combustion of residual biosolids from a high solids anaerobic digestion/aerobic composting process

15
Pergamon Biomass and Bioenergy Vol. 12, No, 5, pp. 367--381. 1997 ~, 1997 ElsevierScienceLtd. All rights reserved Printed in Great Britain Pll: S0961-9534(97)00086-X 0961-9534/97 $17.00 + 0.00 COMBUSTION OF RESIDUAL BIOSOLIDS FROM A HIGH SOLIDS ANAEROBIC DIGESTION/AEROBIC COMPOSTING PROCESS B. M. JENKINS*, M. KAYHANIANt, L. L. BAXTER~ AND D. SALOUR§ *Department of Biological and Agricultural Engineering, University of California, Davis, CA 95616, U.S.A. tDepartment of Civil and Environmental Engineering, University of California, Davis, CA, U.S.A. :~Combustion Research Facility, Sandia National Laboratories, Livermore,CA, U.S.A. §California Energy Commission, Sacramento, CA, U.S.A. (Received 19 September 1995; accepted 12 August 1996) Abstract--A humus consisting of aerobically stabilized anaerobic digester effluent was burned in two laboratory test facilities to evaluate possible problems in using the humus as fuel in boilers, An atmospheric circulating fluidized-bed combustor (FBC) was used to assess possible bed agglomeration when using alumina-silicate bed media. A multi-fuel capable entrained flow combustor (MFC) with electrically heated walls simulating a boiler was used with air-cooled tubular probes to evaluate potential ash deposition on heat exchangers. Low heating value of the humus led to poor temperature control in the FBC when burned alone. Tests of the humus blended 50% by weight with wood were also carried out in the FBC. No evidence of bed agglomeration was found in the FBC with either humus alone or blended with wood at temperatures up to 900~C; the maximum tested. Bed-drain rate was nearly equal to fuel-ash feed rate to maintain constant bed pressure drop due to the high sand fraction in the humus. Deposits collected on the probes in the MFC experiment were enriched in alkali-sulfate, as were the fine fractions of fly-ash samples. Deposition rates were not established due to particle-induced abrasion of deposits from the heavy sand fraction of the fuel, but at least moderate alkali-sulfate deposition can be anticipated if humus is burned in commercial units. © 1997 Elsevier Science Ltd Keywords--Anaerobic digestion; composting; humus; combustion; fouling; agglomeration; fluidized bed; biomass boilers i. INTRODUCTION Anaerobic digestion of organic solids produces a combustible biogas and residual digested solids and cell mass. The latter can be stabilized through a process of aerobic composting, to produce an odor-free humus material. Depend- ing on the characteristics of the original waste and the waste processing used, the humus may be of limited value as a compost or soil amendment. Also, adequate markets for the product simply may not exist or users may be reluctant to commit to a product which could in the future be subject to additional legal or regulatory restrictions. Combustion of the humus would serve as an alternative utilization/ disposal technique, although some of the properties (e.g. heavy metals, supply con- straints) which may make it undesirable for *Author to whom correspondence should be addressed. Tel: (916) 752-1422; Fax: (916) 752-2640. other purposes would similarly influence decisions to use it as fuel. A system employing high-solids digestion (22-30% solids) followed by aerobic composting has been investigated for its technical and economic feasibility in converting the biodegrad- able organic fraction of municipal solid wastes (BOF-MSW: includes paper, yard waste and food waste) to useful products/3 The conversion of BOF-MSW in this fashion has considerable benefits over conventional landfilling, especially in regions such as California where legislation mandating a 50% diversion of MSW from landfills has recently been enacted. 4 The gas (biogas consisting principally of methane and carbon dioxide) produced by the first-stage high-solids anaerobic digestion pro- cess can be used as a fuel for boilers, internal combustion engines, and other thermal appli- cations. The gas may also be processed for the production of methanol or be reformed and shifted to produce a hydrogen-rich feed stream 367

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Page 1: Combustion of residual biosolids from a high solids anaerobic digestion/aerobic composting process

Pergamon Biomass and Bioenergy Vol. 12, No, 5, pp. 367--381. 1997

~, 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain

P l l : S0961-9534(97)00086-X 0961-9534/97 $17.00 + 0.00

C O M B U S T I O N O F R E S I D U A L B I O S O L I D S F R O M A H I G H

S O L I D S A N A E R O B I C D I G E S T I O N / A E R O B I C C O M P O S T I N G

P R O C E S S

B. M. JENKINS*, M. KAYHANIANt, L. L. BAXTER~ AND D. SALOUR§

*Department of Biological and Agricultural Engineering, University of California, Davis, CA 95616, U.S.A.

tDepartment of Civil and Environmental Engineering, University of California, Davis, CA, U.S.A. :~Combustion Research Facility, Sandia National Laboratories, Livermore, CA, U.S.A.

§California Energy Commission, Sacramento, CA, U.S.A.

(Received 19 September 1995; accepted 12 August 1996)

Abstract--A humus consisting of aerobically stabilized anaerobic digester effluent was burned in two laboratory test facilities to evaluate possible problems in using the humus as fuel in boilers, An atmospheric circulating fluidized-bed combustor (FBC) was used to assess possible bed agglomeration when using alumina-silicate bed media. A multi-fuel capable entrained flow combustor (MFC) with electrically heated walls simulating a boiler was used with air-cooled tubular probes to evaluate potential ash deposition on heat exchangers. Low heating value of the humus led to poor temperature control in the FBC when burned alone. Tests of the humus blended 50% by weight with wood were also carried out in the FBC. No evidence of bed agglomeration was found in the FBC with either humus alone or blended with wood at temperatures up to 900~C; the maximum tested. Bed-drain rate was nearly equal to fuel-ash feed rate to maintain constant bed pressure drop due to the high sand fraction in the humus. Deposits collected on the probes in the MFC experiment were enriched in alkali-sulfate, as were the fine fractions of fly-ash samples. Deposition rates were not established due to particle-induced abrasion of deposits from the heavy sand fraction of the fuel, but at least moderate alkali-sulfate deposition can be anticipated if humus is burned in commercial units. © 1997 Elsevier Science Ltd

Keywords--Anaerobic digestion; composting; humus; combustion; fouling; agglomeration; fluidized bed; biomass boilers

i. INTRODUCTION

Anaerobic digestion of organic solids produces a combustible biogas and residual digested solids and cell mass. The latter can be stabilized through a process of aerobic composting, to produce an odor-free humus material. Depend- ing on the characteristics of the original waste and the waste processing used, the humus may be of limited value as a compost or soil amendment. Also, adequate markets for the product simply may not exist or users may be reluctant to commit to a product which could in the future be subject to additional legal or regulatory restrictions. Combustion of the humus would serve as an alternative utilization/ disposal technique, although some of the properties (e.g. heavy metals, supply con- straints) which may make it undesirable for

*Author to whom correspondence should be addressed. Tel: (916) 752-1422; Fax: (916) 752-2640.

other purposes would similarly influence decisions to use it as fuel.

A system employing high-solids digestion (22-30% solids) followed by aerobic composting has been investigated for its technical and economic feasibility in converting the biodegrad- able organic fraction of municipal solid wastes (BOF-MSW: includes paper, yard waste and food waste) to useful products /3 The conversion of BOF-MSW in this fashion has considerable benefits over conventional landfilling, especially in regions such as California where legislation mandating a 50% diversion of MSW from landfills has recently been enacted. 4

The gas (biogas consisting principally of methane and carbon dioxide) produced by the first-stage high-solids anaerobic digestion pro- cess can be used as a fuel for boilers, internal combustion engines, and other thermal appli- cations. The gas may also be processed for the production of methanol or be reformed and shifted to produce a hydrogen-rich feed stream

367

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368 B.M. JENKINS et al.

for fuel cells, although the technical details of these approaches have not been fully developed. The humus produced "by the second-stage aerobic composting and drying process can potentially be used in a number of ways, including soil amendment or as an energy source, such as fuel in commercial boilers.

Assuming that combustion is feasible, its application with thermal and electrical power generation could be integrated into the system as a means to feedback energy to the biological processes, as illustrated in Fig. 1. Alternatively, humus could be sold as fuel for separate generation without integration.

Due to the conversion of the volatile organic portion of the feedstock, the resulting humus is enriched in ash. High ash fuels (and some low ash fuels depending on compo- sition) have frequently proved difficult to burn in commercial systems due to fouling of heat transfer surfaces,-slagging on furnace walls and grates and agglomeration of bed media in fluidized-bed units. The relationships of biomass ash composition to boiler fouling have recently been investigatedJ ~0 This paper reports the results of a preliminary investi- gation into the combustion characteristics of a humus fuel produced by the pilot-scale high solids anaerobic digestion/aerobic composting system operated by the Civil and Environ- mental Engineering Department, University of California, Davis. This investigation had two primary objectives: (1) characterization of the fuel properties, including ash composition and mode of occurrence for ash elements, and (2)

evaluation of fouling and agglomerating be- havior of the humus ash during combustion. In addition, pollutant emissions were moni- tored during laboratory combustion exper- iments completed in fulfilling these objectives.

FACILITIES AND METHODS

Studies of the agglomerating characteristics of humus fuel ash were conducted in the pilot-scale atmospheric circulating fluidized bed combustor (FBC) operated by the Bio- logical and Agricultural Engineering Depart- ment, University of California, Davis. The tests were designed to determine whether the use of humus as fuel in a fluidized bed would lead to bed agglomeration, a common prob- lem with biomass fuels. Ash decomposition investigations were carried out using the Multifule Combustor (MFC) located at the Combustion Research Facility, Sandia Na- tional Laboratories, Livermore California. The MFC was used to evaluate the potential fouling of superheaters and 'other boiler heat transfer surfaces by compounds originating from the humus.

FBC experiments

Two experiments were conducted on humus fuel in the FBC unit. The first experiment used humus alone. Due to difficulties encoun- tered in maintaining steady combustion during this test, a second experiment was conducted using both humus alone as well as humus blended with wood.

Water Air BOF- ~ Digested ~ Residual biosolids

Aeroblo Anaerobic ~ (humus) ~ Utilization/ Digesti°n l f I C°mp°stingVDrying I ,loo,. i l--osooa

. wi • n Optional Co-fired l- "- . . . . . . . ~ . . . . . Fuel(e.g. wood)

--~" ~ I Combustion ] ~ Ash I

Heat

4E"P°!e-~r P°Wer Generatl°n k . . . . . . . . . . . . . . . . . . . . . i i ~ . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Sur'~us/ Alternate

u s e

Fig. l. Schematic integration of residual biosolids (humus) combustion into a high solids anaerobic digestion/aerobic composting and drying system.

Page 3: Combustion of residual biosolids from a high solids anaerobic digestion/aerobic composting process

Combustion of residual biosolids 369

The FBC pilot facility consists of a refrac- tory-lined main reactor, employing in-bed fuel feed, with external solids recirculation through a cyclone and separately fluidized loop-seal H. A spray-type wet-scrubber and demister are located downstream of the cyclone and ahead of the stack to reduce the effluent particle concentration. The system is shown schemati- cally in Figure 2. Fuel metering is done by the primary feed screw, which is loaded uniformly along its length. The metering screw discharges into a duel air-lock and and secondary feeding screw discharging into the bed. Air purge between the two air locks prevents combustion gas from leaking out through the feed system, and accounts for part of the combustion air. The secondary feed screw is driven faster than the primary screw and performs no metering.

The main bed combustion air distributor used for these experiments was a perforated stainless steel plate, with a bed drain located at the center. Fluidizing air, which served as primary combustion air, was blown through the distributor from the wind box and was supplied either from the laboratory compressed air supply or from a ring-compressor. Start-up was accomplished with laboratory air. Following start-up, the air source was switched to the ring-compressor. The flow rate from the ring-compressor was monitored continuously by orifice plate. Secondary air could also be injected into the reactor at various ports along its length. Secondary air flow rates were monitored manually by means of rotameters. Additional combustion air was supplied through the loop-seal fluidization. The flow rate

Stack A

Wet scrubber

& I

Water

Cyclone \ I

I

Demister

~, Sight glass

k

i

-o 3p ~ -- s~ al

I L

1.2 m

l

t

I c m

Secondary Air Port

Instrument Port Fuel metering screw

~ t Airl°ck . = Air purge Airlock Feed screw

e

Propane preheat bumer Windbox

Bed Drain

Fig. 2. Fluidized-bed combustor.

Page 4: Combustion of residual biosolids from a high solids anaerobic digestion/aerobic composting process

370 B.M. JENKINS et al.

of air to the loop seal was monitored manually via rotameters.

The main reactor is approximately 4.5 m tall, with an 18 cm inside diameter. At start-up, the bed material and reactor were preheated using a propane fired burner discharging directly into the wind box. The bed was preheated to a temperature above 600°C prior to introducing test fuel. The bed media used in these experiments was an alumina-silicate sand (elemental analysis, oxide basis, 53% as silica, 44% as alumjina, 3% other) with a mean particle size of 260/~m and specific gravity of 2.55. The bed was originally loaded with 7.4 kg of media, giving a static bed depth of 0.12 m. Details of the media used and the fluidization characteristics of the bed are described by Salour et al. ~2

Thermocouples were situated along the length of the reactor and at various other locations in the system. The thermocouple positions are shown in Fig. 2. Thermocouple T1 was located in the windbox, T2-T4 in the lower bed and T5-T18 upwards above the bed at intervals of approximately 0.3 m. Six strain-gage transduc- ers were used to monitor pressure at various locations in the system, including the orifice plate used to measure inlet air flow rate (P1), loop seal windbox pressure (P2), main bed differential pressure (P3), loop seal to main bed inclined-pipe differential pressure (P4), main reactor windbox pressure (P5), and loop seal bed pressure drop (P6). All sensors were monitored electronically, processed in real time by computer, with final data storage on disk.

Gas concentrations were monitored in the stack past the demister. CO2 concentration was measured continuously by infrared analyzer, and 02 intermittently by paramagnetic analyzer. For the second test, concentrations of NOx were monitored continuously via chemiluminescence and SO2 by UV fluorescence as well. Grab samples of combustion gas were collected periodically in 250 ml glass sample holders from the line downstream of the demister. Grab samples were analyzed by GC for permanent gases and light hydrocarbons (C,-C3).

The first test was terminated early due to low and declining temperatures in the main reactor. With humus alone, there was insufficient heat release to maintain bed temperature above 700°C. The high sand content of the humus led to more accumulation of ash in the bed than anticipated, and consequently high bed pressure drop. Increasing the bed-drain rate did not,

however, overcome the problem of declining bed temperature when firing humus-alone.

The second test lasted a total of 6.7 h and was terminated after exhausting the humus fuel supply. For the first 3 h, humus only was fed to the reactor, but a similar behavior was observed to that of the first test, in that the bed temperature could not be maintained above 700°C even with adequate bed draining to maintain a constant bed pressure drop. The temperature distribution was also observed to become uniform with little difference between bed and lower freeboard. At 3 h into the test, a mixture of wood and humus was introduced. This mixture was made by combining wood and humus in amounts to give equal weight fractions on a dry basis. Because the moisture content of the two fuels were similar, after an additional 0.5h, the mixture was made simply by combining equal weights of the moist fuels. Temperature control was improved with the blends, and freeboard temperature increased to between 800 and 900°C over the next 2 h. The characteristic increase in freeboard temperature above bed temperature when burning high volatile wood fuel was also observed to develop after initiating the blend. The final hour of operation was quite stable and no adjustments were made other than small changes in primary air flow rate to keep freeboard temperature below 900°C, and continued bed draining at intervals of about 20 min. The continuous gas sample line to the CO2 and 02 analysers failed shortly into the test, and repairs were completed only 2 h prior to the end of the experiment.

1.1. M F C experiments

The deposition of inorganic elements on heat transfer surfaces was investigated in the multi-fuel combustor facility (MFC) at Sandia National Laboratories, Livermore, CA. The multi-fuel combustor was originally designed to simulate pulverized coal combustion and ash deposition in utility coal combustors. The combustor, depicted in Fig. 3, is a 4,2 m high vertical tube furnace with 15 cm inside diam- eter. The upper six of seven furnace modules are electrically heated. The furnace is open at the bottom, and discharges across a clear space into the exhaust, that also draws laboratory air for dilution and cooling. A natural gas burner situated at the top of the furnace can supply a pre-heated oxidant stream to the furnace to simulate different combustion stoichiometries. The gas burner was not used in these tests and

Page 5: Combustion of residual biosolids from a high solids anaerobic digestion/aerobic composting process

Combust ion of residual biosolids

t Con"

Preh, Flow

Pneumatic fuel transp(

371

Heat~ Insu Line

Instrun

)le probe

Surface them

Open tesl

\ / 1 Y-- \\

sit probe

otor

nt table

To exhaust blower

Fig. 3. Multi-fuel combustor.

combustion air was supplied directly to the top of the furnace. In this way, no corrections were needed to the measured combustion gas concentrations, as there were no contributions from natural gas combustion. Humus fuel was injected pneumatically via a water-cooled lance inserted through the side of the furnace just below the top (Fig. 3). The fuel was fired downward from a position about 4 m above the furnace exit. For all tests reported here, the furnace wall temperature was set at 900~C to simulate a typical biomass combustor furnace exit gas temperature ahead of the superheaters. A silicon-carbide furnace wall liner was used.

Due to the restricted size of the pneumatic fuel-feed eductor inlet, the fuel as received was

sieved through a 10 mesh screen prior to loading in the feeder. This removed about 10% of the fuel by weight. Samples of the as-received, as-fired, and oversized fuel material were submitted for separate chemical analysis.

Two fuel samples were submitted to chemical fractionation; an analytical techniquC which sequentially leaches the fuel with water, ammonium acetate, and hydrochloric acid to determine the fraction of elemental mass which is water soluble, ion-exchangeable, and acid soluble. The technique is useful for characteriz- ing the mode of occurrence for ash elements. Water soluble and ion-exchangeable alkali (primarily potassium and sodium) are more readily volatilized and more likely to participate

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372 B.M. JENKINS et al.

in fouling of boiler heat transfer surfaces in the convection pass. Submitted for chemical frac- tionation were a sample of the humus as-fired, and a "simulated" sample of the original digester feedstock. The latter sample was made by combining typical weight fractions of individual digester feed components. Excluded from the simulated feedstock blend was dairy manure, which was added to the digester as an inocculant. As shown later, the sodium and chlorine concentrations in the humus were higher than in the simulated feedstock, possibly as a result of contributions from the dairy manure. Chemical fractionations of the simu- lated feedstock and humus samples were conducted to assess changes in readily available alkali and other elements due to the digestion and composting processes. Analyses of fuel, deposits and fly ash, and chemical fractiona- tions were performed by CONSOL Inc (Li- brary, PA).

The combustion gas composition was moni- tored via continuous analyzers by sampling the flow near the bottom of the furnace. An air-cooled sample probe with a ceramic nozzle

was inserted through the side of the furnace at the top of the lower unheated section. CO2, CO, NOx, and S02 concentrations were measured using non-diSpersive infrared (NDIR) analysers, 02 concentration by paramagnetic analyzer. The maximum CO concentration which could be measured was 720 ppmv, and this value was exceeded during part of the experiment. Analysis of total hydrocarbons was intended, but the analyzer response proved erroneous and accurate concentration measurements could not be made. An ash deposit was formed on the tip of the gas sample probe during the experiment. The deposit was collected and submitted separately for analysis.

Three stainless steel, air-cooled, tube-type deposit probes were inserted horizontally across the exit gas flow at the bottom of the furnace. One of the probes was continuously rotated through a cycle of two forward and two reverse revolutions. The other two probes were held stationary. The tube rotation is normally employed to optically monitor deposit compo- sition in situ using emission FTIR. The technique has not been fully developed for

Table 1. Composition and heating value of FBC fuels*

Humus Wood 50/50 blend

Dry blend Moist blend

Moisture (% wet basis) 12.51 9.53 Bulk density (kg m 3) 560 480 Ultimate analysis (% dry fuel)

Carbon 32.37 50.72 Hydrogen 3.83 5.99 Oxygen 26.91 42.49 Nitrogen 1.21 0.30 Chlorine 0.65 <0.01 Sulfur 0.39 <0.01

Proximate analysis (% dry weight) Ash (575°C) 34.64 0.50 Volatile 51.59 83.69 Fixed carbon 13.77 15.81

Higher heating value (MJ kg ~, dry 12.89 19.67 basis) Ash analysis (600°C) % Ash % Dry f0el

SiO2 41.50 14.38 A1203 17.56 6.08 TiO2 2.41 0.83 Fe203 5.38 1.86 CaO 14.87 5.15 MgO 3.65 1.26 K20 4.04 1.40 Na,O 3.94 1.36 SO3 1.56 0.54 P205 2.62 0.91 Undetermined 2.47 0.86

Humus ash fusion temperatures (C ° ) Oxidizing Reducing Initial deformation 1169 1093 Fluid 1254 1153

11.04 11.02 521 520

41.55 41.70 4.91 4.93

34.70 34.83 0.76 0.75 0.33 0.32 0.20 0.10

17.57 17.28 67.64 67.91 14.79 14.81 16.28 16.34

% Ash % Dry fuel % Ash 12.26 0.0613 41.08 41.07 2.83 0.0142 17.35 17.34 0.08 0.0004 2.38 2.38 4.24 0.0212 5.36 5.36

37.08 0.1854 15.19 15.20 5.86 0.0293 3.68 3.68

17.00 0.0850 4.22 4.23 3.16 0.0158 3.93 3.93

11.20 0.0560 1.70 1.70 1.86 0.0093 2.61 2.61 4.43 0.0222 2.50 2.50

*ASTM standard test methods: E870; moisture E871; bulk density E873; ultimate analysis D3176, E775, E776, E777, E778; proximate analysis D1102, E872; heating value E711; ash elemental D3682 (600°C); ash fusibility E953.

Page 7: Combustion of residual biosolids from a high solids anaerobic digestion/aerobic composting process

Combust ion of residual biosolids 373

biomass, and was not used during this experiment. The tube was rotated for compari- son with the stationary probes. A crown deposit, commonly seen on the upstream superheater tubes in commercial boilers, is not normally found on the rotating tube. The tube rotation provides a means of assessing the influence of different deposition mechanisms, particularly condensation, reaction and ther- mophoresis, in comparison with inertial im- paction) All probes were mounted in the same horizontal plane separated by a distance of about 20 ram. Probe surface temperatures were controlled by proximity to the furnace exit and held in the neighborhood of 450°C, typical of upstream superheater surface temperature in a biomass boiler. At the end of the experiment, the deposits from the rotating probe and one of the stationary probes were collected separately and submitted for analysis.

Fly-ash samples were collected from the flow near the bottom of the furnace through a water-cooled, helium-quenched nozzle. Fly ash was collected on 100 mm diameter, 1 /~m pore size polycarbonate filters. After sampling, large (mm size), loose particles representing partially burned fuel particles were segregated into a coarse fraction, with the fine fraction retained on the filter. The size separation between coarse and fine fractions reported later is rather arbitrary on this basis. A total of four fly-ash samples were taken. The third sample clogged very soon after insertion, possibly as a result of incorrect suction flow rate. The small amount of fine materia.l obtained was added into the fine filter deposit obtained on the second sample. The fine filter deposit from the fourth sample was also added to that of the second filter to increase the mass available for chemical analysis. The fine filter deposit from the first sample was submitted separately. In addition to the filter samples, particles exiting the bottom of the furnace were collected simply by inserting a stainless steel cup into the flow below the probes. Unlike the filter samples, no attempt was made to quantify the mass flow rate of these particles.

2. RESULTS AND DISCUSSION

2.1. FBC experiments

Fuel properties for the humus, wood, and blends used during the fluidized-bed combus- tion experiments are included in Table 1. The

blend properties are computed from the separate measured compositions of the humus and wood. The wood used, a white fir, was obtained as clean pulp chips and had very low ash (0.5% dry weight). The principal effect of blending the humus with wood was to increase the organic fraction and heating value of the fuel. The wood ash content was a factor of 70 less than that of the humus, and the changes in ash composition associated with addition of wood were insignificant. The humus had a high ash content, about 35% by weight dry basis. Most of this ash appeared to be in the form of adventitious material (sand and clays). The high alumina concentration in the humus ash supports this conclusion, as aluminum is toxic to plants in high concentrations and rarely observed above a few percent. In this case, much of the potassium was likely included in relatively unavailable forms as feldspars.

Particle size distributions (by sieve) for the humus and wood fuels as-fired are shown in Fig. 4. The humus fuel was considerably finer, with a median size roughly half that of the wood.

A summary of the results obtained from the FBC studies is reported in Table 2. Test results are segregated by fuel type, either humus alone or the humus-wood blend. The humus only results are composites of the first test and the early portion of the second test. A large variation in bed and freeboard temperature was observed when firing humus alone.

The low temperatures were attributed to: (i) low heating value of humus (11.3 MJkg -~ as-fired compared with 17.8 MJ kg -~ as-fired

Humus . . . . . Wood

100.0

90.0 ~ - ~ "~ =~ 80.0 == 70.0 jr" ~= 60.0

= so.o =~ 40.0 /

30.0 E = 20.0 O

10.0 [ / /

0.0 /

0 1 2 3 4

Size (mm)

Fig. 4, Particle size distributions for humus and wood fuels as burned in the FBC.

Page 8: Combustion of residual biosolids from a high solids anaerobic digestion/aerobic composting process

374 B.M. JENKINS et al.

Table 2. Summary of FBC results

Range Average

humus blend humus blend

Fuel flow rate g s- ' 2-6 4-6 4 5 Air/fuel ratio g s- ' 2-10 4-5 5 5

Temperature Lower bed °C 500-700 700-800 600 750 Lower freeboard "C 500-900 800-900 650 850 Upper freeboard °C 300-700 450-650 500 600

Pressure drop Main bed kPa 1-6 2-4 3.5 2.5 Loop seal bed kPa 0-3 0-3 1.0 2.0

Gas analysis CO2 % v/v 2-13 15-17 14"* 16 NOx (as NO2) ppmv 20-100 10-100 50 50 SO2 ppmv 1-6 2-9 3 6 O_~ % v/v 5-15 2-5 7** 3 CO % v/v 1.9-2.1 2-4 2 3 THC* (as CH0 ppmv 950-2500 3000-7500 1725 5800

Emission factort CO ppmv 0.29 THC (as CH4) ppmv 0.03 NOx (as NO.,) ppmv 0.004 SO., ppmv 0.00008

*Total hydrocarbons, by GC. **average unknown due to failure of gas sample line, estimated from measured air/fuel ratio, rEmission factor = mass of pollutant emitted per unit mass of dry fuel.

for the wood); (ii) slugging of the loop seal bed media; and (iii) excessive fuel ash accumulation in the main reactor bed. The latter two conditions were corrected when firing humus alone during the second test, but' the low heating value again resulted in excessively low tempera- tures at sufficiently high fluidizing velocities. A smaller bed grain size was not available and lower velocities were not tested. When the humus/wood blend was introduced in the second test, the temperature fluctuation dimin- ished and the reactor-bed temperature could be well controlled between 800 and 900°C in the upper bed and lower freeboard regions. In contrast to humus alone, the temperature control was considerably more stable when firing the blend. Temperature profiles along the reactor are illustrated in Fig. 5 for two separate time periods representing humus alone and the humus-wood blend during the second exper- iment.

Gas compositions for both fuel types are also shown in Table 2. CO2 concentrations were reduced when firing humus alone due to the higher air/fuel ratios resulting from the un- steady operation. NOx concentrations varied primarily as a result of unsteadiness in the fuel feed rate, especially on start-up or during transition to the blend in the second test. Following stabilization on the blend, gas

composition remained relatively constant. The high CO and hydrocarbon concentrations while firing the blend indicate the lower air/fuel ratio and the lack of complete burn-out for this experiment. Although carbon retention in the bed appeared to be low upon draining, fine particle carbon captured at the wet scrubber was consistent with the higher CO and hydrocarbon

- - x - - Humus o- - - - Blend

7:

:z

.X

X \

I x \

x I

x I

x I

X

o 1

300 400 500 600 700 800 900

J

T e m p e r a t u r e ( °C)

Fig. 5. Vertical temperature profiles along FBC when firing humus alone and humus blended with wood. Temperature depression at 3.2 m height when firing humus alone due to secondary air injection. Height measured above top surface

of distributor plate.

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Combustion of residual biosolids

Table 3. Composition and heating value of MFC humus fuels

375

Sample description As-fired As-delivered Oversize Simulated digester feedstock

Moisture (% wet basis) 18.43 Ultimate (% dry fuel)

Carbon 27.99 30.47 28.64 45.53 Hydrogen 2.68 3.17 2.9 5.18 Nitrogen 1.39 1.44 1.37 1.22 Chlorine 0.617 0.681 0.463 < 0.02 Sulfur 0.32 0.26 0.24 0.12

Proximate (% dry fuel) Ash 46.91 49.18 51.11 8.29 Volatile 42.40 42.64 45.12 76.23 Fixed carbon 10.69 8.18 3.77 15.48

Heating value (MJ kg ~, dry basis) 10.53 10.40 10.07 17.25 Ash (% ash as oxide)

SiOz 51.84 49.02 56.3 36.89 AI20~ 10.83 11.64 9.77 19.95 TiO,_ 1.36 1.55 1.06 I. 16 Fe_,O~ 8.72 8.70 9.55 2.25 CaO 11.57 12.81 9.64 23.78 MgO 3.60 3.78 3.22 3.53 K~O 2.22 2.47 1.93 2.84 Na_,O 2.60 2.64 2.29 3.60 P.,O5 2.37 2.62 1.91 2.58 SO~ 1.33 1.44 1.43 2.33 Undetermined 3.56 3.33 2.90 1.09

emissions. Secondary air inject ion into the f reeboard was not employed with the blend in o rde r to ma in ta in sufficiently high t empera tu res to evalua te po ten t ia l agg lomera t ion .

Emiss ion factors (mass o f po l lu tan t emi t ted per unit mass dry fuel burned) are shown for the s teady ope ra t ing per iod when firing the blend. As indica ted above, the emissions for CO and h y d r o c a r b o n s are high due to the near s to ichiometr ic condi t ions (blend stoichio- metr ic air / fuel ra t io = 4.4, for humus only, s to ichiometr ic air /fuel ra t io = 3.4), much higher than expected for commerc ia l units. N i t rogen

emit ted as NOx accounts for only a b o u t 2% o f fuel ni t rogen, as does sulfur emi t ted as SO_,,

Post- tes t inspect ions o f the reac tor on both exper iments revealed no bed media agg lomera - t ion or ash slagging. Dur ing opera t ion , the bed remained free flowing wi thout evidence o f s intering or agg lomera t ion for bo th humus a lone as well as the blend. The high ash conten t o f the humus required a high bed-dra in rate due to the low e lu t r ia t ion o f fuel ash f rom the bed. Dra in rates dur ing the second test were kept very close to the fuel-ash feed rate in o rde r to ma in ta in main bed pressure d rop cons tant .

Table 4. MFC deposit compositions

Sample description Rotating probe Stationary probe Gas sample probe nozzle

Ultimate (%) Carbon 0.13 Hydrogen < 0.01 Nitrogen 0.02 Chlorine Total sulfur 1.48 0.97 7.44

% as oxide SiO, 28.75 30.61 19.73 AI_,O~ 11.89 12.66 8.91 TiO2 2.01 2.13 1.54 Fe,O3 11.41 11.81 7.86 CaO 16.46 17.32 13.89 MgO 4.09 4.35 3.37 Na,O 3.59 3.3 5.71 K_,O 7.74 6.1 11.98 P:O5 3.72 3.94 3.2 SO, 3.7 2.43 18.59 Undetermined 6.64 5.35 5.22

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376 B. M. JENKINS et al.

Table 5. MFC fly-ash compositions

Filter No. 1 1 2,3,4 2 4 Cup Fraction Fine Coarse Fine Coarse Coarse Coarse

Ultimate (%) Carbon 32.72 Hydrogen 1.99 Nitrogen 0.25 Chlorine Sulfur 0.93 0.58 5.12

% as oxide SiO: 41.62 18.33 15.02 A1203 8.48 3.02 4.90 TiO~ 0.51 0.25 0.88 Fe203 4.80 1.99 4.65 CaO 9.60 4.52 7.83 MgO 2.55 1.13 1.99 Na20 2.81 1.34 6.98 K20 6.13 2.64 18.30 P_,Os 1.13 0.47 1.91 SO3 2.32 1.46 12.80 Undetermined 20.05 64.85 24.74

12.81 12.22 10.39 1.46 0.84 0.37 0.52 0.53 0.49 0.45 0.34 0.31 0.30 0.21 0.16

24.71 34.08 48.73 5.36 7.61 8.70 0.76 0.79 1.02 4.21 5.04 7.59 7.04 8.28 8.38 1.95 1.96 2.99 1.41 2.03 2.09 1.88 1.90 2.08 1.33 1.34 1.62 0.76 0.59 0.33

50.59 36.38 16.47

2.2. MFC experiment

Chemical analyses of the fuels, fly-ash samples and deposits from the MFC experiment are listed in Tables 3-5. Results of chemical fractionation of the simulated digester feedstock and the as-fired humus fuel are shown in Figs 6 and 7, respectively. The humus fuel fired in the MFC had a higher ash content (47% ash) and correspondingly lower heating value than the humus fuel fired in the FBC (35% ash). The reason for the difference is presumably related to additional screening of the fuel done prior to delivery for the purposes of achieving a fine enough grain size for the pneumatic feed eductor on the MFC. The additional screening through.the 10 mesh sieve done just prior to the MFC test caused a reduction in the ash content of the as-fired fuel compared with the delivered fuel (Table 3). The higher ash content (51%) of

the oversized fuel particles can also be seen in Table 3. The reason that the larger particles have higher ash content may be due to the rather high sand content of this fuel. The humus had a substantially higher ash content than the simulated digester feedstock (8%). It is also higher in ash than the anticipated ash content for the simulated feedstock based on the fractional conversion (roughly 50%) of organic matter in the anaerobic/aerobic processes. The ash of the humus contained large amounts of silica, alumina and calcium, indicating the presence of soil and sand particles, most likely as a result of contamination stemming from the use of scraped dairy manure as digester inocculant. The chlorine concentration in the simulated feedstock is quite low in comparison with the concentrations in the humus fractions. The source of chlorine in the humus is likely the dairy manure (including urine, possibly as alkali

100%

90%

80%

70%

60%

50%

40%

39% 20%

l o %

0%

II II

0 0 0 0 03

o o 0 m ~ N o ~E Z

c~ o o

D,,

Fig. 6. Chemical fractionation results for simulated digester feedstock.

Page 11: Combustion of residual biosolids from a high solids anaerobic digestion/aerobic composting process

100%

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

Combustion of residual biosolids

kr [] Residual HAeid So ub e !m Ion Exchangeable ]g Water Soluble

0 0 0 0 m m ~ ~ 0 0

Fig. 7. Chemical ffactionation results for as-fired (MFC) humus fuel.

377

chlorides) that was periodically fed to the digester.

Total fuel consumption was 10.8 kg over 193 min actual running time, giving an average feed rate of 0.93 g s -~. The total deposit mass collected on the rotating probe was 416 mg. For the stationary probe from which deposit was collected, the total mass was 169 mg. The stationary probe insertion length was about half that for the rotating probe, and no deposit formed on the under-side of the stationary probes. These two factors account for the difference in mass on the stationary and rotating probes. When normalized by the probe insertion length and the total time, the rates of deposition were about 0.2 mg m -t s ~ for both probes. On the basis of the active deposit area, however, the rates were 4.8 and 7.8 mg m-2s ~ for the rotating and stationary probes, respectively, the latter indicating the larger build-up on the top surface and the lack of deposit formation on the rear. As expected, the deposit on the rotating probe was rather uniform in height. The stationary probe deposit may have, over additional time, developed more vertical struc- ture similar to the types of deposits seen on superheater surfaces in commercial boilers, where typically an aerodynamic wedge extends upstream from the front surface of the tube. On the basis of the total ash feed (fuel feed

Table 6. Emission factors ( - - ) for measured species at two equivalence ratios (¢), MFC experiment (THC not

measured due to instrument failure)

q~ = 0.3 4) = 0.85

c o 0:0025 so: 0.002 0.005 NOx (as NO_,) 0.01 0.001

~'Instrument range exceeded.

multiplied by the ash fraction of the fuel), and the ratio of probe projected area to furmice exit area, the probes collected about 0.1% of available ash (assuming uniform distribution of ash across the furnace and ignoring aerodynam- ics of the flow)• Whether or not the results are representative of deposition behavior in a commercial boiler is difficult to predict. Confounding the analysis here was the heavy impingement on the probes of the substantial sand fraction in the fuel (comprising much of the ash), which abraded the deposit as it formed. The extent to which this would occur in a commercial boiler depends on the boiler design. In most instances, large particle abrasion would not occur, although abrasion of deposits and tube erosion is common due to smaller particle entrainment.

Oxygen concentration in the exhaust gas was varied from approximately 3 to 15%, represent- ing equivalence ratios ranging from 0.85~).17 (stoichiometric air/fuel ratio = 2.6), by slowly changing the fuel-feed rate. Instantaneous fuel mass flow rates were derived via carbon balance and checked against batch fuel-feed rates measured gravimetrically. With some fraction, xc, of the fuel carbon converted to CO,, ignoring the mass in CO and hydrocarbons as small, the moist fuel feed rate was computed as:

m l : m . i ~ t =

W.o ~-] ((1 - M ) y . ~ + M)Cco:

1 - (1 - l~l)xcy~

m,,Cco, w~w~,. x

(1 -- M)xcyc

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378 B.M. JENKIYS et al.

where M is the fuel moisture content, wet basis, the W, are the molecular weights, the y, are the mass fractions of elements, i, in the dry fuel, rh,, is the mass flow rate of combustion air, and Cco2 is the CO2 concentration in the furnace exit gas, dry basis. A value x c = 1

yields average feed rates close to those measured for each batch (ranging from 0.77-1.45 g s -~ on average), although fly-ash samples reveal some carbon in the coarse, partially burned particles (Table 5). Rather large undetermined fractions in the compositions of fine fly ash also suggest some unreacted carbon, although the appearance (white color) of this fine particle fraction was not consistent with large carbon concentration, unless present as carbonates.

Quite visible on the deposit probes was a white material, seen also in the fine fly-ash fraction, which appeared to condense from the combustion gas. The material appeared not only on the probe surfaces but all other surfaces in the vicinity of the furnace exit. The humus fuel produced a great deal more smoke than other biomass fuels tested in the same furnace. The composition of this material was not specifically determined due to insufficient mass, but fly ash analyses suggest that much of it is composed of alkaline-sulfates.

Emission factors for gas species are listed in Table 6. These are given for two values of the equivalence ratio, q~, equal to the ratio of the stoichiometric air fuel ratio to the actual air/fuel ratio (~b = AFs,o~MAF). At around 0.3 equival- ence ratio (AF = 9, moist basis), the CO emission factor averaged about 0.0025, which is similar to the SO2 emission factor (0.002) at this equivalence ratio. At 0.85 equivalence ratio, the upper range (720 ppmv) of the CO analyzer was exceeded. The NOx emission factor at the low equivalence ratio is of the order of 0.01, or about 25 times larger than that from the fluidized-bed, and 10 times larger than the emission factor at ~b---0.85 in the MFC. Element balances show NOx to account for 25% of fuel nitrogen at ~b = 0.30, and 3% of fuel nitrogen at ~b =0.85. This trend in NOx emission is consistent with other fuels and suggests most of the NOx arises from conversion of fuel nitrogen. ~3 ~5

The SO2 emission factor was also much higher from the MFC compared with the FBC. The lower values from the FBC are likely the result of sulfur absorption in the wet scrubber. The MFC employed no scrubbing of the combustion

gas. Sulfur balances for the MFC account for 26% of fuel sulfur as SO, at ~ = 0.3, and 64% as SO, at 4, = 0.85. The lower SO2 concen- tration at lower equivalence ratio in the MFC might be the result of greater alkaline capture at lower flame temperatures as the fuel-feed rate was reduced. Sulfur concentrations in fly ash account for between 15-60% of fuel sulfur depending on sample.

Results of the chemical fractionation tests are summarized in Figs 6 and 7 for the simulated digester feedstock and the as-fired humus fuel, respectively. Shown are the fractions removed by leaching sequentially with water, ammonium acetate (for ion exchangeability), and hydro- chloric acid. The residual fraction is that remaining after the final acid leach. The simulated digester feedstock results are fairly typical of biomass fuels, with silica, alumina and titanium essentially inert, a large fraction of iron being acid soluble, calcium and magnesium ion exchangeable, and sodium, potassium, phos- phorous and sulfate being water soluble or ion exchangeable. The alkali metals Na and K both yield 70% or better extraction by water alone. The high residual fraction of magnesium is somewhat unusual, perhaps indicating the presence of feldspars or other soil-derived material.

The humus fuel fractionates quite differently compared with the digester feedstock. A lower fraction (about half that of the original feedstock) of the alkali is water soluble, although the total water soluble and ion exchangeable potassium remains high ( > 80% for K and 60% for Na). Much less of the calcium is ion exchangeable, as is the case with phosphate and sulfate. These trends suggest the presence of large amounts of feldspars and other soil materials in the ash of the humus fuel compared with the original digester feedstock. However, the amount of readily available alkali and sulfate implies that fouling may still be a problem with humus fuel. Concentrations of water soluble K20, S, and Na20 are 0.43, 0.11 and 0.46% of the dry humus, respectively. By comparison, the simulated digester feedstock contains 0.17, 0.06 and 0.24%, respectively. Concentrations of readily available (water soluble) alkali and sulfur are, therefore, about twice as high in the humus compared with the original digester feedstock. The high concen- tration of chlorine in the humus, possibly from the manure as indicated earlier, is also troublesome from a fouling perspective.

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Combustion of residual biosolids 379

Chlorine is thought to facilitate the formation of alkali deposits, and contributes to corrosion.

One implication of these results is that very little leaching or extraction of water soluble forms of elements takes place in the digester or composter. The high solids digester has no liquid effluent stream. Any internally leached materials are, therefore, retained in the solids passed to the aerobic composter and eventually into the humus. Loss of organic matter through volatilization enriches the ash constituents in the humus, without particular changes in the total water soluble quantities, if not volatilized along with organic matter. Reactions among components likely occur in the digestion or composting steps, but differences between the composition of the simulated digester feedstock and what was actually fed to the digester preclude any strong conclusions in this regard.

The relative abundances (deposit/fuel concen- trations) of oxides in the deposits compared to the as-fired fuel ash are shown in Fig. 8. The deposit compositions are enriched in potassium and sulfur and depleted in silicon compared with the fuel ash. Concentration ratios for other elements are near unity, indicating a capture of fuel ash in addition to selective deposition on the probes. The deposit accumulated on the ceramic gas sampling probe nozzle is distinc- tively enriched in what appears to be a

potassium-sulfate. The difference in compo- sition between the gas sampling nozzle and the deposit probes is likely due to the differences in surface temperatures and possibly to the location and size of the probes. The gas probe nozzle temperature runs close to furnace temperature. The smaller gas sampling nozzle, situated directly in the furnace, may have been subject to greater abrasion of less tenacious materials and was, thus, enriched in sulfated particles.

The undetermined fractions of the deposit compositions are about 5% by weight. Carbon- ate (not analyzed) may account for some of this, but the low carbon concentration found in the rotating probe deposit (Table 4) suggests carbonate cannot account for the entire undetermined fraction. Sample sizes were not large enough to analyze chlorine, although it also may be present in the deposits.

Fly ash compositions (Table 5) in general support the observations concerning the deposit compositions. The undetermined fractions are, however, in general quite large. The fine fly-ash fractions were of insufficient mass to analyze for carbon. Carbon in the coarse fractions accounts for one-fourth to two-thirds of the undeter- mined mass, still insufficient to close the mass balance. Chorine, where analyzed, also does not close the balance.

[ ] Rotating probe [ ] Stationary probe [ ] Gas probe

16

14

12

o u ,-' I 0 ¢= '1o c

8 m

w

4

2

0 e~ m ~ r~ 0 0 0 0 m e~ 0 0 0 0 m ~ c~ ~ 0 0 m ~ I- • Z ,,

Fig. 8. Abundance of oxides in MFC deposits relative to fuel-ash composition. Rotating and stationary probes are air-cooled probes simulating boiler superheater tubes; gas probe refers to deposit collected on

ceramic gas sampling nozzle at near furnace temperature.

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380 B. M. JENKINS et al.

[ ] Coarse [ ] Fine

14

12

O

" 10 I " D ¢ -

8

6 .~_

ee 2

0 l i ~ l l l ~ t l i ~ l l t ~ l l i ~ E l l t ~ ; i I I ;ll[~'/zlt e~ ,N ~ 0 0 0 0 m e~

0 0 0 0 m m ~ ed 0 0 - - • Z Q.

Fig. 9. Abundance of oxides in MFC fly-ash filter fractions relative to fuel-ash composition. Normalized concentrations of fly ash from filter number 2 (coarse fraction) and composited fine fractions from filters

2, 3 and 4.

Fly-ash compositions were normalized to the total reconstructed mass and compared by species to the fuel-ash composition. The relative oxide abundances for the coarse fraction of filter sample 2 and the composited fine fraction from filters 2, 3 and 4 are shown in Fig. 9. The coarse fraction is not remarkably different from the fuel ash in composition. 3 The fine fly-ash fraction is enriched in alkali species and sulfate, a result consistent with the deposit analyses. Similar results were observed for the coarse fractions from other samples except filter 1. This filter sample shows about equal enrichment (three times) of potassium and sulfate in both the coarse and fine fractions. Such enrichment is missing from coarse fractions of other samples, in particular the sample for which only coarse material was collected (cup). The difference in the case of the first filter sample may simply be the result of inadequate separation of fine and coarse fractions. In any case, the fine fractions from fly-ash samples appear enriched in alkali sulfate, a result consistent with the composition of deposits collected on the probes. The trend for alkali and sulfates to be concentrated in the fine fraction of fly ash is also consistent with other combustion systems. If the sulfates are assumed to arise from alkali vaporization, condensation and subsequent sulfation, there is much greater surface area for condensation and reaction on fine particles than on larger ones. The sulfates, when formed this way, also tend to bind

particles together and give deposits greater tenacity, cf. the enrichment on the smaller gas sampling probe nozzle. Alkali-sulfate depo- sition may, therefore, be anticipated when burning humus fuel at similar temperatures. Rates of deposition under full-scale conditions remain to be determined.

3. CONCLUSIONS

FBC experiments suggested no substantial bed agglomeration potential associated with the humus alone or blended. This conclusion is rather weak for the case of humus alone, because of the unsteady combustion with this fuel resulting predominantly from its low heating value and correspondingly low heat release rate. When blended with wood fuel at 50% by weight and fired at more typical bed temperatures (800-900°C), the humus did not exhibit any tendency to agglomerate the bed, although the amount of humus available was limited and the test was rather short (2 h). The results suggest, however, that where bed let-down (draining) is routinely employed, agglomeration is not likely to be a substantial problem. Let-down rates are likely to be high due to the high ash content of this fuel.

Chemical fractionation tests reveal that the fraction of readily available (water soluble) alkali is reduced in the humus fuel compared with the simulated digester feedstock, although the fraction of total water soluble plus ion

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Combustion of residual biosolids 381

exchangeable alkali remains nearly the same. However, total alkali in the humus (2.26% dry fuel) was elevated above that in the digester feedstock (0.53% dry fuel), as was water soluble alkali (0.89% in humus, 0.41% in digester feedstock). The simulated feedstock did not include dairy manure that was fed intermittently to the digester, and direct assessments of the influence of the digestion/composting system on elemental compositions cannot be made. Much of the alkali in the fuel burned in the multi-fuel combustor (MFC) was likely present as feldspars and other soil particles, and not available to participate in deposition other than by inertial impaction and sticking. However, deposits and the fine fractions of fly-ash samples were enriched in an alkali-sulfate, visually apparent as a fine textured white material. Deposition of alkali-sulfates can, the'refore, be anticipated under full-scale conditions, although the rates of deposition have yet to be established. Deposition rates measured in the MFC were likely influenced by sand/ash impaction and particle induced abrasion.

For commercial use, the humus would perhaps best be fired in combination with another more conventional fuel, such as wood. At small blending ratios the humus may be an acceptable fuel, although increased fuel rates would be required to maintain capacity given a reduction in the heating value of the fuel blend. The high pollutant emission rates observed during the FBC experiments with blends may be due partly to the humus, but the operation was by no means optimized and secondary air inadequate most of the time (in an attempt to retain high freeboard temperatures). At smaller blending ratios and in better controlled facilities (e.g. commercial power boilers), CO and hydrocarbon emissions should be reduced substantially. NOx and SO2 emission rates are similar to other biomass fuels on the basis of the nitrogen and sulfur concentrations. The NOx emission factors and trends in emission rate with varying equivalence ratio are consistent with fuel nitrogen as the predominant source of NOx.

Acknowledgements--The work reported was funded under a research contract from the California Energy Commission. The authors are grateful for this support. The authors acknowledge and appreciate the assistance of S. Hardy and D. Rich in conducting the FBC experiments, and G. Sclippa in conducting the MFC experiments. The assistance of P. Rosendale and V. Conrad from CONSOL Inc is also gratefully acknowledged.

REFERENCES

1. Kayhanian, M., Lindenauer, K., Hardy, S. and Tcho- banoglous, (3., Two-stage process combines anaerobic and aerobic methods. BioCycle., 1991, 32, 48 53.

2. Kayhanian, M., Lindenauer, K., Hardy, S. and Tchobanoglous, G., The recovery of energy and production of compost from the organic fraction of MSW using the high-solids anaerobic digestion/aerobic biodrying process. Final report prepared for California Prison Industry Authority, Folsom, CA, 1991b.

3. Kayhanian, M. and Tchobanoglous, (3., Innovative two-stage process for the recovery of energy and compost from the organic fraction of municipal solid waste (MSW). Water Sci. Tech., 1993, 27, 133-143.

4. California Legislature, Assembly Bill 939. Chapter 1095, Statutes o[" California. Legislative Council, Sacramento, CA. 1989.

5. Miles, T. R., Miles, Jr., T. R., Baxter, L. L., Bryers, R. W., Jenkins, B. M. and Oden, L. L., Alkali deposits found in biomass power plants - - a preliminary investi gation of their extent and nature. Summary report for the National Renewable Energy Laboratory, NREL Subcontract TZ-2-11226-1, Golden, CO, 1995. Also L. L. Baxter, T. R. Miles, T. R. Miles, Jr., B. M. Jenkins, D. C. Dayton, T. A. Milne and R. W. Bryers, The behavior of inorganic material in biomass-fired power boilers--field and laboratory experiences. Alkali De- posits Found in Biomass Power Plants, Vol. II. National Renewable Energy Laboratory, Golden, CO, 1996.

6. Baxter, L. L., Ash deposition during biomass and coal combustion: a mechanistic approach. Biomass and Bioenergy, 1993, 4, 85 102.

7. Baxter, L. L., Miles, T. R., Miles, Jr., T. R., Jenkins, B. M., Richards, (3. H. and Oden, L. L., Transformations and deposition of inorganic material in biomass boilers. In Second Int. Con[~ on Combustion Technologies for a Clean Environment, ed. M. G. Carvalho, l, Biomass II, pp. 9-15, Commission of European Communities, Lisbon, Portugal, 1993.

8. Jenkins, B. M., Baxter, L. L., Miles, T. R., Miles, Jr., T. R., Oden, L. L., Bryers, R. W. and Winther, E., Composition of ash deposits in biomass fueled boilers: results of full-scale experiments and laboratory simu- lations. ASAE Paper No. 946007, ASAE. St. Joseph, MI, 1994.

9. Miles,T. R., Miles, Jr., T. R., Baxter, L. L., Jenkins, B. M. and Oden, L. L., Slagging problems with biomass fuels. First Biomass Conference of the Americas, Burlington, VT, 1993.

10. Miles, T. R., Miles, Jr., T. R., Bryers, R. W., Baxter, L. L., Jenkins, B. M. and Oden, L. L., Alkalis in alternative biofuels. FACT 18, Combustion Modeling, Scaling, and Air Toxins, pp. 211-220, ASME, New York, 1994.

11. Salour, D. and Jenkins, B. M., Thermal conversion of rice straw in a pilot-scale fluidized bed reactor. Final Report, Contract #500-86:013, California Energy Commission, Sacramento, CA, 1994.

12. Salour, D., Jenkins, B. M., Vafaei, M. and Kayhanian, M., Control of in-bed agglomeration by fuel blending in a pilot-scale straw and wood fueled AFBC. Biomass and Bioenergy, 1992, 4, 117-133.

13. Jenkins, B. M. and Baxter, L. L., Uncontrolled pollutant emissions from biomass combustion under simulated boiler furnace conditions. AFRC-JFRC Pacific Rim Int. Conf. on Environmental Control o[" Combustion Processes. Maui, Hawaii, October, 1994.

14. J.H. Seinfeld. Atmospheric Chemistry and Physics of Air Pollution. John Wiley and Sons, New York, 1986.

15. Bowman, C. T., Chemistry of gaseous pollutant formation and destruction. In Fossil Fuel Combustion. eds W. Bartok and A. F. Sarofim. Chap. 4, John Wiley and Sons, New York, 1991.