Pyrolysis as a technique for separating heavy metals from hyperaccumulators. Part II: Lab-scale pyrolysis of synthetic hyperaccumulator biomass

Available online at www.sciencedirect.com

Biomass and Bioenergy 25 (2003) 651 – 663

Pyrolysis as a technique for separating heavy metals from hyperaccumulators. Part II: Lab-scale pyrolysis of synthetic hyperaccumulator biomass Lakshmi Koppolua;∗ , Foster A. Agblevorb , L. Davis Clementsc a Biological

Systems Engineering, University of Nebraska-Lincoln, 217 L.W. Chase Hall, Lincoln, NE 68583, USA b Renewable Products Development Laboratories, 5100 N 57th, Lincoln, NE 68507, USA c Biological Systems Engineering, Virginia Polytechnic Institute, 212 Seitz Hall, Blacksburg, VA 24060, USA Received 20 May 2002; received in revised form 17 March 2003; accepted 21 March 2003

Abstract Synthetic hyperaccumulator biomass (SHB) impregnated with Ni, Zn, Cu, Co or Cr was used to conduct 11 experiments in a lab-scale 7uidized bed reactor. Two runs with blank corn stover, with no metal added, were also conducted. The reactor was operated in an entrained mode in a oxygen-free (N2 ) environment at 873 K and 1 atm. The apparent gas residence time through the lab-scale reactor was 0:6 s at 873 K. The material balance for the lab-scale experiments on N2 -free basis varied between 81% and 98%. The presence of a heavy metal in the SHB decreased the char yield and increased the tar yield, compared to the blank. The char and gas yields appeared to depend on the form of the metal salt used to prepare the SHB. However, the metal distribution in the product streams did not seem to be in7uenced by the chemical form of the metal salt used to prepare the SHB. Greater than 98.5% of the metal in the product stream was concentrated in the char formed by pyrolyzing and gasifying the SHB in the reactor. The metal concentration in the char varied between 0.7 and 15.3% depending on the type of metal in the SHB. However, the metal concentration was increased 4 to 6 times in the char compared to the feed. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Hyperaccumulator; Biomass; Phytomining; Phytoremediation; Heavy metals; Recovery; Pyrolysis

1. Introduction 1.1. Rationale Phytoremediation has received increasing attention in recent years. A brief introduction to hyper∗

Corresponding author. E-mail address: [email protected] (L. Koppolu).

accumulators, phytoremediation, phytomining and details of the preparation of synthetic hyperaccumulator biomass (SHB) used in this study is presented in the Arst part of the paper [1]. Economic estimates for metal phytoremediation ranges between $25 and $100 per ton of soil [2]. The cost of phytoremediation compares favorably [2] when compared with remediation per ton of soil for other technologies currently being used or studied such as chemical treatment ($100 – $500), soil washing ($75 –$200), in-situ soil 7ushing

0961-9534/03/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0961-9534(03)00057-6

652

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

($40 –$190), reagent vitriAcation ($75 –$90), thermal vitriAcation ($250 –$425), thermal desorption ($150 – $500), thermal treatment ($170 –$300), electrokinetics ($20 –$200), incineration ($200 –$1500), and landAlling ($100 –$500). However, should large-scale metal phytoremediation be demonstrated as economically and technologically viable, disposal of the hyperaccumulator biomass becomes a relatively important issue. The heavy metal concentrations in the harvested biomass will require further processing to immobilize or beneAcially utilize the metals; otherwise, their disposal would have to be as a toxic waste in a controlled landAll. Economic separation of metal from hyperaccumulator biomass is of great importance for an emerging technology called phytomining as well. Though this technology is in its infancy, certain developments that might render phytomining technology a feasible proposition have been suggested [3]. These include using gene technology, conventional plant breeding, fertilizer amendments to increase the hyperaccumulator biomass, and chelate addition to soils to increase the metal uptake by hyperaccumulators. Potential applications for phytoremediation were the basis for the selection of Zn, Cu and Cr synthetic hyperaccumulators as candidates for this study. These metals are among the most prevalent pollutants at most of the contaminated sites in the United States. The National Priority List of 1986 [4] suggests a total of approximately 1000 sites that pose signiAcant environmental or health risk. Forty percent [5] of these sites have metal related problems. Of these, the number of sites polluted by Pb, Cr, As, Cd, Cu, and Zn decreases in that order. Ni and Co hyperaccumulators have been suggested as potential opportunities for phytomining [3]. Phytomining for two metals simultaneously using a single crop of hyperaccumulator is also a possibility for Ni–Co and Co–Cu combinations. Hence, the Anal selection of the Ave heavy metals (Ni, Zn, Cu, Co and Cr) was based on a combination of all of the above factors. This work is an evaluation of the eHcacy of pyrolysis for recovering the accumulated metals as an “ore” or “metal concentrate” for further reAning. The work used both lab- and pilot-scale reactors for the pyroly-

sis of SHB. The current paper describes the lab-scale experiments. Pilot-scale experiments are described in Part III of this paper. While hyperaccumulation of the metals in the plants can be viewed as the Arst concentration step in the metal recovery process, pyrolysis can be considered as the second step leading to further concentration and the conversion of the metal chemistry into a form that is suitable for recovery. The rationale for using this approach as the method of choice is (a) reduction in the volume and weight of biomass to be processed, (b) energy recovery in the form of syngas produced from the pyrolysis operation, (c) achieving on-site (geographical location where the biomass is produced) processing of the biomass using a portable pyrolysis unit and (d) production of a char/ash residue that recovers the metals in the form of a metal concentrate that may be processed using commercially available metallurgical operations.

1.2. Objectives The speciAc objectives of this work are three fold: 1. Determine the feasibility of using pyrolysis to produce metal concentrate (Ni, Zn, Cu, Cr, Co) from SHB. 2. Study the distribution of the metal among the product streams by documenting the overall; carbon, ash and metal component material balances using conventional, established operating conditions for biomass pyrolysis. 3. Investigate the eMect of the chemistry (SHB prepared using metal citrate and acetate) on the Anal metal product from SHB after the pyrolysis process. The lab-scale 7uidized bed reactor used in this study was based on an original design from National Renewable Energy Laboratory (NREL, Golden, CO). The original reactor was designed for two diMerent studies, (a) conversion of biomass feedstocks to pyrolytic oils for diesel fuel applications [6] and (b) to scale up data generated from molecular beam mass spectrometric studies of lignocellulosic materials [7].

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

2. Materials and methods 2.1. Lab-scale experiments SHB is referred to as “feed” in this paper. Thirteen pyrolysis experimental runs were conducted in the lab-scale 7uidized bed reactor. The Arst two blank runs were done using corn stover feed with no metal incorporated into the biomass. To establish experimental variation, duplicate experiments with the Nickel acetate SHB were performed; all other measurements on the metal SHB were single experiments. The eleven SHB samples were the acetates and citrates of the following metals: Ni, Zn, Cu, Co, and Cr. The experiments were labeled with the following key: L = lab-scale experiment; MT = the metal used (e.g., NI, ZN, CU, CO, CR or BLNK for the biomass substrate with no metal impregnation); LI = the ligand attached to the metal, either AC (acetate) or CT (citrate); X = 1 or 2 depending on the number of the experiment with a speciAc metal. 2.1.1. Sample preparation A Thomas ScientiAc (model 3383-L10) grinder with a 0:841 mm opening (20-mesh) sieve was used to grind the SHB. The particle size required for the lab-scale reactor system was based on earlier studies [6]. Sieving of the milled SHB was done for 20 min using a sieve shaker and 20:32 cm (8 in) brass sieve pans. These were stacked from top to bottom in the following order: 28 mesh pan with a cover, 35 mesh, 60 mesh, 65 mesh, 80 mesh, and the bottom pan. A typical particle size distribution by weight of the SHB is shown in Table 1. 2.1.2. Reactor setup A schematic of the lab-scale pyrolysis reactor setup is shown in Fig. 1. The reactor was connected to a K-TRON K2 model T20 volumetric screw feeder that was loaded with about 120 g of the milled and sieved SHB. The feeder was sealed with a gas tight cover before the carrier gas 7ow in the system began. A Cole–Parmer digital display pressure gauge was attached to the feed tank cover to indicate a build up of pressure due to any blockage in the system. A box constructed of plexiglass at the auger outlet from the feeder allowed visual observation of the feed entrain-

653

Table 1 Typical particle size distribution of SHB for lab-scale experiments Mesh

Sieve opening (mm)

Weight (%)

28 35 60 65 80

0.595 0.420 0.250 0.210 0.177

10 47 32 8 3

ment into the jacketed feed pipe. Air-cooling through the external jacket of the feed pipe was required, because conduction of heat from the reactor heated the feed pipe. A feed rate of 2.02–2:17 g min−1 was used for all runs. The feeder was calibrated with blank corn stover, prior to the experiments. The nitrogen carrier gas 7ow rate used to carry the corn stover from the feeder into the reactor inlet was 6:5 l min−1 . Preheated nitrogen carrier gas was also introduced from the bottom section of the reactor at a rate of 19:75 l min−1 . Thus, the total nitrogen carrier gas 7ow rate through the reactor was 26:25 l min−1 . The gas 7ow rates through the feeder and the bottom of the reactor were recorded using Cole Parmer 33115/33116 series gas mass 7ow controllers that were previously calibrated. The 36:83 cm long reactor was constructed using schedule 40 stainless steel pipe with an internal diameter of 5:08 cm. A porous metal gas distributor was located at the interface between the reactor and the gas preheater zone. The reactor was externally heated at the top, middle and bottom using a three zone electric furnace. The estimated minimum 7uidization velocity for a particle of size 0:595 mm (mesh 28) at 873 K, which corresponds to the reactor operating temperature, was 5:1 cm s−1 . This velocity corresponds to a gas 7ow rate of 6:18 l min−1 through the 5:08 cm reactor. The calculated apparent gas residence time in the reactor at 873 K, which is the operating temperature inside the reactor, is 0:6 s. Once gas 7ow through the system was started, heating of the gas preheater, reactor, cyclone and the transfer lines between the reactor and the cyclone was initialized. Temperatures were controlled using an Omega Multiscan/1200 data acquisition system linked to a Microsoft Windows based acquisition

654

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

Plexiglass window Jacketed feed pipe Feed tank

Electrostatic Precipitator

out

Condenser T4

Feeder screw

T8

in

T3 in

Reactor

Preheater

T9

T2 T5

T1

Cyclone

T6

T10

out

T7

Ash box

Tar box 1

Tar box 2 Gas Filter

Nitrogen tank

Gas sampling valve Exit gas Wet test meter

Fig. 1. A schematic of the reactor setup for the lab-scale experiments.

application called TEMPviewTM on a personal computer. Ten thermocouples (see Fig. 1) were used to monitor the temperatures at T1: preheater, T2: reactor bottom, T3: reactor middle, T4: reactor top, T5: cyclone top, T6: cyclone bottom, T7: Ash box, T8, T9, and T10: transfer lines. The heating of the gas preheater, top, middle and bottom of reactor, transfer lines, and the top and bottom of the cyclone was controlled at set points of 423 K, 893 K, 873 K, and 573 K, respectively. The reaction temperature chosen for the study was 873 K. This temperature was selected based on the optimum conditions indicated for biomass pyrolysis in the spouted bed pilot-scale reactor from work done by Janarthanan and Clements [8,9]. A typical reactor temperature proAle for the lab-scale experiments is shown in Fig. 2. The 7ow of water through the condenser was started after the reactor approached a thermal steady-state. When the observed variation in the temperature from

Fig. 2. A typical reactor temperature proAle for the lab-scale experiments.

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

the set point in the core section of the reactor was less than 5%, the reactor was said to have approached a thermal steady-state condition. The feeder was started and then electric current to an electrostatic precipitator was switched on. The exit gas from the electrostatic precipitator was Arst passed through a coalescing Alter (Reading Technologies Inc., PA) to trap the tar particles and then through a Precision ScientiAc Petroleum Instruments (model number 63125) wet test meter to measure the total volume of the gaseous product stream. The temperature of the exit gas was measured with a thermometer at the wet test meter. Gas samples were collected after 5 min, 20 min, and 35 min in 15:24 cm × 15:24 cm (6 in × 6 in) Tedlar J gas sampling bags. The feeder was frequently tapped with a mallet to aid steady 7ow of the SHB through the twin-screw feeder into the jacketed feed tube. The pressure gauge on the feeder cover was constantly monitored to ensure that there were no blockages resulting in backpressure in the system. Experiments with blank corn stover and Ni acetate SHB were done in duplicate to study the repeatability of the experiments. These reactions were carried out for 20 –28 min. For all other experiments, single runs were carried out for about 40 min. After the reactions were completed, the feeder was stopped and then all heaters were shut down. The electrostatic precipitator was disconnected. The system was 7ushed with nitrogen gas for a minimum of 15 min, and then the gas 7ow was terminated. The Anal steps in the reactor shut down procedure were to stop water 7ow through the condenser and air7ow through the jacketed feed pipe. The unreacted SHB was collected from the feed tank. The diMerence between the quantity loaded into the feed tank and the unreacted SHB was the weight of the SHB pyrolyzed. For each reaction, char from the reactor and the ash box were collected in a ZiplocJ bag as the solid product stream. A small quantity of char from the transfer lines between the reactor and cyclone and some Ane particles adhering to the sides of the cyclone were also collected and added to the char in the ZiplocJ bag. The condenser, the electrostatic precipitator, transfer lines between the cyclone and the gas Alter, tar box 1, tar box 2, and the gas Alter were all washed with acetone and the products collected in a beaker. This beaker was placed in a ventilated fume

655

hood until all the acetone was evaporated at room temperature. This liquid or tar product stream was then transferred into a vial for storage. 2.2. Product analysis The feed (blank corn stover or SHB) and the product streams (char, tar, and gas) collected from the lab-scale pyrolysis reactor were analyzed as described in the following paragraphs. A sample of 0.05 –0:1 g of the blank stover, the SHB, the char or the tar was weighed into a 50 ml Erlenmeyer 7ask. The char was collected as friable porous particles from the reactor that had to be ground into a Ane powder before weighing. The sample was digested for 10 min at room temperature (300 K) in 5 ml conc. H2 SO4 (ACS reagent, assay: 95 –98%). Then, 5 ml of 30% H2 O2 was added and the mixture was further digested at room temperature for 15 min. Hydrogen peroxide was added slowly to the sample, since the solution in the 7ask tended to spill over due to rapid eMervescence. The contents were then transferred into a digestion tube and maintained at 423 K on a digestion block for 20 min. The solution was clear after cooling and was diluted with distilled water to a Anal volume of 15 ml. The above procedure was adapted from the technique given by Jones and Case [10] for plant tissue analysis. For the char samples, solid undigested particles were allowed to settle to the bottom of the tube before transferring about 7 ml of the solution into glass vials for analysis. The Soil and Plant Analytical Lab, UNL, NE analyzed the samples in duplicate with a Perkin–Elmer 460 atomic absorption spectrometer (AAS) for the appropriate metal. The SHB and char were also analyzed for the moisture, carbon, and ash content. Tar samples could not be analyzed for the moisture, ash or carbon content due to the makeup of the matrix of these samples. Gas samples collected from the lab-scale experiments were analyzed using a Shimadzu gas chromatograph (model GC-14A) with a thermal conductivity detector (TCD) at 398 K. Three columns were used to separate the C1 –C4 hydrocarbons and the Axed gases N2 , O2 , and CH4 . The columns were: Porapak N, 2 m × 1=8 in, 80/100 mesh; MS-5A (molecular sieve 5A), 2 m × 1=8 in, 60/80 mesh; Haysep Q, 2 m × 1=8 in, 100/180 mesh.

656

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

The method of separation used is described as follows: Column 1 (Porapak N) separated the sample into two fractions: N2 , O2 , CH4 and CO which eluted quickly as one peak that was directed to column 2 (MS-5A) which separated these gases. The other fraction of the sample, which moved very slowly, was composed of CO2 , C2 , C3 , and C4 gases and was directed to column 3 (Haysep Q) which separated them into various components. An automatic valve control using the Shimadzu CLASSVP program accomplished the column switching. The carrier gas was helium at a 7ow rate of 25 ml min−1 . Other parameters used were: oven temperature at 323 K, run time of 30 min, injection temperature of 473 K. The gas sample was injected automatically using a 2 ml gas-sampling loop. Calibration curves were drawn using standard Scotty II Analyzed gases supplied in 14 l cylinders at 240 psig and 294:1 K. 3. Results and discussion Each experiment was assigned a Run ID and the observations corresponding to each experimental run are presented in Tables 2–6 and Fig. 3. 3.1. Overall material balance and gas composition The inputs, outputs and the overall material balance for the lab-scale experiments are presented in Table 2. The typical temperature at which the inlet carrier gas 7ow rate was measured was 302 K. The measured outlet gas 7ow rate was 27.1–27:5 l min−1 at a typical exit temperature of 306 K. The exit gas density was calculated using the Soave–Redlich–Kwong (SRK) equation of state [11] at a pressure of 1 atm and 294:1 K (standard conditions at which the gas 7ow rate is indicated by the mass 7owmeter), using the gas composition given in Table 3, and the nitrogen content of the gas mixture. Since, the mass 7owmeter was calibrated using air=N2 gas, correction for the gas density was not necessary. The average exit gas density for the 13 experiments was 1:1603 kg m−3 . The various components identiAed and quantiAed in the gas samples on an N2 -free basis were CH4 , CO2 , C2 H2 , C2 H4 , and C2 H6 as shown in Table 3. The elution times of the N2 and CO peaks in the gas chro-

matograms were identical and quantiAcation of CO was not possible. Therefore, the quantity of carbon monoxide in the samples was calculated based on the literature [6] value of CH4 =CO = 0:16 for corn stover pyrolysis. The calculated CO concentration was subtracted from the measured N2 concentration from the gas chromatogram, to give the net N2 concentration in the exit gas mixture. A sensitivity analysis of ±20% change in the CH4 =CO ratio resulted in less than 1.5% change in the carbon balance. Hydrogen is usually a product of pyrolysis and it is possible that hydrogen was not produced in significant quantities to enable detection in the gas chromatogram. The yield of H2 is no more than 1.5% by weight at 1073 K [12]. The pyrolysis was done at 873 K. At this temperature not all the liquid tar fraction in the system was gasiAed, leading to lower yields of hydrogen. By assuming that 1% by weight of the N2 -free product gas stream was hydrogen, the H2 composition (mole%) in the gas was calculated using the ratio H2 =CH4 = 1:6. A sensitivity analysis of ±20% change in H2 composition indicated less than 0.1% change in the overall material balance. The average gas composition (mole%) on an N2 -free basis for all the 13 lab-scale runs was: H2; calculated = 13:4%, CH4 = 8:4%, CO2 = 22:08%, COcalculated = 52:2%, C2 H2 = 3:4%, C2 H4 = 0:5%, C2 H6 = 0:02%. Visual observation of the 7ow of SHB through the plexiglass window revealed that the feed entrained into the reactor on an intermittent rather than a continuous basis. So the quantity of biomass pyrolyzed in the reactor followed a step function with a cyclic frequency. It was suspected that the observed gas composition was dependent on the time when gas sampling was done. To overcome this problem, sampling was done at three speciAed times during each run and the average of these three compositions was used in the calculations. However, after the gas analysis was done for the samples collected at various times during the reaction, it was observed that the average variation in the gas composition (mole%) on a nitrogen-free basis was minimal for H2; calculated (±0:25%), CH4 (±0:1%), C2 H2 (±0:6%), C2 H4 (±0:25%), and C2 H6 (±0:3%). The maximum variation was observed for CO2 (±2%) and COcalculated (±1%). Analysis of the gas sample lost due to leaks from the Tedlar J gasbags was also done. This revealed that the loss was approximately 4% in a 24-h period for all the gas components on

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

657

Table 2 Inputs, outputs and overall material balance for lab-scale experiments Run ID

LBLNK1 LBLNK2 LNIAC1 LNIAC2 LZNAC1 LCUAC1 LCOAC1 LCRAC1 LNICT1 LZNCT1 LCUCT1 LCOCT1 LCRCT1

Run time (min)

25 28 26 21 41 38 40 40 41 38 40 40 42

Input wet

basis

(g)

Output wet

Feed

N2

Char

Tar

Gas

N2 -free gas

51.2 56.6 53.1 44.2 86.4 78.3 84.0 82.7 88.1 79.4 85.6 83.1 91.0

762 853 792 640 1249 1158 1219 1219 1249 1158 1219 1219 1280

9.5 10.1 9.5 7.3 13.8 11.7 12.6 13.8 14.5 12.6 12.1 12.0 15.1

5.1 6.1 11.0 9.0 15.5 11.7 19.6 13.6 14.7 15.1 16.2 17.0 15.9

791 885 818 661 1299 1212 1265 1260 1297 1203 1263 1265 1323

29.1 31.8 25.9 20.8 49.5 53.7 45.8 41.1 47.4 45.5 44.0 46.4 43.6

basis

(g)

Missing mass (g) N2 -free

Mass balance (%) N2 -free

8.0 9.1 7.2 7.5 8.4 1.9 6.7 14.9 12.3 6.8 14.0 8.5 17.2

84 84 87 83 90 98 92 82 86 91 84 90 81

Table 3 Nitrogen free exit gas composition (mole%) for lab-scale experiments Run ID

H2

LBLNK1 LBLNK2 LNIAC1 LNIAC2 LZNAC1 LCUAC1 LCOAC1 LCRAC1 LNICT1 LZNCT1 LCUCT1 LCOCT1 LCRCT1

15.8 14.2 11.4 12.2 13.3 14.2 13.7 13.7 12.9 14.0 12.8 12.6 12.9

calculated

(%)

CH4 (%)

CO2 (%)

COcalculated (%)

C2 H2 (%)

C2 H4 (%)

C2 H6 (%)

9.9 8.9 7.1 7.6 8.3 8.9 8.5 8.6 8.1 8.8 8.0 7.9 8.1

10.3 18.0 34.2 29.0 22.8 16.4 19.8 19.5 24.7 18.7 25.2 26.0 22.5

61.7 55.6 44.4 47.5 52.0 55.6 53.3 53.7 50.5 54.7 50.1 49.3 50.5

1.9 2.5 2.7 3.4 3.1 3.6 4.2 3.9 3.3 3.5 3.3 3.8 5.4

0.5 0.7 0.3 0.3 0.5 0.9 0.5 0.6 0.5 0.3 0.7 0.4 0.6

0.0 0.0 0.0 0.0 0.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0

a mole basis, resulting in no relative changes in the gas composition. Since, all the samples were analyzed within this period it was safe to assume that the measured gas composition was equal to the actual gas composition. The blank corn stover and the SHB-Ni acetate were run in duplicate. The repeatability of these experiments was within 4%, based on the N2 -free mass balance. Thus, subsequent lab-scale experiments were not done in duplicate. The overall mass balance closure for all the runs was above 99%. As N2 accounted for more than 93% of the input and the output streams,

an N2 -free mass balance was also done. This varied between 81% and 98% as shown in Table 2. 3.2. Product yields The total yields of the various product streams are shown in Fig. 3. Yield is deAned as the ratio of the output (weight) of the product of interest to the amount (weight) of the feed material input, expressed as a percentage. The char yield varied between 14.1% for the SHB prepared with Cu citrate and 18.5% for the blank corn stover. The diMerence in char yields between the

658

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

Table 4 Moisture, ash, and carbon content of feed, char, and tar calculated for lab-scale experiments Run ID

Feed (wt%)

LBLNK1 LBLNK2 LNIAC1 LNIAC2 LZNAC1 LCUAC1 LCOAC1 LCRAC1 LNICT1 LZNCT1 LCUCT1 LCOCT1 LCRCT1

Char (wt%)

Tar (wt%)calculated

Moisturea:r

Ashm:f

Carbona:r

Moisturea:r

Ashm:f

Carbona:r

Asha:r

Carbona:r

6.8 6.8 7.9 7.9 6.9 7.4 7.2 7.1 7.3 7.0 6.9 7.0 6.5

3.9 3.9 5.5 5.5 4.0 4.3 4.3 3.6 4.4 4.0 3.8 3.2 3.6

33.2 33.2 30.1 30.1 33.2 34.8 31.2 30.3 34.2 31.9 33.4 31.4 30.4

6.0 6.0 6.2 5.9 5.6 5.5 5.8 4.7 4.0 4.9 4.1 4.8 3.8

15.1 17.2 23.6 24.2 18.6 22.1 20.8 17.4 21.3 18.9 18.9 16.0 17.6

60.1 63.4 53.1 58.1 50.4 50.1 55.5 62.6 53.6 52.0 54.9 56.6 67.3

10.3 7.2 5.4 6.5 5.3 6.0 4.4 3.5 4.5 4.6 5.3 3.8 3.0

6.2 7.3 10.9 13.9 20.5 10.2 10.5 7.3 31.4 11.2 33.8 11.3 7.0

a.r: as received; m.f: moisture free.

Table 5 Ash and carbon balance for lab-scale experiments Run ID

Ash balance

Carbon balance

Feed in (g) Char out (g) Closure (%) Tar out; calcu (g) Feed in (g) Char out (g) Gasout (g) Closure (%) Tar out; calcu (g) LBLNK1 LBLNK2 LNIAC1 LNIAC2 LZNAC1 LCUAC1 LCOAC1 LCRAC1 LNICT1 LZNCT1 LCUCT1 LCOCT1 LCRCT1

1.9 2.1 2.7 2.2 3.2 3.1 3.3 2.8 3.6 3.0 3.1 2.5 3.0

1.3 1.6 2.1 1.7 2.4 2.4 2.5 2.3 3.0 2.3 2.2 1.8 2.6

72 79 78 74 75 78 74 83 82 77 72 74 84

0.5 0.4 0.6 0.6 0.8 0.7 0.9 0.5 0.7 0.7 0.9 0.7 0.5

duplicate blank corn stover runs and the Ni-SHB runs was 0.7% and 1.4%, respectively. The mean yield for the two blank stover runs was 18:2 ± 0:5%. For the Ni-SHB runs, the mean yield was 17:2 ± 0:97%. Presence of a metal at high concentrations in the corn stover decreased the char yield by a maximum of about 4.4%. The lowest char yields resulted for SHB containing Cu and Co, followed by Zn, Ni and Cr.

17.0 18.8 16.0 13.3 28.7 27.2 26.2 25.1 30.1 25.3 28.6 26.1 27.6

5.7 6.4 5.0 4.2 7.0 5.9 7.0 8.6 7.8 6.6 6.6 6.8 10.2

11 12 10 8 19 20 17 15 18 17 16 17 16

98 98 92 91 89 96 92 96 85 93 81 93 96

0.3 0.4 1.2 1.3 3.2 1.2 2.0 1.0 4.6 1.7 5.5 1.9 1.1

Statistical testing was carried out using the procedure [13] described in the appendix. The mean char yields for Cu-SHB and Co-SHB were signiAcantly diMerent from that of blank SHB at 90% conAdence level. A comparison between the mean char yield of metal acetate SHB and metal citrate SHB shows no diMerence. Based on this analysis, it can be concluded that the metal salt used to prepare the SHB did not

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

659

Table 6 Metal balance and metal composition of feed, char and tar in lab-scale experiments Run ID

Metal conc. in feed (mg kg−1 )

Total metal in feed (mg)

Metal conc. in char (mg kg−1 )

Total metal in char (mg)

LBLNK1-Ni LBLNK1-Zn LBLNK1-Cu LBLNK1-Co LBLNK1-Cr LBLNK2-Ni LBLNK2-Zn LBLNK2-Cu LBLNK2-Co LBLNK2-Cr LNIAC1 LNIAC2 LZNAC1 LCUAC1 LCOAC1 LCRAC1 LNICT1 LZNCT1 LCUCT1 LCOCT1 LCRCT1

34 30 20 11 25 34 30 20 11 25 22,700 22,700 29,600 13,000 7150 2610 7850 14,000 4890 4340 1380

1.74 1.54 1.04 0.54 1.26 1.93 1.70 1.15 0.60 1.40 1205.4 1002.2 2558.3 1017.9 600.7 215.8 691.6 1110.9 418.3 360.6 125.6

105 100 102 51 72 114 114 104 51 86 111,800 126,600 153,100 75,300 36,300 13,100 38,600 73,600 29,600 17,400 7440

0.99 0.94 0.96 0.48 0.68 1.15 1.15 1.05 0.51 0.87 1062.1 924.2 2112.8 881.0 457.4 180.8 558.9 927.4 358.2 208.8 112.3

have any eMect on the char yields for the same metal species. The tar yield for the 13 runs was in the range 10.0 –23.3%. The minimum tar yield was observed for the blank corn stover runs. The diMerence between the tar yields for the two blank corn stover runs was 0.8%. The presence of a metal in the corn stover increased the tar yield for all the metals studied. The maximum tar yield (23.3%) observed was for the SHB prepared with Co acetate. This yield was 125% greater than the tar yield for blank corn stover. The tar yields for the various SHBs were higher than the blank corn stover yields by the following amounts: Ni acetate and Co citrate—98%, Zn citrate and Cu citrate—82%, Zn acetate and Cr citrate—70%, Cr acetate and Ni citrate—60%, and Cu acetate—45%. It appears that the metal in the SHB acted as a catalyst and promoted tar formation. The gas yield for the 13 runs was determined on an N2 -free basis and varied between 47.0% for the SHB prepared with Cr citrate and 67.7% for the SHB prepared with Cu acetate. The average gas yield for the

Metal conc. in tar (mg kg−1 ) 143 101 18 15 85 107 68 15 11 61 200 240 1920 22 62 15 190 110 200 58 12

Total metal in tar (mg)

Metal balance (%)

0.73 0.52 0.09 0.07 0.44 0.65 0.42 0.09 0.07 0.37 2.2 2.2 29.8 0.3 1.2 0.2 2.8 1.7 3.2 1.0 0.2

99 95 102 103 89 94 92 100 97 88 88 92 84 87 76 84 81 84 86 58 90

two blank corn stover runs was 55.5%. The diMerence in the yields between the two blank runs was 0.6%. A comparison of the gas yields between the blank corn stover runs and the SHB showed that there was a decrease in gas yield for SHB prepared with Ni acetate, Cr acetate and Cr citrate and an increase in gas yield for SHB-Cu acetate. Zinc and Co metal did not in7uence gas yields. SHB prepared using Ni citrate and Cu citrate decreased the gas yield, but the eMect was relatively small. A statistical analysis was not performed to study the diMerences in the gas yield due to the assumptions made in calculating the gas composition. 3.3. Ash and carbon component balance Three diMerent component balances, ash, carbon and metal, were done for each experiment to provide insights into the pyrolysis behavior of the SHB. The ash and carbon component balances are discussed in this section. The moisture and carbon content in the feed and char, shown in Table 4, are given on a wet weight

660

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

Fig. 3. Product yields for the lab-scale experiments.

basis, while the ash content is presented on a dry weight basis. The average moisture, ash and carbon contents of the feed were 7.1%, 4.2%, and 32.1%, respectively. The moisture content of the char varied between 3.8% and 6.2%. The average moisture content of the char from SHB prepared with metal acetate salt and metal citrate salt was 5.6% and 4.3%, respectively. Thus, metal citrate appeared to make the char more hydrophobic than the metal acetate salt. The typical moisture content of the char from metal-doped feed was lower than that of char from blank corn stover. The ash in the char varied between 15.1% for blank corn stover and 24.2% for the SHB prepared using Ni acetate. Carbon content observed in the char was in the range of 50.1– 67.3%. SHB prepared using Cu acetate appeared to have the highest eHciency in oxidizing carbon in the char while the SHB prepared using Cr citrate had the least eMect. The amount of ash present in the char as a percent of the total ash in the feed and the amount

of carbon present in the char and gas as a percent of the total carbon in the feed is presented in the ash and carbon component balance in Table 5. The distribution of carbon in feed, char and gas is also shown in Table 5. The amount of ash and carbon in the tar could not be quantiAed due to matrix restrictions. Acetone was used as the solvent to wash and collect the tar from the tar boxes (see Fig. 1). Though the tar samples were exposed to air to volatilize the acetone, residual quantities of acetone remaining in the tar hindered the measurement of ash and carbon. Based on a 100% mass balance closure for the ash and carbon component balances, the total ash and carbon in the tar is presented as Tar out; calculated in Table 5 and Tar (wt%)calculated in Table 4. The ash mass closure varied between 72% for the blank corn stover run to 84% for the SHB-Cr citrate run. The carbon mass closure varied between 81% for SHB-Cu citrate and 98% for the blank corn stover. It should be noted that the ash and carbon content in

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

the tar fraction was not included in the above mass closures. The ash and carbon content by weight in tar was calculated assuming that all the ash and the carbon, which was not present in the char or gas product streams, would be present in the tar. As shown in Table 4, ash varied between 3% and 10.3% by weight (average of 5.4%), while carbon varied between 6.2% and 33.8 wt%. 3.4. Metal component balance and metal distribution in the product streams The distribution of metal in the product streams and the metal balance for each type of feed is given in Table 6. The SHB was prepared so that it would represent the metal concentrations in actual hyperaccumulators [1]. The concentration of metal in the feed on a wet basis is given in Table 6. These concentrations are lower than the concentrations presented in the Arst part of this paper due to the size reduction operation of corn stover prior to performing the lab-scale pyrolysis experiments. Some of the impregnated metal was lost in ground SHB Ane particles (¡ 80-mesh) that were discarded. For the blank corn stover runs, the metal component mass closure was between 88% and 103%. The distribution of metal in mg in the product stream for the blank corn stover runs showed that the average metal content in the char was higher (Ni—61%, Zn—69%, Cu—92%, Co—66%, Cr—88%) than the tar (Ni—39%, Zn—31%, Cu—8%, Co—34%, Cr— 12%). However, for SHB doped with a metal, more than 98.5% of the metal in the product stream was contained in the char. The concentration (mg kg−1 or ppm) of metal in the char increased by greater than four times as compared to that in the SHB feed. Cu concentration for SHB prepared with acetate and citrate salts increased nearly six fold in the char. From a metallurgical perspective, the char from each of the SHB feed is a metal concentrate. Irrespective of whether the SHB was prepared from metal acetate or metal citrate, almost all of the metal was contained in the solid product stream, namely the char. Metal component balance for the SHB containing a doped metal varied from 58% for Co citrate to 92% for Ni acetate. However, the metal component balance was typically between 80% and 90% for most of the SHB feeds, as shown in Table 6. For Zn and Cu the

661

metal salt used to prepare the SHB did not aMect the metal component balance. The Ni and Cr component balance for SHB prepared using citrate salt was nearly 9% lower and 6% higher, respectively, compared to the SHB prepared with acetate salt. The Co component balances were unusually low (acetate salt—76%, citrate salt—58%). This is attributed to the possible volatilization of the metal in the SHB into the heating zone of the reactor. Subsequent coating of the interior of the reactor with the metal in the downstream section of the system or escaping of the metal in the exit gas would lead to loss of metal from the char. It is also speculated that these contributed to the less than 100% metal component balance observed for all the runs. This eMect was also observed by Helsen et al. [14] during the pyrolysis of chromium copper arsenate (CCA) treated wood. 4. Conclusions A summary of the observations for the pyrolysis experiments is presented in Table 7. The overall material balance closure was greater than 99% for all the experiments. On an N2 -free basis, the mass closure was 81–98%. The average gas composition on an N2 -free basis (mole%) was H2; calc = 13:4%, CH4 = 8:4%, CO2 = 22:08%, COcalc = 52:2%, C2 H2 = 3:4%, C2 H4 = 0:5%, C2 H6 = 0:02%. The char yield was between 14.1% and 18.5%. A decrease in char yield was observed for SHB containing a doped metal compared to the blank corn stover. There was no aMect on the char yield due to salt type (acetate or citrate) used to prepare the SHB. The tar yield varied between 10.0% and 23.3%. While the type of salt used to prepare the SHB did not aMect the tar yield, the metal in the SHB increased the tar yield. Gas yield varied between 47% and 67.7%. The gas yield for lab-scale experiments for SHB prepared with metal acetate and metal citrate increased in the following order Ni ¡ Cr ¡ Co ¡ Blank run ¡ Zn ¡ Cu and Cr ¡ Cu ¡ Ni ¡ Co ¡ Blank run ¡ Zn, respectively. Ash component balance for lab-scale experiments was between 72% and 84%. Carbon component balance was between 81% and 98%. The carbon

662

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

Table 7 Summary of lab-scale observations Observation

Lab-scale experiment

General

Gas composition

• • • • • •

Product yields

•

Overall material balance

• • Ash component balance Carbon component balance Metal component balance

• • • • • • • • • • • •

Reactor ID = 5:03 cm (2 in); Length of heated reactor zone = 36:83 cm (14:5 in) Mode of heating—external furnace heating Apparent gas residence time at 873 K = 0:6 s ¿ 99% N2 -free mass balance = 81–98% Average gas composition (mole%) on an N2 -free basis: H2; calc = 13:4%, CH4 = 8:4%, CO2 = 22:08%, COcalc = 52:2%, C2 H2 = 3:4%, C2 H4 = 0:5%, C2 H6 = 0:02% Char yield: 14.1–18.5%; Presence of a heavy metal in the SHB decreases the char yield compared to the blank run; Char yield for Cu ¡ Co ¡ Zn ¡ Ni ¡ Cr ¡ Blank run; No eMect on the char yield from the type of salt (acetate or citrate) used to prepare the SHB Tar yield: 10.0 –23.3%; Presence of a heavy metal in the SHB increases the tar yield compared to the blank run; No trend observed based on the type of salt used to prepare the SHB on the tar yield Gas yield: 47– 67.7%; Gas yield for SHB prepared with metal acetate: Ni ¡ Cr ¡ Co ¡ Blank run ¡ Zn ¡ Cu; Gas yield for SHB prepared with metal citrate: Cr ¡ Cu ¡ Ni ¡ Co ¡ Blank run ¡ Zn 72–84% 81–98% Average carbon content (wt.) of char: 57% Blank corn stover runs: 88–103% SHB doped with a metal: 58–92% Metal concentration (mg kg−1 ) in the char (“ore” or metal concentrate) varied between 0.7% and 15.3% depending on the type of metal in the SHB Greater than 98.5% of the metal in the product stream is present in the char for experiments done with SHB doped with a metal irrespective of the form (acetate or citrate) of the metal in the SHB Metal concentration in the char increased by 4 – 6 times compared to the SHB feed Metal component balance for SHB-Ni acetate greater by 9% compared to SHB-Ni citrate Metal component balance for SHB-Zn and SHB-Cu nearly the same irrespective of the metal salt used to prepare the SHB Co component balances unusually low due to possible volatilization of metal, however metal component balance for SHB-Co acetate greater than SHB-Co citrate by 18% Metal component balance for SHB-Cr citrate greater by 6% compared to SHB-Cr acetate

in the tar was not accounted for due to matrix considerations during carbon analysis. Metal component balance closure for the blank corn stover runs varied between 88% and 103%. In the case of SHB doped with a metal, the closure was between 58% and 92%. Possible volatilization of the metal and re-deposition on the reactor interior surface is suspected to account for the low cobalt mass closures. More than 98.5% of the metal in the product stream was present in the char stream. Compared to the metal concentration in the SHB feed, the concentration in the char increased 4 – 6 times. The concentration of the metal in the char was between 0.7% and 15.3%. Char with such high concentrations of metal can be considered a rich “ore” or metal concentrate, which can be processed for

possible separation of the metal in a conventional ore-processing unit.

Appendix: Statistical analysis for di erences in treatments for lab-scale experiments Statistical analysis in this section addresses two different issues for the lab-scale experiments: (a) diMerence in product yields (char, tar and gas) resulting from presence of a metal in the SHB compared to the blank run; (b) diMerence in product yields (char, tar and gas) resulting from pyrolysis of SHB prepared with acetate and citrate salt.

L. Koppolu et al. / Biomass and Bioenergy 25 (2003) 651 – 663

The statistic used in this discussion is called the least signiAcant diMerence (lsd). It involves the analysis of variance as a test criterion for planned comparisons of paired means [13]. The various groups considered in this statistical analysis consist of unequal number of replications. The calculations indicate the presence or absence of a signiAcant diMerence at a particular conAdence level between the compared means. The procedure for calculating the lsd is given by the following equations: Treatment number = i Replication number in treatment i = ij Number of replications in treatment i = ri Observation (yield) recorded for each sample = xij

xi = xij and x = xi ; j

xi2

=




i

xij2

and

j

(x)2 =




2

x =




[3]

[4] [5]

[6]

[7]

xi2 ;

j

((xi )2 =ri );

[8]

j

xTi = xi =ri : Error sum of squares, ESS = x2 − (x)2 . Degrees of freedom within treatments, df = j (ri − 1).  lsd=t(conAdence level; df ) ESS2 ((1=r1i )−(1=r2i )), where, t = table value for conAdence level chosen and df r1i and r2i =number of replications for means being compared. References [1] Koppolu L, Clements LD. Pyrolysis as a technique for separating heavy metals from hyperaccumulators—Part I: Preparation of synthetic hyperaccumulator biomass. Biomass and Bioenergy 2003;24(1):69–79. [2] Glass DJ. Economic potential of phytoremediation. In: Raskin I, Ensley BD, editors. Phytoremediation of toxic metals—

[9] [10]

[11] [12] [13] [14]

663

using plants to clean up the environment. New York, NY: Wiley; 2000. Brooks RR, Robinson BH. The potential use of hyperaccumulators and other plants for phytomining. In: Brooks RR, editor. Plants that hyperaccumulate heavy metals—their role in phytoremediation, microbiology, archaeology, mineral exploration and phytomining. New York, NY: CAB International; 1998. National Priorities List Fact Book. HW 7.3. US Environmental Protection Agency. Washington, DC, 1986. p. 94. FUorstner U. Land contamination by metals—global scope and magnitude of problem. In: Allen HE, Haung CP, Bailey GW, Bowers AR, editors. Metal speciation and contamination of soil. Boca Raton, FL: Lewis Publishers; 1995. p. 1–33. Agblevor FA, Besler S, Wiselogel AE. Production of oxygenated fuels from biomass: impact of feedstock storage. Fuel Science and Technology International 1996;14(4): 589–612. Agblevor FA, Evans RJ, Johnson KD. Molecular beam mass spectrometric analysis of lignocellulosic materials. I: herbaceous biomass. Journal of Analytical and Applied Pyrolysis 1994;30:125–44. Janarthanan AK, Clements LD. GasiAcation of wood in a pilot-scale spouted bed gasiAer. In: Bridgewater AV, Boocock DGB, editors. Developments in thermochemical biomass conversion, vol. 2. New York, NY: Blackie Academic & Professional, 1997, p. 945 –59. Janarthanan AK. Design, construction and operation of a pilot-scale spouted bed biomass gasiAer. MS thesis, University of Nebraska-Lincoln, Lincoln, NE, 1996. Jones JB, Case VW. Sampling, handling, and analyzing plant tissue samples. In: Westerman RL, editor. Soil testing and plant analysis. Madison, WI, USA: Soil Science Society of America, Inc.; 1990. p. 389–427. Clements LD, Millwright SS, Puckett JH, Osmani AS. Texas Instruments Software: TI-95 Chemical Engineering Library Guidebook. Texas Instruments, 1986. Arpiainen V, Lappi M. Products from the 7ash pyrolysis of peat and pine bark. Journal of Analytical Pyrolysis 1989;16:355–76. Steel RGD, Torrie JH, Dickey DA. Principles and procedures of statistics: a biometrical approach. New York: McGraw-Hill; 1997. Helsen L, Bulck EVD, Broeck KVD, Vandecasteele C. Low-temperature pyrolysis of CCA-treated wood waste: chemical determination and statistical analysis of metal input and output; mass balances. Waste Management 1997;17(1): 79–86.