Ind. Eng. Chem. Res. 2010, 49, 11825–11831
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A New Pyrolysis of Metal Hydroxide-Mixed Waste Biomass with Effective Chlorine Removal and Efficient Heat Recovery Shigeya Hayashi,* Hiroshi Amano, Toyoaki Niki, and Morihisa Yokota UBE Industries, Ltd., 1978-96, Kogushi, Ube, Yamaguchi, 755-8633, Japan
Kazuhiro Mae Department of Chemical Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto, 615-8510, Japan
Waste woody biomass samples including poly(vinyl chloride) (PVC), which were mixed with metal hydroxides as additives, were carbonized at 500 °C to investigate the catalytic effect of the additives on pyrolysis products and to elucidate the mechanism of biomass carbonization. The results showed that the yield of char significantly increased, whereas tar evolution was suppressed by the influence of metal hydroxides, even with the coexistence of PVC. Moreover, the interaction behaviors between the biomass structure including PVC and metal hydroxides were further investigated. It was clarified that the dehydration reaction to form a cross-linked structure in biomass by the effect of metal ion and the neutralization reaction between PVC and metal hydroxides were simultaneously occurred during the carbonization. The presence of chlorine component, which is one of corrosive substances, was confirmed in char structure by chlorine analysis. Therefore, a flushing method with warm and cold water was applied, to investigate the possibility of chlorine removal. The results showed that flushing with warm water and cold water are both effective for the removal of chlorine component in the char. Finally, the new pyrolysis process of NaOH-mixed waste biomass materials, including proposed heat recovery facilities, can be suggested as an effective system of biomass utilization for energy savings and CO2 reduction. 1. Introduction Through the issue of global warming, focus has been placed on CO2 reduction in the industrial process, using renewable and clean technology with waste thermal energy recovery in existing process systems. Woody biomass has been expected to be one of green fuel resources for future power generation, because of its CO2-neutral characteristics. Three thermochemical conversion technologies have been mainly considered as practical methods to produce various forms of energy and electricity effectively:1 (i) gasification under high temperature, (ii) liquefaction under high pressure, and (iii) pyrolysis. Pyrolysis is one of the promising technologies to produce combustible char, which can be practicably transported, because of its relatively high bulk density, and subsequently can be utilized in energy conversion process such as gasification. However, tar products obtained during pyrolysis, which is sometimes called carbonization, may easily cause trouble with combustible gas utilization and ultimately can damage equipment and piping systems in the process.2-6 To avoid the problems of tar products, tar removal methods under wet and/or dry conditions, including catalytic tar decomposition methods, have been studied and developed by several researchers.7 McKee has reported that alkali-metal carbonates, oxides, and hydroxides decomposed tar in catalytic gasification effectively.8 Zhang et al. have reported that nickelsupported silica showed sufficient activity for the catalytic cracking of tar at low temperature (550 °C).9 However, because a large amount of coke deposited on the catalyst surface, it was mentioned that the activity of these catalysts was short-lived.9-11 At present, Ni catalyst is one of promising catalysts for biomass tar decompositionl; however, sulfur, chlorine, and other effects of catalyst poison, and deactivation by coke decomposition from heavy tar, are cited as being problems. On the other side, many * To whom correspondence should be addressed. E-mail: 30210u@ ube-ind.co.jp.
research workers have studied the reduction of tar by conducting a pretreatment of biomass including coal. Zeng et al. have studied both thermal and steam pretreatment of Loy Yang brown coal at low temperature.12,13 The yield of tar decreased, while char yield increased, when coals were thermally pretreated in a helium atmosphere above 250 °C, and then pyrolyzed in a Curiepoint pyrolyzer at 900 °C. When pyrolyzing coals pretreated with steam above 250 °C for 30 min, the overall tar yield decreased less than that for helium pretreatment. That is, the tar yield was 6% when pyrolyzing coals pretreated with steam at 350 °C, and 15% when pyrolyzing coals pretreated with helium at 350 °C. This is because the hydrolysis reaction among weak bonds such as ester or ether bonds proceeded during steam pretreatment, which enhanced cross-linking reaction.12,13 Moreover, the effect of inorganic materials, particularly the ash content in a biomass structure, on pyrolysis behavior has been investigated by Gray et al.14 The results showed that the addition of calcium reduced the yield of tar by 13% at 330 °C and by 40% at 460 °C. The untreated samples that had ash components gave 30%-50% less tar than the acid-washed samples. An increase in the yields of aqueous products and gas corresponded with reduction of the tar yield.14 Focusing on the catalytic effect of alkali-metal compounds, Wang et al. has investigated the influence of several sodium compounds, such as NaOH, NaCl, Na2CO3, and Na2SiO3 (including TiO2 and HZSM-5), on biomass pyrolysis behavior. Four types of sodium compounds resulted in a shift of devolatilization to lower temperature with more exothermic reactions, because more char formation occurred during pyrolysis.15 Consequently, we have further studied the effect of sodium hydroxide (NaOH) on the pyrolysis profile, to clarify the validity of the proposed method for tar suppression during pyrolysis. Through pyrolysis of a NaOH-loaded Douglas fir at 500 °C, a relatively high yield of char at 32 wt % and no tar products
10.1021/ie1008234 2010 American Chemical Society Published on Web 10/08/2010
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were obtained, because the dehydration reaction to form crosslinked structure was further proceeded by the effect of added alkali metal.16,17 This suggests that the new biomass pyrolysis process with effective tar suppression and energy recovery can be proposed by adding metal hydroxide additives to the biomass structure. However, the waste from construction sites often includes waste woody board that has wallpaper (which is made from poly(vinyl chloride), PVC) affixed to it, or waste woody chips including crushed pipe made from PVC, and so on. Thus, treatment of the chlorine content, which is one of the contaminants in practical biomass resources, is one the problems of waste biomass utilization for energy generation. At present, the utilization of waste woody biomass that contains pipes or sheets made of PVC is still limited, because the corrosive substances such as chlorine in the biomass can easily lead to equipment damage. Therefore, it is necessary to investigate the removal methods of these contaminants when the combustible products obtained from pyrolysis will be further practically used for a better efficient energy recovery system. In this study, alkali-metal hydroxides and alkali-earth metal hydroxides were used as additives in waste biomass that contained PVC, to investigate the most appropriate conditions for tar suppression and energy recovery during carbonization. Analysis of the chlorine content in carbonized char, including its removal method, was carried out following the discussion on the feasibility of a practical process. The interaction behaviors among major components in additive-mixed waste biomass were also investigated, to clarify its reaction mechanism during carbonization. Finally, the evaluation of new process efficiency was comparatively determined to verify the feasibility of the proposed carbonization process of waste biomass with the optimized amount of additive components and the most appropriate energy recovery system. 2. Experimental Section 2.1. Samples Preparation of Biomass Mixtures with Additives. In this study, the mixture samples of biomass and PVC were simply prepared to obtain a chlorine content of 4 wt % in the samples, which is less than the maximum chlorine content limit of 5 wt % in general biomass waste. Japanese cypress timber sieved to 210-500 µm was used as raw biomass, and then it was mixed with PVC (Taiyo Polyvinyl Chloride, TH1000). The PVC powder, which consisted of 45 wt % chlorine, was pulverized to an average grain size of 170 µm prior to mixing with the raw biomass. Moreover, metal hydroxide additives such as Mg(OH)2, Ca(OH)2, NaOH, and KOH, were further added to PVC-mixed and raw woody biomass samples (∼100 mg), to investigate their effects on carbonization behavior and its pyrolyzed char. Metal hydroxide-mixed woody biomass samples, containing 8 wt % metal ions, were prepared by two different methods: (i) dry blending and (ii) wet blending. Woody biomass samples were physically mixed with three additives (NaOH, Mg(OH)2, and Ca(OH)2) without solvents as a dry-blending method. For a wetblending method, two types of metal hydroxides (NaOH and KOH) were simply mixed with woody biomass samples using theappropriateamountofpurewater,thenthemetalhydroxide-mixed woody biomass samples were dried under vacuum for 24 h at 70 °C prior to use in carbonization experiments. 2.2. Carbonization Process and Char Analysis. Figure 1 shows the scheme of apparatus used for biomass carbonization (pyrolysis). The biomass mixture samples were carbonized at a heating rate of 5 °C/min from room temperature to different target temperatures (300, 400, and 500 °C) in a nitrogen flow
Figure 1. Schematic of an apparatus used for woody biomass carbonization (pyrolysis).
Figure 2. Yield of char carbonized at 300-500 °C using a woody biomass mixture mixed with various metal hydroxide additives, using dry- and wetblending methods.
that was maintained at 200 cm3/min, and then immediately cooled after holding at the final temperatures for 1 min. For char analysis, the concentration of chlorine that remained in carbonized char was measured using oxygen flask combustion and ion chromatography. The weight of char, based on raw biomass, was determined by deducting the total weight of three componentss(i) metal hydroxides in char, (ii) chlorine content in char, and (iii) char obtained by carbonizing only PVC componentsfrom the weight of char carbonized from each biomass mixture sample. Heat treatments of raw biomass mixed with metal hydroxides and PVC to carbonization temperatures were separately conducted to determine the weight of metal hydroxides and char obtained by carbonizing only PVC, respectively, whereas the weight of chlorine content in carbonized char was calculated from its measured concentration. Moreover, X-ray diffraction (XRD) was performed to analyze the distribution of metal and chlorine ion in carbonized char. 3. Results and Discussion 3.1. Effect of Metal Hydroxide Additives on Biomass Char Yield. The yield of char obtained from carbonization of PVC-mixed woody biomass samples added with various metal hydroxides were plotted with different carbonization temperatures to preliminarily investigate the effect of each additive behavior on char property as shown in Figure 2. The mixing method of each additive with woody biomass samples (dryblended and wet-blended) was also compared in this graph, particularly NaOH-blended samples using the two methods. It is evident in Figure 2 that the char yield of PVC-mixed woody biomass samples without additives (BM + PVC) carbonized at 300 °C was 62.4 wt %, which was significantly higher than those of metal hydroxide-blended samples via the wet-blending method: 55.0 and 52.9 wt % for NaOH and KOH,
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Figure 3. Weight ratio of chlorine content in char carbonized at 300-500 °C using woody biomass mixture samples mixed with various metal hydroxide additives.
respectively. In contrast, when the samples were further carbonized to higher temperatures (400 °C), the chars obtained from the samples blended with NaOH and KOH via the wet-blending method have relatively higher yields, of 42.8 and 36.9 wt %, respectively, whereas the char yield of the samples without additives decreased extremely to 24.6 wt %. These results are in agreement with those reported by Ohmukai et al.,16,17 that char yield significantly increased with the suppression of tar evolution via the effect of alkali-metal hydroxides added in a cellulose structure. Moreover, the metal hydroxide additives may effectively cause cross-linking in the biomass structure, via dehydration, to obtain a higher char yield, even in the coexistence of PVC in the woody biomass samples. Furthermore, the char yields obtained from the samples blended with metal hydroxides via the wet-blending method were still kept at a relatively higher level (37.5 and 33.7 wt % for NaOH and KOH, respectively), which were ∼1.3-1.5 times higher, compared to those of samples without additives. The samples mixed with Mg(OH)2, Ca(OH)2, and NaOH using the dry-blending method showed approximately the same tendency of char yields to change with the carbonization temperature as that of the samples without additives. This indicates that the mixing process of biomass and metal hydroxides, using water to obtain homogeneous samples, as in the wet-blended method, is reasonably necessary to gain a relatively higher char yield after carbonization, particularly when using NaOH as a potential additive. 3.2. Analysis of the Chlorine Content in the Char Structure. Since the chlorine components from PVC in the biomass mixture samples must be removed as contaminants, clarification of the existence of chlorine in the char structure is necessary, using quantitative and qualitative analysis. The parameter that is referenced as the weight ratio of chlorine content was quantitatively evaluated using the ratio between the weight of chlorine in char after carbonization and the initial weight of chlorine in the samples before carbonization. Figure 3 shows the relationship between the weight ratio of chlorine content and the carbonization temperature for biomass mixture samples blended with various metal hydroxide additives by the two different methods. For the samples without additives, the weight ratio of chlorine content in the char was extremely low (
