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Behavior of Heavy Metals in Steam Fluidized Bed Gasification of Contaminated Biomass Michal Syc,* Michael Pohorely , Michal Jeremias, Martin Vosecky , Petra Kameníkova, Siarhei Skoblia, Karel Svoboda, and Miroslav Puncochar Environmental Process Engineering Laboratory, Institute of Chemical Process Fundamentals of the ASCR, v. v. i., Rozvojova 135/2, Prague 6—Suchdol, Czech Republic
bS Supporting Information ABSTRACT: Heavy metal phytoextraction by growing energy crops such as flax could be a promising approach for remediation of brownfields with energy benefits. The present paper deals with flax (40 wt %) and hardwood (60 wt %) cogasification with particular focus on the distribution of heavy metals to both solid and gaseous gasification products. The blended fuel was gasified by steam in a fluidized bed gasifier at about 850 °C and steam to biomass ratio of 1.01 kg kg1. Concentrations of selected heavy metals (Cd, Cu, Ni, Pb, and Zn) were determined in bed ash, cyclone ash, and in downstream producer gas. From the analysis, it follows that under the given experimental conditions the subsequent order of heavy metals volatility can be found: Cd (mostly in producer gas) > Pb > Zn > Cu > Ni. Heavy metals concentrations in the producer gas (nitrogen free, dry gas) were determined in the range of 0.374.2 mg m3.
1. INTRODUCTION Substitution of fossil fuels by biomass is one of the main worldwide challenges and items of current energy research. This trend brings about some global drawbacks because energy crops seize forests and soils used for food production. Therefore, energy crops growing on unused soils in industrial agglomeration and/or contaminated soils and brownfields could avoid the above-mentioned disadvantages. Moreover, it is well-known that some plants have phytoremediation ability. Therefore, decontamination and/or stabilization of given sites by their growing could be achieved. The most progressive approach seems to be growing of energy crops with heavy metal phytoextraction ability. Two kinds of plants can be used for heavy metal phytoextraction—hyperaccumulators and fast growing species. High phytoextraction ability as well as low biomass production are characteristic features for hyperaccumulators.1,2 Fast growing species have lower phytoextraction ability than hyperaccumulators, but the total biomass production is significantly higher.3 The remediation capacity of plants, i.e. total quantity of metal which plants are capable of absorbing from soil to above-ground parts, is surprisingly similar when hyperaccumulators and fast growing species are compared.4,5 Certain species of short rotation coppice or energy crops meet the requirements for the continuous phytoextraction process. Willows (Salix spp.) growing on brownfields with increased metal uptake were investigated by Lord et al.6 Vervaeke et al.7,8 reported positive effects of willows growing on soils contaminated by dredged sediment disposal. Various poplar genotypes are also well-suited for heavy metal phytoextraction.9,10 Unterbrunner et al.11 advised that also adult poplar and willow species can be used. Besides, not only dendroid species accumulate heavy metals. Angelova et al.12 investigated the phytoextraction ability of flax, hemp, and cotton. The highest uptake of heavy r 2011 American Chemical Society
metals was found for flax. Moreover, the ability of flax to accumulate Cd from agriculture soils with natural baseline values was reported many times before.1316 A lot of other fast growing species are mentioned in relation to higher heavy metal uptake, for example: sorrel,17 some weed species,18,19 rape,20 maize,21 sorghum,22 sunflower,22 etc. Heavy metal phytoextraction is feasible and economically profitable only with additional treatment of produced biomass.20 Composting and compaction are suggested as postharvesting pretreatment methods for biomass weight reduction and as transport cost-saving measures.23,24 Thermochemical conversion (e.g., combustion or gasification and pyrolysis) is proposed as the most appropriate energetic and final disposal method for energy crops.23,24 Lievens et al. studied heavy metals distribution during fast pyrolysis of contaminated birch, sunflowers, and willows.2527 Very few combustion tests have been carried out with biomass containing higher concentrations of heavy metals than their natural baseline values defined by Kabata-Pendias.28 Moreover, most combustion studies are based on thermodynamic equilibrium calculations29 and/or incineration simulations,30 despite the fact that input thermodynamic data vary often.31 However, some trends in heavy metals behavior during fluidized bed combustion32 or cocombustion33 of biomass with other fuels are known. Another way of thermochemical conversion of energy crops is gasification. One of the most commercialized biomass gasification technologies is the allothermal steam fluidized bed gasification, where steam is used as the gasification agent. This process produces calorific gas suitable for novel technologies for efficient Received: December 9, 2010 Revised: April 13, 2011 Published: April 13, 2011 2284
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Table 1. Ultimate and Proximate Analysis of Blended Fuel and Its Components (Raw State) components
hardwood
flax
Table 2. Major Ash-Forming Elements Content in Blended Fuel and Its Components (Raw State)
blended fuel
ash-forming elements 1
hardwood
flax
blended fuel
9.4 ( 0.3
6.4 ( 0.9
8.1 ( 0.4
Al (mg kg )
265 ( 60
3770 ( 860
1670 ( 340
ash (wt %) combustibles (wt %)
0.9 ( 0.1 89.7 ( 0.5
25.6 ( 0.9 67.9 ( 1.2
12.3 ( 0.4 79.6 ( 0.6
Ca (mg kg1) Fe (mg kg1)
4900 ( 1070 402 ( 101
11600 ( 2400 1580 ( 390
7590 ( 1150 871 ( 167
volatiles (wt %)
75.9 ( 0.7
54.0 ( 0.6
65.8 ( 0.5
K (mg kg1)
1290 ( 390
15500 ( 3200
6990 ( 1290
fixed carbon (wt %)
13.8 ( 0.4
13.9 ( 0.4
13.8 ( 0.3
Mg (mg kg1)
703 ( 156
1800 ( 370
1140 ( 170
moisture (wt %)
higher heating value (MJ kg1)
17.6
14.1
16.2
Mn (mg kg1)
48.6 ( 10.4
95.7 ( 23.5
67.4 ( 16.7
lower heating value (MJ kg )
16.0
13.0
14.8
Na (mg kg1)
200 ( 46
4930 ( 1000
2090 ( 400
carbon (wt %)
44.2
33.9
40.1
P (mg kg1)
193 ( 47
3140 ( 640
1370 ( 260
oxygen (wt %)
41.3
23.9
34.3
Ti (mg kg1)
11.7 ( 2.9
66.6 ( 16.1
33.7 ( 6.7
hydrogen (wt %) nitrogen (wt %)
5.49 0.201
4.11 1.87
4.94 0.76
sulfur (wt %)
0.3 μm at temperatures higher than the tar dew-point. Producer gas was subsequently cooled in an air cooler and passed through the two serial impingers with a solution of HNO3 (3.3 wt %) and H2O2 (1.5 wt %). The temperature of the producer gas in sampling point was 380 430 °C. Heavy metals were analyzed separately in the solid and liquid part of the collected samples. The solid parts of the samples (fly ash and glass filter) were digested in three steps by HNO3/HCl/HF and diluted by distilled water to 100 mL. The content of heavy metals in digests as well as in the liquid phase (sorptive solutions) was determined by ICP-OES (PerkinElmer Optima 2000 DV). Seven samples, each collected during 1 h, were obtained during 11.5 h-long gasification experiment. Ash samples (cyclone and bed ash) were also decomposed by digestion in three steps by HNO3/ HCl/HF and diluted by distilled water to 100 mL. Heavy metals contents in the digests were determined by ICP-OES (Perkin-Elmer Optima 2000 DV). Analysis of certified reference material (BCR 038 fly ash from pulverised coal) confirmed applicability of the analytical procedure and results.
3. RESULTS AND DISCUSSION Producer Gas. The main process parameters relating to producer gas such as gas yield, lower heating value (LHV), and composition are shown in Figure 2 (recalculated on nitrogen free, dry gas conditions). At a chosen steam to biomass ratio (1.01 kg kg1), an average dry gas yield 1.19 ( 0.04 m3 kg1 with a lower heating value of 14.6 ( 0.4 MJ m3 was obtained. The estimated fuel carbon conversion to producer gas components was 90%. The composition and yield of producer gas was relatively stable. Only slight evolution toward higher gas yield and lower LHV can be observed. This is caused by slight increase of H2 and CO2 concentration and by the decrease of CO, CH4, and C2C7 hydrocarbons (CxHy) concentration in the producer gas (see Figure 2). This effect can be attributed to the accumulation of ash material in the sand fluidized bed. The ash material with significant concentrations of Al, Ca, Fe, K, Mg, Mn, and Na (see Table 2) can play a catalytic role by shifting the water gas reaction equilibrium toward higher concentration of H2 and CO2
Figure 3. Heavy metal distribution.
and lower concentration of CO. The slight decrease of CH4 and higher organic compounds concentration can be attributed to the catalytically enhanced steam reforming reaction.55,56 The tar in the producer gas was also evaluated. According to the standard definition of the tar protocol,53,54 tar is the sum of hydrocarbons with molecular weight equal or higher to toluene. The average concentration of tar in the producer gas was constantly 11.4 ( 0.4 g m3; however, the proportion of different tar species changed during the experiment. The concentration of heavy tars decreased considerably, accompanied by the increase of lighter tar concentration. As heavy tars condensate at higher temperature, the tar dew point decreased. The table with detailed tar concentration is provided in the Supporting Information. Behavior of Heavy Metals. The heavy metal balance closure was in the range of 5085% with the exception of Cd for which it was 110%. Due to analyses and balance uncertainty, the heavy metal closure is acceptable and comparable with other relevant studies.32,46 For further discussion of heavy metal distribution an assumption that heavy metals found in output flow streams are 100% of the total amount was used. The obtained heavy metal distribution is depicted in Figure 3. 2287
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Energy & Fuels Cadmium. The most volatile of the selected heavy metals during biomass steam gasification was Cd. Almost 90% of Cd was determined in the producer gas. There was less than 5% and 8% in the bed ash and the cyclone ash, respectively. From the results it is obvious that at the working temperature of the cyclone (ca. 450550 °C), there is ca. 90% of Cd species in the gaseous phase or on fine particles passing through the cyclone. At the temperature of producer gas sampling (380430 °C), a high content of Cd was still in the gaseous phase, namely 8090% of Cd was found in the absorption solution of the sampling apparatus and only 1020% on fine particles of fly ash. Significant Cd species condensation occurred at temperatures lower than 380 °C. Cd behavior is in good agreement with model predictions which stated that at temperature above 380 °C the main cadmium specie is Cd(g);57,58 hence, Cd volatility above this temperature is not affected by other elements. Cd contents in the bottom ash and the cyclone ash were found below the limit of detection (LOD) of ICP-OES; therefore for the estimation of Cd distribution, the LOD value was used as the representative content for the both streams. Hence, Cd distribution can be possibly shifted toward producer gas. The above-mentioned fact can be also the reason for exceeded Cd output. Lead. The second most volatile heavy metal was Pb. The major portion of Pb (ca. 77%) was found in cyclone ash separated at temperatures 450550 °C. Only minority of lead (ca. 7%) remained in the bed ash and ca. 16% of Pb amount passed through the hot cyclone in the producer gas. Therefore, it could be concluded that Pb was dominantly volatilized at a reactor temperature of 855 °C and also partially deposited on small particles entrained with the producer gas from the reactor. Models at temperatures above ca. 620 °C predict for Pb to be mainly in gaseous form regardless of probable chemical speciation.5759 Therefore volatilization of lead and subsequent condensation at temperatures lower than in the reactor but higher than in the cyclone seems to be more relevant lead behavior. The observed behavior of Pb in a reducing atmosphere and high temperatures is in good accordance with theoretical calculations.5759 An insignificant Pb chemical speciation effect on its volatility is predicted at lower temperatures as well because of very similar volatility of possible Pb species in a reductive environment. Very quick condensation is predicted when temperature decreases below ca. 600 °C whereas at ca. 500 °C prevailing solid Pb species can be expected. Generally, according to mentioned calculations, it was possible to predict dominant Pb condensation at working temperatures of the cyclone, and this fact was experimentally verified in our tests. Gas to solid partitioning of Pb in sampling point of producer gas at 380430 °C implies that the majority of Pb passed through the cyclone deposited on particles. Zinc. Zn was found predominantly in cyclone ash (55 wt %). Approximately 20% of Zn was found in bed ash and 25% in producer gas, although zinc is stated as very volatile in reductive conditions, comparable to Cd.58 According to model predictions, zinc should be present mainly in gaseous forms with prevailing Zn(g) above a temperature of 850 °C.43,58,60 The predicted behavior was experimentally verified by gasification experiments of biomass46 as well as other fuels.58,61,62 To explain the differences between published data and observations made in this study, the complex composition of the reaction system has to be considered. Zn volatility is sensitive to chlorine and sulfur content, S/Cl ratio, and the content of alkali metals.60,62 Sulfur and chlorine
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increase the volatility of zinc. On the contrary, alkali metals decrease Zn volatility because they form heavy metal volatile compounds preferentially with S and Cl. In our experiments, the fuel ratio of (S þ Cl)/(alkali metals) was ca. 0.3 (w/w), i.e. the alkali metals are in strong abundance, which means also stronger influence on decreasing the volatility of Zn. This influence itself would not explain the presence of 20% of the total Zn amount in bottom ash, because Zn should be in Zn(g) form at reductive conditions above 850 °C. Further reactions that have to be considered are the following homogeneous reactions in the gaseous phase. The oxidation of volatile zinc by carbon dioxide to solid zinc oxide and carbon monoxide (reaction 1) and especially the homogeneous oxidation of gaseous zinc by steam to solid zinc oxide and hydrogen (reaction 2) proceed at temperatures above 750 °C and are described for pyrometallurgic processes.6365 ZnðgÞ þ CO2 ðgÞ ¼ ZnOðsÞ þ COðgÞ
ð1Þ
ZnðgÞ þ H2 OðgÞ ¼ ZnOðsÞ þ H2 ðgÞ
ð2Þ
In this environment with steam abundance (steam/hydrogen molar ratio in producer gas is ca. 2.5) that is present in the gasification reactor, volatilized zinc can be transformed to solid form by homogeneous oxidation reactions to the form of ZnO. Moreover, ZnO can react with H2S and HCl and form zinc sulphide and/or zinc chloride and steam (reactions 3 and 4). ZnOðsÞ þ 2HClðgÞ ¼ ZnCl2 ðgÞ þ H2 OðgÞ
ð3Þ
ZnOðsÞ þ H2 SðgÞ ¼ ZnSðgÞ þ H2 OðgÞ
ð4Þ
However, because of the high partial pressure of steam in the reaction system and the high content of alkali metals, which decreases the partial pressure of HCl and H2S, the balance of the reactions is shifted toward reactants, which means that ZnO is relatively stable. Moreover, the stable character of formed ZnO can be also supported by the formation of zinc silicates or mixed oxides. Therefore, the reason for lower volatility of Zn in our experiments should be mainly a partial formation of stable and solid zinc oxide by homogeneous oxidative reactions in gaseous phase caused by high partial pressure of steam (and subsequently by low S þ Cl/alkali metals ratio). Nickel and Copper. Ni and Cu were found to be the least volatile of the studied heavy metals. More than half (53%) of Ni was found in the bed ash, more than a quarter (26%), in the cyclone ash, and one-fifth (21%), in the producer gas. The Cu behavior was ambiguous, without a prevailing concentration in one of the gasification product streams; nearly the same amount of Cu was observed in the bed ash and the cyclone ash. Generally, for both Cu and Ni, lower volatility is stated in a reductive than in an oxidative environment.66,67 Equilibrium calculations predicted that at temperatures of about 850 °C, no significant volatilization of Ni and Cu occurs under reducing conditions.61,68 The effect of Cl content on the volatility of Ni is described to be quite insignificant in contrast to Cu for which strong increase of volatility is assumed in the presence of chlorine.66 However, due to alkali metals predominance over Cl content, suppression of Cl effect on Cu volatility can be expected. Therefore, in accordance with model predictions, we presume that Cu and Ni are nonvolatilized and deposited on ash particles. 2288
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Table 5. Concentrations of Heavy Metals in the Producer Gas (Dry Gas, Nitrogen Free) Cd
Cu
Ni
Pb
Table 6. Heavy Metals Content in the Bed Ash and the Cyclone Ash
Zn
bed ash 1
producer gas 0.37 ( 0.26 1.2 ( 0.3 0.80 ( 0.50 1.2 ( 0.8 4.2 ( 0.8 (mg m3)
In our experiments, approximately the half of Ni and Cu (53% and 41%, respectively) remains in the reactor bed deposited on the bed ash and the next 26% of Ni and 45% of Cu deposited on the cyclone ash. Verveake et al.46 found during downdraft gasification of contaminated willows that ca. 74% of Ni and 83% of Cu remained in the bottom and bed ash, and another 19% of Ni and 11% of Cu remained in the cyclone ash while in the producer gas was only 7% of Ni and 6% of Cu. The reason for the above-mentioned different distribution is the one order of magnitude lower emissions of solid particles from the countercurrent downdraft fixed bed gasifier in comparison with a fluidized bed gasifier. Therefore, nonvolatilized nickel and copper species deposited on solid particles in the downdraft fixed bed gasifier in contrast with the fluidized bed gasifier remain in the bed whether in the bottom ash or bed ash. In the case of the fluidized bed gasifier, solid particles (dust) emissions are significant and the distribution of Ni and Cu was mainly given by their partitioning on particles with different diameters. During coal gasification, the Ni concentration increase on solid particles was observed with increasing particle diameters from 0.3 to 10 μm.59 The Ni behavior is in accordance with the mentioned study which reported an increase in Ni concentration with increasing particle size. The trend of Cu concentration as a function of particle size under gasification conditions was not found in the literature. Heavy metals volatilization during steam gasification of used blended fuel is not as high as was published for woody biomass fluidized bed air gasification69 or predicted in other literature.40 The distribution of heavy metals is given by several factors, among which the most important is chemical speciation of metals and the dynamics of fluidization, including the influence of the agglomeration effect of solid particles. The high content of alkali metals in the fuel influences both mentioned factors. The influence of fluidized bed material on the heavy metals distribution is also mentioned;70 however, in our experiments, it cannot be significantly expected because only silica sand was used. Another factor influencing the distribution of heavy metals can be the gasification agent; however, comparable experiments cannot be found in the literature, which makes more profound discussion about this subject difficult. Summarily, it can be concluded that during steam gasification the volatility of analyzed heavy metals decreases in subsequent order: Cd (mostly in producer gas) . Pb > Zn > Cu > Ni. The volatility of heavy metals affects the ratio of heavy metals transferred from the fuel to the producer gas. However, it does not indicate how much of a particular metal is present in the producer gas. Thus, concentrations of selected heavy metals in the producer gas downstream of the cyclone are indicated in Table 5. Any decisive limits for further producer gas valorization are not known. Heavy metals in deposits can initiate corrosion, and they are also catalytic poisons as well; hence, effective cleaning prior to producer gas utilization must be employed. Heavy Metals in Ashes. The content of heavy metals in the bed ash and the cyclone ash is presented in Table 6. Observed
cyclone ash
Cd (mg kg )
