ) t h an it w o u ld be in the a b s e n c e o f these gases ( G r e e n & Wyatt , 1965 ; Mitchel l , 1989) . Inc reas ing a n t h r o p o g e n i c emis- s ions o f g r e e n h o u s e gases, pa r t i cu la r ly CO2, CH4, N 2 0 , an d c h l o r o f l u o r o c a r b o n s (CFCs) , a re thus e x p e c t e d to cause add i t i ona l g lobal warming .

T h e use of cel lulosic b iomass (CB) as a r enew- able fuel in the r e f o r e s t a t i o n m o d e is a m o n g the

220 A. Green, H. van Ravenswaay, J. Wagner, B. Green, T. Cherry, D. Clauson

most promising approaches to greenhouse mitiga- tion by reducing carbon dioxide buildup (Adler & Schwengels, 1989; Walter et al., 1989). The United States has an abundant supply of biomass which potentially could replace fossil fuels by sup- plying some 10-15 EJ of our total national energy use of about 84 EJ (Young et al., 1988; Zerbe, 1988). Unfortunately, as noted earlier, the tech- nology and operations required to burn wood chips or any solid fuel are much more com- plicated and hence more costly than those required to burn natural gas or distilled oil. Thus market forces do not currently favor increased use of CB. To overcome this problem we have proposed co-firing non-hazardous waste (NHW) in a heat recovery combustion system with locally available cellulosic biomass (CB) as a base load. Natural gas, a high hydrogen/carbon fuel, serves as a variable load and helps to control the emis- sions of particulates and organic gases (this topic will be the subject of future reports). With a favor- able economy of scale, the low-cost CB combines with the negative-cost NHW (tipping fees) and moderate-cost NG to provide a versatile, econo- mically competitive domestic fuel. A national strategy which addresses the problems of waste disposal, combustion system emissions, our exces- sive oil imports, and increasing greenhouse gases should find NHW-CB-NG fuel to be a helpful option.

ACKNOWLEDGEMENTS

This work was supported by the Tennessee Valley Authority's Southern Regional Biomass Energy Program (Robert Brooks, scientific monitor) and the Florida Governor's Energy Office (Randall Zipser, scientific monitor). The authors would also like to thank M. Jackson and D. Copeland of Sunland at Gainesville, and G. Hemp, G. Schaffer, W. Phillips, R. Gaither, and R. Cremer, University of Florida administrators, who have made major 'in-kind' contributions to this effort. We also thank the Precon Corporation of Gainesville for donat- ing the gunnite machine which now serves as a biomass feeder and our colleagues D. Rockwood, G. Prine, and E Mislevy and the Gainesville Regional Utilities for providing the cellulosic bio- mass we have used. Finally we thank R. Nipper, R. Nefzger, F. Natour, L. Pinkham, R. Kramel, S. Davis, B. Warren, C. Hill, R. Brown, C. Rollins, and J. Kellum for providing the non-hazardous waste essential to these co-firing experiments.

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