Scale-up study on combustibility and emission formation with two biomass fuels (B quality wood and pepper plant residue) under BFB conditions

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32 (2008) 1311 – 1321

Available at www.sciencedirect.com

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Scale-up study on combustibility and emission formation with two biomass fuels (B quality wood and pepper plant residue) under BFB conditions Atif Ahmed Khana,, Martti Ahob, Wiebren de Jonga, Pasi Vainikkab, Peter Johannes Jansensa, Hartmut Spliethoffc a

Department of Process and Energy, Section Energy Technology, Faculty 3ME, Delft University of Technology, Leeghwaterstraat 44, NL-2628 CA, Delft, The Netherlands b VTT Processes, P.O. Box 1603, 40101 Jyva¨skyla¨, Finland c TU Munich, Lehrstuhl fu¨r Thermische Kraftanlagen, Boltzmannstraße 15, D-85748 Garching, Germany

art i cle info

ab st rac t

Article history:

Combustion of two biomass fuels: demolition wood (DW) and pepper plant residue (PPR),

Received 21 April 2007

was investigated from an emission viewpoint in a 20 kWth fluidized bubbling bed reactor

Received in revised form

and a 1 MWth fluidized bubbling bed test boiler. Fluidization velocity and boiler output were

23 March 2008

varied in the larger facility whereas they were kept constant in the smaller reactor.

Accepted 27 March 2008

Traditional flue gases were analyzed. In addition, impactor measurements were carried out

Available online 27 May 2008

to determine the mass flow of the finest fly ash and toxic elements. These measurements

Keywords: Fluidized bed Biomass Toxic emissions Particulates Heavy metals Capsicum annuum

were compared with EU emission directives for biomass co-incineration. It was possible to combust DW without operational problems. However, the DW was contaminated with lead, which tended to get strongly enriched in the fine fly ash. Pb tends to be adsorbed on the measurement line surfaces stronger than many other toxic elements and therefore proved difficult to collect and measure. Enrichment of Pb in the fine fly ash can be weakened by cofiring DW with PPR. Increasing the share of PPR up to 50% markedly reduces the toxic metal concentration in the finest fly ash. This, however, leads to increased mass flow of fine fly ash and increases the potential risks of operational problems such as bed agglomeration and fouling. & 2008 Elsevier Ltd. All rights reserved.

1.

Introduction

Biomass combustion or co-combustion with fossil fuels can significantly reduce CO2 emissions from energy production. Compared with the combustion of hard coal, the CO2 emissions can be reduced by 93% [1] with biomass combustion. In addition, biomass (co)-combustion also brings an additional advantage by greenhouse gas mitigation by avoiding CH4 release from the otherwise landfilled biomass residues (sewage sludge, manure, etc.). Most biofuels contain

lower sulfur and nitrogen contents, and additionally, the alkaline ash from biomass captures some of the SO2 produced during co-combustion. Blending of low-cost biomass fuels can also result in the reduction of fuels costs [2–5]. However, biomass utilization in energy production is not free from problems. The consistent supply of biomass and its heterogeneous and variable composition offer challenges. The importance of emission control has increased sharply due to increased need of energy from combustion. The community has reacted with stricter environmental norms

Corresponding author. Tel.: +31 15 2786987; fax: +31 15 2782460.

E-mail addresses: [email protected], [email protected] (A.A. Khan). 0961-9534/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2008.03.011

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putting additional pressure to research. NOx (mainly NO) and CO are the most important gaseous pollutants with emission limits during biomass combustion. NOx emissions are strongly dependent on fuel-N content and air staging [6–8]. CO emissions can be controlled with the fuel air ratio and other operating parameters (combustor load, fluidization velocity, mixing, etc.). The particulate emissions in large power plants are usually well under control because those units have been equipped with effective dust separators like electrostatic precipitators (ESP) or bag house filters (BHF), which can keep dust emissions below the emission limits. However, these emissions are of major concern for smaller units (up to 10 MWth) as the capital costs of ESP and maintenance costs of BHF are high. Because of the increased interest to burn CO2-neutral fuels in heating and power plants, research to apply suitable combustion technologies (bubbling and circulating fluidizing bed, grate firing, pulverized fuel) has become more intensive in the recent years. Among the available technologies, fluidized bed combustion (FBC) has been regarded as one of the most efficient [9–11]. FBC technology offers a number of advantages like lower environmental impact, high combustion efficiency, etc. but its capacity in terms of fuel flexibility gives it the competitive edge over others. However, several operational problems should be solved with demanding biomass (such as bed agglomeration, slagging, fouling and corrosion of the heat transfer surfaces). These problems originate from particular elements like alkalies and chlorine. The sources of demolition wood (DW), also called B quality wood (BQW) include timber yards, packaging and scrap furniture. In consequence, DW can include paints, wood preservatives and fire retardants with toxic elements and compounds. Paints can include lead (Pb), cadmium (Cd) and other toxic metals. Preservatives that increase the resistance of the wood to decay, insects, and marine borers include creosote, pentachlorophenol, copper-8-quinolate, copper naphthenate and borate, and salt compounds made with

copper, chromium, arsenic and zinc. Chemicals used as fire retardants include borates, phosphates, ammonium sulfate and resins (melamine, urea and dicyanodiamide). The pepper plant residue (PPR) is the residue woody stems of the capsicum pepper plant. Capsicum pepper here refers primarily to Capsicum annuum L. It is an herbaceous annual crop species that reaches a height of 1 m and has glabrous or pubescent lanceolate leaves, white flowers, and fruit that varies in length, color, and pungency depending upon the cultivar. Native to America, this plant is cultivated almost exclusively in Europe and the United States, and The Netherlands is one of the large exporters of pepper and chillies in Europe [12–14]. The goal of the whole investigation was to detect and solve possible operational problems (incomplete burnout, bed agglomeration, fouling and slagging) as well as emission issues associated with the co-combustion of two Dutch fuels: DW and PPR. This article focuses on (1) the effect of fuel and combustion parameters (air staging ratios, temperature profiles, etc.) on gaseous emissions (CO and NO) and (2) particulate emissions by characterizing the fly ash. Experiments were conducted using an electrically stabilized 20 kWth BFB pilot reactor (VTT, Finland) and a 1 MWth experimental BFB boiler (TU Delft, Netherlands).

2.

Experimental studies

2.1.

Description of the combustor setups

2.1.1.

TU delft, 1 MWth bubbling fluidized bed boiler

Stack

The experimental bubbling fluidized bed boiler (1 MWth—max), designed for the combustion of solid fuels, (Fig. 1) was engineered and constructed by the Dutch company Crone. The vertical fluidized bed section consists of a vessel with a square geometry existing of two parts: the bed and the freeboard zone. The fluidized bed is cement-lined and has an

Sc

r

ew

c

v on

ey

Natural gas pilot burner

or Crone Boiler Freeboard

Biomass feed

Cyclone

Bed

Fan Fig. 1 – Schematic picture of the TU Delft’s 1 MWth BFB experimental setup.

ARTICLE IN PRESS BIOMASS AND BIOENERGY

area of 1.1 m2 and its total length (including the bed) is approximately 2.36 m. The bed zone has a distribution plate consisting of a large number (1 0 0) of air tuyeres, which ensure an even distribution of the combustion air into the bed. Secondary air is introduced in the splashing zone with welldistributed air nozzles (396 holes of 4 mm diameter uniformly distributed over a skeleton of 40 air pipes). The fluidized bed section also has a moveable internal heat exchanger that can be inserted into the bed zone. In its top position, the distance between heat exchanger and bottom air distributor is 47 cm. Downstream of the vertical freeboard section, a horizontal boiler part is situated that contains the heat exchanger, which is of the shell and tube type. The flue gas line was equipped with a cyclone to remove the coarse fly ash (cyclone cut size 10 mm) from the flue gas. The setup is a hot water boiler typically designed for greenhouse heating. The heat was transferred with two external coil heat exchangers to a cooling tower. The designed feeding equipment enabled to burn three solid fuels simultaneously. Options for both top and side feeding existed. However, the former was used to achieve better fuel distribution. A probe and an analysis system was set up to study the particles in the flue gas downstream of the cyclone. The fine particulate matter was first extracted isokinetically from the duct and then led to the fly ash measuring equipment. An 11-stage Pilat Mark 5 cascade impactor was used to collect the fly ash for elemental analysis and to determine the size distribution in the range of 0.2–30 mm. The aerodynamic particle size distribution was measured with glass fiber substrates and a back-up filter. The back-up 47 mm (diameter) glass fiber filters were used to measure the total mass concentration of the fine fly ash fraction. The substrates and filter were gravimetrically analyzed to characterize the size distribution and particle loading. CO, CO2 and NO were measured continuously during the experiments with NDIR and O2 with a paramagnetic method. FTIR measurements of SO2, HCl, HCN, NH3, NO2, N2O, H2O and some hydrocarbons in the flue gas were conducted with a resolution of 0.125 cm1. For the quantification of condensable and water soluble species, heated Teflon tubing was used, which was kept at 150 1C. The gas cell temperature was 150 1C.

2.1.2.

VTT’s 20 kWth bubbling fluidized bed reactor

The reactor (Fig. 2) has a bed diameter of 0.16 m and a freeboard diameter of 0.23 m. The freeboard is 3.5 m in length. The inner parts with flame contact were made of ceramics and quartz. Bed material was natural sand, particle size 0.1–0.6 mm, mean diameter 0.33 mm and composition (wt%) Na2O3 3.0, K2O 2.3, MgO 0.59, CaO 2.3, Al2O3 11.8, Fe 1.4, SiO2 77.5. The mean gas velocity in the reactor is about 0.5 m s1, corresponding to a total residence time of 7–8 s. This flow velocity is sufficient to transport particles of 4100 mm to the cyclone. The larger particles will be deposited on the walls of the freeboard. Fly ash particles of larger than 10 mm (cyclone cut size) will be separated by the cyclone. Fly ash of o10 mm fly ash is collected by the filter. The residence time in the pilot 20 kWth BFB is higher than in a typical BFB boiler because the upper half of the freeboard

32 (2008) 1311 – 1321

1313

was designed to be a well-controlled zone for various samplings and observations (like deposit and fly ash probing). Dry flue gas was analyzed with traditional on-line analyzers (O2, CO2, CO, NO, SO2) and wet and hot (180 1C) flue gas by FTIR (CO2, CO, NO, NO2, CH4, SO2, H2O, HCl). A Dekati low-pressure Impactor (DLPI) with 12 stages (see fly ash sampling from the 1 MWth facility) was used to measure the mass flow of fine fly ash in fractions and to fractionate the fine fly ash before the analysis. The sampling flow rate was 10 l/min and the sampling site was between the cyclone and filter.

2.2.

Fuels

Table 1 compares the proximate and ultimate analysis results of the fuels from TUD (1 MWth) and VTT (20 kWth experiments). Fixed carbon and oxygen content were calculated by difference. DW has a much lower ash content and higher content of volatiles compared with PPR. Both of these parameters have a significant influence on the combustion process. Due to high volatile content, biomass ignites and burns faster than coal with comparable particle size. The differences in the composition of fuels (DW and PPR) burnt using the TUD and VTT setups, were due to the heterogeneous nature of these fuels. DW and PPR pellets used for experiments in a 1 MWth BFB were of dimensions of 12 and 8 mm diameter, respectively. For combustion experiments in a 20 kWth BFB, both fuels were crushed to particle size of o5 mm and fuel moisture was increased up to 30% for the experiments. XRF analysis of PPR reveals high shares of reactive fly ash [15]. Ash contents were determined at 550 1C in order to oxidize the organic matrix without vaporizing the toxic metal studied. This high amount of alkali (K2O) is expected to form low melting compounds, resulting in bed agglomeration. Table 2 gives the XRF analysis of PPR and DW ash. The fuels used in the 20 kWth reactor were collected from the big bags (1000 kg) combusted later in the 1 MWth facility. In spite of such a collection, differences were found between the fuel composition (see Table 1). Fig. 3 shows the metal concentrations in the investigated fuels from the VTT sample. Therefore, some differences are also possible between the toxic metal concentrations of the fuels combusted in the 20 kWth and 1 MWth reactors. Fig. 3 indicates that the DW sample was significantly enriched with Pb and Cu. Relatively high concentrations of Cr and Mn were also found. Proper analysis of the toxic elements from the feedstocks of the 1 MWth BFB would have required several sequential samplings during each test run because the fuel composition in a largescale experiments can vary as a function of time. This is a highly recommended procedure for future research (in spite of rather high analysis costs).

2.3.

Experimental procedure

2.3.1.

TU Delft 1 MWth BFB

The boiler is started up by combusting natural gas. A moveable pilot burner above the bed and a few bottom natural gas nozzles heat up the top sand layer, which subsequently transfer the heat to the sand layers. At about 50 1C, small

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To stack Gas cooling Sampling port

Deposit probe Observation port

Temperature control

Gas sample

Gas probe

Sampling port

Bag filter

Heating zone 4 Sampling port Cyclone

Obervation port/ Deposit probe

Heating zone 3

Tertiary air optional Tertiary air optional

Obervation port Tertiary air (preheated) Fuel container 2 Fuel container 1 Additive container

Heating zone 2/ Cooling zone 2 Obervation port Heating zone 1/ Cooling zone 1 Obervation port

Secondary air (preheated)

BED made of quarz

Nitrogen Air

Primary gas heating PC control and data logging system

Fig. 2 – Schematic picture of the VTT’s 20 kWth BFB experimental setup.

biomass doses are fed in order to decrease the start-up time. Airflow rate is kept close to the minimum fluidization velocity. The biomass feed and airflow rates, both are gradually increased with reference to the bed temperature. At about 550 1C, natural gas burning is stopped and the system runs on biomass only. In the typical steadystate operation of the boiler, air is blown at room temperature from the bottom of the bed through the distribution plate. The boiler uses sand as a fluidization medium, typically 480 kg of sand batches were used for different experiments. The flue gas released in the bed flows through the splashing region to the freeboard section. Secondary air is introduced in to the splashing zone in the air-staging experiments. The flue gases then flow to the horizontal tube section of the heat exchanger where heat is removed by the water in the shell section. Finally, the flue gases are passed through the

cyclone to remove largest portion of the fly ashes. The following process variables were monitored during the experiments:

 Temperature along the boiler length and at other impor-

 

tant locations for example, cyclone, water inlet temperature and air temperature. Flue gas emission analysis, CO, CO2, O2 and NO. Combustion air, water and feed flow rates.

Air staging experiments were done with sand of mesh size 0.5 mm, while for the other experiments 0.8 mm was used. This was deemed necessary to keep proper mixing in the bed [16]. DW and PPR in co-combustion experiments were fed on weight percentage basis. In cocombustion experiments, the time in achieving steady state

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Table 1 – Fuel analysis of demolition wood and pepper plant residue Parameter

Pepper plant residues

Demolition wood, Bquality

VTT

TUD

VTT

TUD

Proximate analysis Moisture (%), as received Volatile matter (%), db Ash, 550 1C (%)

9.3 62.7 19.90

6.5 64.7 14.44

9.7 78.2 1.65

9.1 76.5 1.68

Ultimate analysis, db Carbon (%) Sulfur (%) Hydrogen (%) Nitrogen (%) Chlorine (%)

37.9 0.44 4.52 2.31 0.18

36.1 0.49 4.26 2.72 0.13

50.6 0.08 5.94 0.95 0.09

50.3 0.00 6.91 1.03 0.07

Heating value HHV (MJ kg1) LHV (MJ kg1)

14.87 13.89

16.90 14.46

20.49 19.19

18.89 18.54

Table 2 – XRF analysis of B quality wood and pepper plant residue Pepper plant residue Mass % ash

B quality wood Mass % ash

12.6 4.9 2.0 0.2 7.4 32.2 0.9 24.6 0.5 5.2

20.4 3.5 2.2 0.3 7.5 27.5 4.8 10.5 2.5 11.1

SiO2 AL2O3 Fe2O3 Mn MgO CaO Na2O K 2O TiO2 P 2O 5

Wt % Fuel Ash

Demolition wood Pepper plant residue

0.2 0.15 0.1 0.05 0 Cu

Co

Cr

Mn

Ni

Pb

V

As

2.3.2.

Sb

Fig. 3 – Metal concentrations in demolition wood (DW) and pepper plant residue (PPR).

was longer than during combustion of a single biomass. This dynamic behavior is due to the large amount of fluidization medium (sand 480 kg).

1315

VTT’s 20 kWth BFB

The running-up stage was started with electric heaters only. The combustion air and its staging were adjusted to the desired values during the start-up phase. After reaching a desired fluidizing air temperature, the fuel feeding was started, after which the feeding rate was carefully increased to the set value. The temperature distribution in the whole reactor was finally adjusted to the set value with the electric heaters located around the bed and freeboard. The mass of bed sand was 3 kg and its sampling was possible during the experiment through a separate line. Staging was kept constant (prim./sec./tert. ¼ 70/30/0). The goal was to keep the bed temperature at 850710 1C and the mean temperature in the freeboard at about 900 1C during the experiments, but no attempts were made to eliminate temperature peaking close to the addition points of secondary air because such peaks exist in full-scale BFB boilers.

3.

Results and discussion

Optimization experiments and studies concerning the influence of operating variables on DW combustion were carried out in the previous work [16]. Work presented here, is a comparative study of lab (VTT 20 kWth) to pilot (TU Delft 1 MWth) scale combustion setups. Thus, tests in this experimental campaign were concentrated on the combustion and co-combustion of DW and PPR. Experiments using the 20 kWth reactor were conducted at constant output and operating conditions. Table 3 shows the operating conditions and flue gas analysis for different experiments carried out using the 1 MWth boiler.

3.1.

0.3 0.25

32 (2008) 1311 – 1321

Temperature distribution along the furnace

Fig. 4 shows the temperature distributions in the 20 kWth BFB. In spite of the small size of this pilot, its furnace conditions (temperature, O2 distribution and residence time) simulate better a large BFB (420 MWth) than a small BFB (o10 MWth) boiler. The bed temperatures were kept at 820–840 1C except during the experiment with 50%DW50%PPR where bed temperatures were lower (770–820 1C) to avoid bed agglomeration due to the high shares of PPR. The freeboard temperature varied more than the bed temperature during the experiments. The freeboard temperature peaks were most prominent with 100%DW combustion, which has a very low ash and high volatile content, and were weakest with 50:50 DW–PPR mixture, where the ash content was the highest. The peak rather high in the furnace with DW alone was probably due to the combustion of the pyrolysis gases. Fig. 5 shows the furnace temperature distribution for different experiments in the 1 MWth boiler. For all the experiments, the temperature decreased along the distance from the bottom of the boiler due to the presence of the internal heat exchanger in the splashing zone. Experiments 6–10 were conducted with different degrees of air staging while there was no staging in experiments 1–5. The observed high splashing zone and freeboard temperatures in experiment 1, with DW alone, were due to the high fluidization velocity (1.4 m s1), which increases the bed length and consequently the temperatures in the sections that follow. In experiments 2

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Table 3 – Operating conditions and flue gas analysis Tests 1b 2c 3d 4d 5d 6d 7d 8d 9d 10e

Vf m s1

Output MWth

Bed T 1C

Splash zone T 1C

Freeboard T 1C

O2 %

CO2 %

NOa ppm

COa ppm

Pri. air %

Sec. air %

1.40 1.40 1.40 1.20 1.00 1.20 0.90 0.66 0.70 0.50

1.00 1.04 1.09 1.03 0.82 1.04 0.86 0.71 0.80 0.76

849 851 843 852 849 847 848 852 847 835

709 760 748 751 723 723 681 657 653 693

569 613 597 574 516 585 521 515 528 554

6.9 4.0 5.4 4.1 4.7 5.7 5.2 5.5 5.5 5.2

13.0 13.5 14.0 16.2 14.8 14.1 14.2 13.2 13.2 13.6

232 472 274 126 138 182 124 97 107 133

115 22846 9448 4635 6886 5869 8266 4679 4327 1187

100 100 100 100 100 79 70.4 61.9 60.2 47.7

– – – – – 21 29.6 38.1 39.8 52.3

a

Normalized at 6%O2. 100% BQW. c 50%PPR 50% BQW. d 75%DW25%PPR. e Moveable heat exchanger at 41 cm.

100DW

12000

50DW 50PPR

10000

75DW 25PPR

300 NO

CO

250

8000

200

6000

150

4000

100

2000

50

0

0 0

750

800

850

900

1000

950

NO (PPMV)

9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0

CO (PPMV)

Residence time (s)

b

10

20

30

40

50

60

Secondary Air %

Temperature (°C)

Fig. 4 – Temperature profiles in the VTT 20 kWth reactor for different runs. O2-concentration 5 vol% (dry).

Residence time (s)

Exp-1 Exp-6 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 500

550

Exp-2 Exp-7

600

Exp-3 Exp-8

Exp-4 Exp-9

650 700 750 Temperature (°C)

800

Exp-5 Exp-10

850

900

Fig. 5 – Temperature profiles in the boiler for different runs, k1.2–1.5.

and 3–5, with fuel compositions 50DW%50PPR% and 75DW%25PPR%, respectively, the temperatures in splashing zone and freeboard were even higher than in experiment 1. These high temperatures in splashing zone and freeboard are not only due to the high fluidization velocity but also due to the presence of high concentrations of unburned CO originated from the low reactivity of the ash-contaminated PPR.

Fig. 6 – Effect of air staging on DW emissions. Bed temperature 850 1C, k1.36–1.39, Exp. no. 6–10.

The high temperature in these sections indicates that the CO is still reacting with the residual oxygen. The temperatures in the splashing zone and freeboard are lower in the staged combustion experiments (experiment 6–10) compared with the experiments with no staging due to lower fluidization velocities. The temperatures in the splashing zone and the freeboard at first decrease and then increase with increasing degree of air staging. This increase in temperature with increase in secondary air ratio indicates that significant combustion is also occurring in these sections due to the availability of fresh oxygen from secondary air.

3.2.

Flue gas emissions

Co-combustion experiments (all experiments except 1, see Table 3) of PPR and DW produced very high CO emissions in the 1 MWth BFB. Fig. 6 shows the effects of air staging on CO and NO emissions during co-combustion of 75DW% and 25PPR% in the 1 MWth boiler (Exp. 6–10). All air-staging experiments were conducted with lower fluidization velocity (0.5–1.2 m s1) and with smaller sand particle size (0.5 mm), respectively. The mobile heat exchanger was set at 41 cm (6 cm lower than its top position) for experiment 8 as it was necessary to run the reactor at higher load.

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BIOMASS AND BIOENERGY

CO & NO Emissions (PPMV) at 6% O2

100000 CO

10000

NO

1000 100 10 1 D

0

10

D

T

DW

R PP

5

0

10

W2

D 75

D

T

-VT

-TU

VT

TU

DW

5

W2

D 75

T

-VT

-TU

R PP

R PP

0

W5

D 50

R PP

0

W5

D 50

Fig. 7 – Comparison of CO and NO emissions from TUD and VTT setups.

Air staging tends to reduce CO and NO emissions. Running the system at lower fluidization velocity (with increased residence time) greatly reduces CO emissions (Exp. 3–4). NO emissions are lower for both air-staging and lower fluidization velocity experiments. Also, the use of a lower fluidization velocity reduces combustor loading and the use of lower mesh size sand (in order to have good mixing at low fluidization velocities) becomes compulsory. Co-combustion experiments with air staging produce acceptable emissions of NO taking into account the small size of the BFB boiler, and better but still unacceptable CO emissions (experiment 10). However, CO emissions peaked during 30% secondary air staging (Fig. 6). Comparison of experiments 4 and 6 (un-staged and staged with 20% secondary air), show that introduction of secondary air reduces the temperature in splashing zone and freeboard. Further increase in the degree of air staging results in the increase of both the splashing zone and the freeboard temperature indicating significant conversion of CO in these sections. It should be pointed out that the reduction of CO emissions can also be achieved without air staging, to a certain extent by running the system at lower fluidization velocity, but at the cost of boiler loadings. Decreasing fluidization velocity will result in increasing residence time and consequently lower CO emissions and boiler loadings. NO emissions, on the other hand, show a typical minimum (for a primary air lambda) for 38% secondary air ratio. Further increase in the degree of air staging results in the increase of NO emissions [1,4,7]. These observations indicate that the desired reduction in CO emissions can possibly be achieved by air staging without significantly compromising the boiler loadings. High CO emissions from PPR fuel can be attributed to a number of factors including its high ash content followed by the accumulation of this ash on surfaces of the burning fuel particles, weakening oxygen penetration to the combustible part of the particle. The partially combusted char particles found in the bed (during abrupt shutdowns), also support this assumption as all had a thick layer of ash on the surface. Thus, running the system at lower fluidization velocity with air staging provides a longer residence time in the bed and

thus lower emissions. Another advantage, besides lower NO and CO emissions with air staging, is the higher loading capacity. The major differences between the two reactors are residence time and temperature profile where the smaller reactor resembles more of a larger (420 MWth) furnace. Fig. 7 shows the comparison of CO and NO emissions in 1 MWth and 20 kWth reactors. All the 20 kWth results have been obtained at identical operating conditions. The smaller reactor produces much less CO and less NO than the experimental boiler for co-combustion experiments. However, the 1 MWth produces lower CO emissions for 100%DW probably indicating the better mixing achieved in the larger reactor bed due to high fluidization velocity. DW shows fast combustion kinetics because the ash does not form a layer on the reactive particle surface, which decreases oxygen diffusion. Very short residence time, at high temperature, in the bed and just above the bed is sufficient for the satisfactory burnout of the gases and pyrolysis products from DW. Therefore, the CO emissions are even lower in the 1 MWth BFB compared with the 20 kWth BFB. In contrast, with PPR shared blends, the high-temperature zone for several seconds in the freeboard is essential to improve the burnout of the gases. Combusting 50DW%50PPR% in the 1 MWth setup appeared to be impossible for higher boiler thermal loadings due to unacceptable CO emissions. It is clear from these comparative tests that in order to burn higher ratios of PPR, the residence time needs to be increased and temperatures in splashing zone and the freeboard should be kept high in the 1 MWth boiler. Preheated secondary air and removal of internal heat exchanger can help in maintaining a high temperature profile. Lower shares of PPR (o25%) can be combusted without significantly compromising the boiler loadings.

3.3.

Particulate emissions

The mass flows of particulates of different size ranges were measured and sampled downstream the cyclones (with cut size of about 10 mm) of the 1 MWth and 20 kWth reactors. The results were combined to obtain three larger particle size

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1400 2.4-10.0

1200 1000

mg/Nm3

Total dust emission limit = 50 mg/Nm3

0.4-2.4 <0.4

800 600 400 200 0 D

orm

N ion

iss

Em

T

-VT

-TU

W 0D

10

10

W 0D

D

TU

R-

P 5P

2

DW

T

VT

R-

P 5P

2

DW

75

75

D

TU

R-

P 0P

5

5

DW

T

VT

R-

P 0P

DW

50

50

Fig. 8 – Dust emissions from VTT and TUD reactors for different runs. 14

40 <0.56

30

0.56-2.7

TUD

2.7-16.0

25 20 15 10

12 Enrichment factor (-)

Enrichment factor (-)

35

10

<0.4

VTT

0.4-2.4 2.4-10.0

8 6 4 2

5

0

0

Sb Co Cr Cu Mn Ni Pb V As

Sb Co Cr Cu Mn Ni Pb V As

Fig. 9 – Toxic metal enrichment factors in TUD and VTT Impactor samples for 100%DW.

50 40

60 <0.56

TUD

0.56-2.2 2.2-7.8

30 20 10

Enrichment factor (-)

Enrichment factor (-)

60

50 40

<0.4

VTT

0.4-2.4 2.4-10.0

30 20 10 0

0 Sb Co Cr Cu Mn Ni Pb V As

Sb Co Cr Cu Mn Ni Pb V

As

Fig. 10 – Toxic metal enrichment factors in TUD and VTT Impactor samples for 75%DW25%PPR.

ranges. Results of measured overall dust load for each experiment from both installations are presented in Fig. 8. The measured emissions of particulates of o10 mm from both the reactors were many times higher than the current EU emission limit for dust. The overall loads of particulates of o10 mm were lower in the 1 MWth reactor than in the 20 kWth reactor, which might have been due to a more effective cyclone in this facility. However, a similar trend was measured also in particle size ranges 0.4–2.4 and o0.4 mm, which cannot be affected by the mentioned cyclones. Fig. 8

shows how rapidly the mass flow of the fine fly ash increases especially in the 20 kWth BFB with the share of PPR containing much more ash, chlorine and potassium than the DW sample (see also Tables 1 and 2). In consequence, it is suggested that the hotter freeboard of the 20 kWth BFB was the main reason for the higher mass flows of the finest (o2.4 mm) fly ash from this reactor. Tgas was about 900 1C for a few seconds in the freeboard of the 20 kWth BFB, whereas it steadily decreased above the bed (850 1C) and decreased promptly in the freeboard to o600 1C in the 1 MWth boiler (compare Figs. 4

ARTICLE IN PRESS BIOMASS AND BIOENERGY

in a particular fly ash fraction by the concentration of this element in the fuel ash (see Fig. 3). The concentrations of Ni, As and Sb were very low in the studied fuels and therefore the high enrichment factors for these species are only shown and not discussed and are considered inaccurate. Also, the calculated enrichment factors for As were clearly unrealistic (450) and therefore are presented as 50 and regarded as inaccurate. The fact that the toxic elements were analyzed only from the fuel samples combusted in the smaller reactor contributes also to the inaccuracy in the comparison of the enrichment factors. Pb, Cr and Cu tended to be enriched in the fine fly ash. For Cr and Cu the enrichment was strongest with 100% DW but weakened with the increasing share of PPR. Instead, Co and Mn showed weak enrichment with all the blends. For 100% DW, the results from 20 kWth and 1 MWth BFB’s are in a good agreement with Cr, Cu and Mn. In contrast, the measured enrichment is clearly stronger for Pb in the 1 MWth BFB. For other blends, the trend is similar: the 1 MWth reactor shows stronger enrichment for Pb in the fine fly ashes compared with the 20 kWth BFB. The construction of the isokinetic measurement system differed in the two reactors: the 1 MWth BFB clearly has lower wall surface to volume ratio. Consequently, it is possible that

and 5). For example, the alkali compounds are partly in vapors at 900 1C and will be found as particles ofo1mm on the impactor stages in 20 kWth BFB. As for the fly ash fraction of 42.4 mm, the share of this fly ash flow in relation to the total ash flow to the furnace is dependent on the fluidization velocity, which was clearly higher in the 1 MWth facility compared with the 20 kWth facility (excluding the last three experiments in the 1 MWth experiment). The share of fly ash is also dependent on the shape and configuration of the combustion chamber and the amount and distribution of the combustion air. The carry over in the combustors installed with excess-air (secondary air and tertiary air) injection facility depends upon how the fluidization air and excess-air is distributed [17–20]. The difference in the flow regime, cyclone efficiency and reactor geometry accounts for the differences in measured emissions of dust of 2.4–10 mm from the two reactors.

3.4.

Toxic metal emissions

Figs. 9–11 show the enrichment factors of toxic elements in fine fly ash fractions from both setups. Enrichment factors were calculated by dividing the concentration of a particular element

18

60 <0.55

50

TUD Enrichment factor (-)

Enrichment factor (-)

1319

32 (2008) 1311 – 1321

0.55-2.5 2.5-14.0

40 30 20

16

<0.4

14

0.4-2.4

VTT

2.4-10.0

12 10 8 6 4

10

2 0

0 Sb Co Cr Cu Mn Ni Pb V

Sb Co Cr Cu Mn Ni Pb V

As

As

Fig. 11 – Toxic metal enrichment factors in TUD and VTT Impactor samples for 50DW%50PPR%.

12.00 Emission limit for the sum = 0.5 mg/Nm3

10.00

mg/Nm3

8.00 2.4-10.0

6.00

0.4-2.4 <0.4

4.00 2.00 0.00 rm

io

o nN

iss

Em

D

0

10

D

T

DW

0

10

R PP

DW

75

R PP

R PP

50

50

DW

50

T

-VT

-TU

R PP

25

25

DW

75

D

T

-VT

-TU

VT

TU

DW

DW

50

Fig. 12 – Toxic metal concentration in TUD and VTT impactor samples.

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BIOMASS AND BIOENERGY

Pb might have adsorbed to the line walls and therefore the long length of 20 kWth BFB flue gas line could have contributed to the lower Pb concentrations measured. In the future, toxic elements sampling from the 20 kWth BFB will be conducted just after the freeboard at significantly higher flue gas temperature to avoid wall losses. Also the larger amount of particulate matter carryover served to dilute the metal concentration in the fly ash [19,21]. As shown in Fig. 8, the dust emissions from 20 kWth BFB were a number of times larger than 1 MWth experiments. Therefore measurements from the 1 MWth BFB show higher enrichment factors for Pb in fly ash. Fig. 12 depicts the sum of toxic metals in fly ash for different runs for both 1 MWth and 20 kWth setups. As in 1 MWth BFB, impactor measurements were carried out at different flow rates, the size distribution of fly ash for every experiment is also different. However, the differences are only of few microns and therefore in Fig. 6 it is represented as same particles classes as in the 20 kWth measurements. The 1 MWth measurements indicate that the highest emissions are observed for the 100% DW run, while the 20 kWth measurements indicate the same for 75DW%25PPR%. Measurements carried out using the 1 MWth BFB clearly show that with increasing share of PPR fuel, the total toxic metal emissions can be significantly reduced. Results from the 20 kWth measurements on the other hand, show that cocombusting a smaller share of PPR(25%) with DW results in the increase of toxic metals emissions concentration due to the three-fold increase in the dust emissions (see Fig. 8) and an insignificant decrease in the toxic metals emission fraction in the ash (see Figs. 9–11). Upon further increasing the PPR share, the reduction in toxic metals emission fraction in ash becomes significant enough to reduce the toxic metal emissions concentration albeit the total dust emissions increases further. The toxic metal emissions for all fuel compositions and from both setups are greater than the emission limit. The emission limit of 0.5 mg/Nm3 for the sum of nine toxic elements is applied in waste incineration or co-incineration units.

4.

Conclusions

Comparative studies on co-combustion of different biomass types at lab (VTT) and pilot (TUD) scale were carried out. The TUD 1 MWth boiler represents typical small boilers (o10 MWth) with a shallow bed and smaller freeboard. On the other hand, the VTT 20 kWth BFB typically represents bigger units (420 MWth) with a longer freeboard. The results include useful information on how design and operating variables can be optimized to realize essential improvement in biomass (co)-combustion processes. Both types of units exist in large numbers for applications from greenhouse boiler to power plant units. DW was contaminated with lead, which tended to get strongly enriched in the fine fly ash fraction. Enrichment of Pb in the fine fly ash can be weakened by co-firing DW with PPR. Increasing the share of PPR up to 50% markedly reduces the toxic metal concentration in the finest fly ash. This, however, leads to increased mass flow of fine fly ash and increases the

32 (2008) 1311 – 1321

potential risks of operational problems such as bed agglomeration and fouling. Small shares of PPR can be co-combusted without significant loss of boiler load. Combusting 50DW%50PPR% in the 1 MWth BFB was impossible for higher boiler loadings due to unacceptable CO emissions. In order to burn higher ratios of PPR, the residence time in the bed area needs to be further increased and the temperatures in the splashing zone and freeboard should be kept high. Preheated secondary air and constructional improvements such as the removal of the characteristic internal heat exchanger can help in maintaining a high-temperature profile in 1 MWth BFB.

Acknowledgments European Union is acknowledged for funding the research in the framework of NNE5 (Project no.: E5-2001-00601) via the project ‘Safe co-combustion and extended use of biomass and biowaste in FB plants with accepted emissions’ (Contract ENK5-CT2002-00638, FBCOBIOW). R E F E R E N C E S

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