J Mater Cycles Waste Manag (2006) 8:109–115 DOI 10.1007/s10163-006-0152-y
SPECIAL FEATURE: ORIGINAL ARTICLE Tohru Kamo · Kanji Takaoka · Junichiro Otomo Hiroshi Takahashi
© Springer 2006
3rd International Symposium on Feedstock Recycling of Plastics & Other Innovative Plastics Recycling Techniques (ISFR 2005)
Production of hydrogen by steam gasification of dehydrochlorinated poly(vinyl chloride) or activated carbon in the presence of various alkali compounds
Received: October 20, 2005 / Accepted: February 2, 2006
Abstract Steam gasification of dehydrochlorinated poly(vinyl chloride) (PVC) or activated carbon was carried out in the presence of various alkali compounds at 3.0 MPa and 560°C–660°C in a batch reactor or in a semi-batch reactor with a flow of nitrogen and steam. Hydrogen and sodium carbonate were the main products, and methane and carbon dioxide were the minor products. Yields of hydrogen were high in the presence of sodium hydroxide and potassium hydroxide. The acceleration effect of the alkali compounds on the gasification reaction was as follows: KOH > NaOH > Ca(OH)2 > Na2CO3. The rate of gasification increased with increasing partial steam pressure and NaOH/C molar ratio. However, the rate became saturated at a molar ratio of NaOH/C greater than 2.0. Key words Hydrogen · PVC · Waste · Steam · Sodium hydroxide · Alkali compounds
Introduction Waste plastics collected in response to laws regarding recycling of containers and packing materials consist mainly of thermoplastic resins, such as polyethylene and a small percentage of PVC. These commingled plastics have been treated in coke ovens, blast furnaces, and by gasification. Most of the shredder dust derived from waste electronic
T. Kamo (*) National Institute of Advanced Industrial Science and Technology (AIST), Research Institute for Environmental Management Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Tel. +81-29-861-8043; Fax +81-29-861-8427 e-mail:
[email protected] K. Takaoka Department of Applied Chemistry, School of Engineering, The University of Tokyo, Tokyo, Japan J. Otomo · H. Takahashi Department of Environmental Chemical Engineering, Kogakuin University, Tokyo, Japan
equipment or end-of-life vehicles is disposed of in landfills, except when the gold or palladium content of the dust is high; this is because the removal of toxic materials such as heavy metals and halogenated compounds and the recovery of useful materials are difficult. The shortage of final disposal sites and the contamination of soil and groundwater through leaching from landfills are serious societal problems. The development of new technology for removing toxic compounds and converting shredder dust to useful products is required not only for the reduction of environmental pollution loads but also for the effective use of natural resources. Recently, waste plastics have been recognized as an important domestic energy resource as a result of the rapid inflation of crude oil prices. The power generation efficiency of fuel cells is 30%–65%, which is not especially high in comparison with the efficiency of a system that combines a 1500°C gas turbine with integrated gasification. Technology for the safe transport and stable storage of hydrogen has not yet been established, and fuel cells have many technical problems that limit their practical application. However, fuel cells are expected to be the standard distributed energy source in the future, owing to their compact and silent operation, low transport losses, and high total energy efficiency, which exceeds 80% when combined with a cogeneration system. Hydrogen gas has been produced industrially by the steam reforming of natural gas or by partial oxidation of naphtha. In the steam reforming process, nickel catalyst supported on alumina is mainly used. However, steam reforming cannot be applied to waste plastics because carbon deposition derived from heavy hydrocarbon fractions or chlorine contained in the waste plastics deactivate the catalyst. Partial oxidation can be used for conversion of heavy hydrocarbons, such as coal or waste plastics, into hydrogen. The Ebara–Ube process (EUP), a partial oxidation process, has been operating at Ube and Kawasaki to produce hydrogen for ammonia synthesis from waste plastics collected from recycling programs. However, the process is not suitable for small-scale hydrogen supply plants for fuel cells because the reaction is carried out at temperatures above 1000°C and the energy loss is very high.
110
Alkali compounds have been used as a catalyst to produce synthesis gas (hydrogen and carbon monoxide) by the gasification of fossil resources such as coal or biomass.1,2 In the presence of excess calcium hydroxide, hydrogen can be produced at high yields from coal and other organic materials with steam at the comparatively low temperature of 700°C.3–5 However, the reaction must be carried out at more than 10 MPa of steam partial pressure to prevent decomposition of Ca(OH)2 to CaO. Recently, an interesting technology was proposed in which hydrogen can be produced from water and carbon under relatively mild conditions in the presence of excess sodium hydroxide.6,7 Because reactions are carried out in the presence of alkali, the production of hazardous halogenated organic compounds is expected to be negligible, even if halogenated compounds such as PVC and bromine-containing flame retardants are present in the waste; the halogen elements are recovered in the form of stable, safe mineral salts. In this study, we used dehydrochlorinated PVC and activated carbon as models for solid residue containing chlorine derived from shredder dust, and we investigated the effects of various alkali compounds and partial steam pressures on the rate of gasification of these materials.
Experimental PVC (60 g; Sigma-Aldrich, Tokyo, Japan) was dehydrochlorinated at 440°C for 60 min under a nitrogen pressure of 10 MPa. The product was distilled under vacuum to remove hydrogen chloride and volatile materials.Analytical data for the activated carbon (Wako, Osaka, Japan), PVC, and dehydrochlorinated PVC are shown in Table 1. Two reactor types were used in this work (Fig. 1). The influence of the sodium hydroxide on the yield of products from the gasification of activated carbon or PVC was investigated using a batch reactor (dotted line in Fig. 1) and the rate of gasification of activated carbon or dehydrochlorinated PVC was measured using a semi-batch reactor with a flow of nitrogen gas and steam. The inner surface of the reactor was coated with gold (thickness, 0.5 mm). In the gasification reaction of activated carbon carried out in the batch reactor, the reactor (Hastelloy C, 43.1 ml) was charged with activated carbon (0.5 g), sodium hydroxide (0–8.0 g), and ultrapure water (0–5.0 g). In the reaction of PVC, the reactor was charged with 1.2 g of PVC and 5.8 g of sodium hydroxide because some of the sodium hydro-
Table 1. Elemental analyses of activated carbon, poly(vinyl chloride) (PVC), and dehydrochlorinated PVC
Activated carbona Activated carbonb PVC Dehydrochlorinated PVC a b
C
H (wt%)
N
H/C (mol/mol)
Ash (wt%)
82.6 76.7 38.5 92.8
2.4 1.6 4.7 4.1
1.0 0.3 0.0 0.0
0.35 0.25 1.44 0.53
0.7 4.2 – 0.1
used in batch reactor used in semi-batch reactor
Fig. 1. Diagram of the semibatch and batch (dotted line) gasification reactors
Mass flow controller
Thermocouple
Nitrogen gas Pressure gauge
Valve Filter
pressuremaintenance valve
Thermocouple Reactor Heat insulator Pipe Distilled water
Sample Gold image heater Gas-liquid separator
Degasser
Pump
Preheater part of batch type reactor
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Results and discussion The distribution of gaseous products from gasification of PVC and activated carbon in the batch reactor are shown in Fig. 2. Hydrogen gas was the predominant gaseous product, and methane and carbon dioxide were minor products. The yields of products were derived from the molar ratios of the products and the carbon in the original sample. The yields of hydrogen from the gasification of PVC at 650° and 700°C were only 18% and 27%, respectively, and a large amount of solid carbon was deposited on the inner wall surface of the reactor. On the thermal decomposition of PVC under atmospheric pressure using a thermobalance, PVC converted to residue through four steps during heating
100 Yield (%, 100 x mol/mol-C in feed)
xide is consumed by the hydrogen chloride produced in the decomposition of PVC, and only 42% (w/w) of PVC corresponds to hydrocarbon. The partial steam pressure depended on the initial amount of water added, and the pressure was calculated from the total pressure using the equation of state for an ideal gas. The reactor was heated to 650°C over a period of 15 min and then kept at the designated temperature for 30 min. Gas samples were collected in an aluminum-laminated gas bag, which was immersed in a water bath after the reaction. The volume of the gaseous products was determined from the amount of water displaced from the bath. Gas products were analyzed with a gas chromatograph equipped with a packed column (GC14B, activated carbon, 3 m; Shimadzu, Kyoto, Japan). The semi-batch reactor was charged with a sample (activated carbon or dehydrochlorinated PVC, 0.5 g) and sodium hydroxide (0–7.0 g). In the experiments to determine the effect of the various alkali compounds, NaOH was replaced with potassium hydroxide (KOH, 7.0 g), calcium hydroxide [Ca(OH)2, 4.6 g], or sodium carbonate (Na2CO3, 6.6 g). The reactor was heated to 560°–660°C over a period of 13 min and the temperature was maintained for 150 min. Ultrapure water was pumped into the reactor at 0–0.3 g/min through a degasser, and nitrogen gas was introduced at 60– 470 cm3/min by means of a mass flow controller. The overall flow rate of the gas mixture was maintained at 19.1 mmol/min, and the reaction pressure was 3.0 MPa. In all experiments, a mixture of steam and nitrogen was injected into the molten sodium hydroxide through a pipe to promote the contact of gas, liquid, and solid samples.The partial steam pressure was controlled by changing the ratio of water and nitrogen gas. Gaseous products flowing from the reactor were analyzed by gas chromatography every 13 min. The gaseous products were collected in an aluminumlaminated gas bag, and the total amount of each gas product was derived from the volume and composition of the collected gaseous product. After the reactions, ultrapure water was added to the solid product in both reactors, and the water-soluble products were separated from the residue by filtration. The solid sample on the filter was dried at 110°C under vacuum for 12 h.
50
0 650°C
700°C
PVC Hydrogen
650°C
700°C
Activated carbon Methane
Carbon dioxide
Fig. 2. Distribution of gaseous products from gasification of poly(vinyl chloride) (PVC) and activated carbon in a batch reactor
to 1100°C. The weight of solid residue decreased to 4% of the initial weight in nitrogen gas or to 2% of the initial weight in the presence of steam.8 The yield of residue increased under high nitrogen pressure during the thermal decomposition of PVC.9 The low hydrogen yield and the deposition of solid carbon imply that volatile matter derived from thermal decomposition of PVC may have adhered to, and carbonized on, the inner surface of the reactor, and that the physical contact between the sample and the molten sodium hydroxide was insufficient for gasification. In contrast, the yields of hydrogen from gasification of activated carbon at 650° and 700°C were 67% and 101%, respectively, and little solid carbon was observed in the reactor after gasification. The high yields of hydrogen from the activated carbon suggest that physical contact with sodium hydroxide is important for hydrogen formation during gasification. In this work, dehydrochlorinated PVC was used mainly to minimize the influences of volatile matter. In the gasification of activated carbon in the presence of sodium hydroxide, the yields of hydrogen and methane increased with partial steam pressure and the yield of residue decreased. However, the yields of all products became saturated at pressures higher than 3.0 MPa (Fig. 3). As the NaOH/C molar ratio increased, the yields of hydrogen and methane increased and the yield of residue decreased (Fig. 4). The saturation of product yield at higher partial steam pressures and higher amounts of sodium hydroxide additives implies that the gasification rate was
112
Yield ( %, carbon base in original sample)
200
150
100
50
0 Ca(OH)2 Fig. 3. Effect of partial steam pressure on yields of products from gasification of activated carbon in the batch reactor (650°C, 30 min, NaOH/C = 3.7)
Hydrogen
Na2CO3 Methane
KOH
NaOH
Carbon dioxide
Fig. 5. Effect of alkali compounds on yields of products from gasification of activated carbon in the semi-batch reactor (600°C, 150 min)
high enough for the reaction to proceed nearly to completion.We suspect that some of the activated carbon remained unchanged even at high partial steam pressure because of poor physical contact between the molten sodium hydroxide, activated carbon, and steam in the batch reactor. The yields of products derived from gasification of activated carbon in the presence of various alkali compounds are shown (Fig. 5). These experiments were carried out using the semi-batch reactor at 600°C for 150 min at a partial steam pressure of 1.7 MPa. In these experiments, twice the amount of alkali required to fix all the carbon in the activated carbon as carbonate was added. In the presence of Ca(OH)2, Na2CO3, KOH, NaOH, the yields of hydrogen were 25%, 26%, 162%, and 165%, respectively. Calcium hydroxide is effective in the HyPr-RING process,3,4 which is carried out at 700°C; however, the reaction temperature of 600°C in this study was too low to produce high yields of hydrogen. The hydrogen yields indicate that KOH and NaOH are effective for the production of hydrogen during the gasification of activated carbon at 600°C. The gasification rate per unit mass of remaining sample (r, min−1) is defined by Eq. 1, where dX/dt represents the rate of weight loss, X is the degree of conversion (−), t (min) is the reaction time, k (min−1) is the reaction rate constant, and (Psteam)n is an exponential function of the partial steam pressure. Fig. 4. Effect of NaOH/C ratio on yields of products from gasification of activated carbon in the batch reactor (650°C, 30 min, Ph2o = 0.4– 3.7 MPa)
r=
1 Ê dX ˆ n Á ˜ = k(Psteam ) Ë ¯ 1 ( - X ) dt
(1)
113
We assumed a first-order reaction rate for the gasification according to the volume mode10 as the simplest reaction model. The gaseous hydrogen concentration in the reactor (C H, mol/l) and the hydrogen flow rate from the reactor (F H, mol/min) are defined by Eqs. 2 and 3, where V, v, and R H are the volume of the reactor,l the total gas flow rate (l/min), and the rate of hydrogen production (mol/min), respectively. In this anaylsis, RH was derived from the gasification rate of carbon in the solid sample (dMC/dt in Eq. 4) because the amounts of hydrogen and carbon consumed by secondary methane formation were negligible during the reaction. Equation 5 was derived from Eqs. 1–4 by using a complete mixture model, where MC is the amount of carbon in the sample (mol), MC0 is the amount of carbon in the initial sample (mol). f is a space rate constant (v/v) (1/min). F H can be approximated by Eq. 6 because f (0.94 min−1 600°C under 3.0 MPa) is fairly large compared to r (0–0.05 min−1), and the flow rate of hydrogen from the reactor is almost equal to the rate of hydrogen production by gasification [exp(−rt) >> exp(−ft)]. dCH = -FH + RH dt FH = vCH
V
(2) (3)
dMC = rMC dt rf M exp( -rt ) - exp( - ft ) FH = ( f - r ) C0 RH ª
(
ln(FH ) = A - rt : A is a constant
)
(4)
Fig. 6. Effect of alkali compounds on the gasification rate of activated carbon in the semi-batch reactor (650°C, Ph2o = 1.7 MPa)
(5) (6)
Table 2. Rate of gasification of activated carbon (r) at 600°C
The time dependence of the flow rate of hydrogen (FH) produced from activated carbon in the presence of various alkali compounds is also shown (Fig. 6). The reaction time was measured from the time at which the temperature of the outer wall of the reactor reached the designated value. According to Eq. 6, the rates of gasification (r) in the presence of various alkali compounds were derived from the slopes of the lines shown in Fig. 6. The gasification rates in the presence of Na2CO3, Ca(OH)2, NaOH, and KOH were 0.0019, 0.0067, 0.0239, and 0.0730 min−1, respectively (Table 2). The yields of hydrogen produced from gasification in the presence of Na2CO3 and Ca(OH)2 were negligible at 600°C and a reaction time of 150 min because the rate of gasification was very low (Table 2). The rate of gasification was highest in the presence of KOH; however, the gasification rate was comparatively high even in the presence of NaOH. The distribution of products from the gasification of activated carbon and dehydrochlorinated PVC at various temperatures is shown in Fig. 7. The yields of hydrogen and the conversion increased with temperature, becoming saturated at temperatures greater than 600°C.Traces of carbon dioxide were detected around 650°C. The amount of hydrogen produced corresponded to twice the molar amount of carbon in the original sample, which suggests that the hydrogen gas produced in these experiments was recovered almost completely and gasification proceeded by means of the reaction shown in Eq. 7 under our experimental conditions:
r (min−1) Na2CO3 Ca(OH)2 NaOH KOH
C + 2NaOH + H 2O Æ Na 2CO3 + 2H 2
0.0019 0.0067 0.0239 0.0730
(7)
In the reactions of activated carbon and dehydrochlorinated PVC with sodium hydroxide, the gasification rate increased with partial steam pressure (Fig. 8). The reaction order with respect to the partial steam pressure was calculated to be 0.69 by means of a least-squares method using Eq. 2. The reaction order with respect to steam partial pressure for gasification was close to the value obtained for gasification of char11,12 derived from coal. The rate of hydrogen production increased linearly as the NaOH/C ratio increased, becoming saturated at ratios greater than 2 (Fig. 9). Because activated carbon and dehydrochlorinated PVC were solid under our experimental conditions, there was an upper limit to the efficiency of physical contact between the sample and liquid sodium hydroxide, even when excess sodium hydroxide was added. Saturation of the hydrogen production rate at NaOH/C ratios higher than 2 implies that the contact efficiency of NaOH and carbon had reached a maximum. These experimental results imply that physical contact of carbon, steam, and sodium hydroxide is very important for the reaction
114
0.04
-1
r (min )
0.03
0.02
0.01
0.00 0.0
2.0
4.0
6.0
NaOH/C (mol/mol)
Fig. 7. Effect of temperature on conversion and the yields of products from gasification of activated carbon and PVC (NaOH/C = 3.2–3.9, Ph2o = 1.7 MPa)
Fig. 9. Effect of NaOH/C ratio on the gasification rate of activated carbon (open circles) and dehydrochlorinated PVC (solid circles) at 600°C (Ph2o = 1.7 MPa)
leading to hydrogen production. Molten sodium hydroxide penetrates into particles of the activated carbon under our experimental conditions.13 The hydrogen production reaction occurs not only on the outer surface of the particles but also inside the particles, as indicated by the similar rate of hydrogen production observed irrespective of the diameter of the sample particles.
0.05
0.04
Conclusion -1
r (min )
0.03
0.02
0.01
0.00 0.0
1.0
2.0
3.0
Partial steam pressure (MPa) Fig. 8. Effect of partial steam pressure on the gasification rate (r) from activated carbon (open circles) and dehydrochlorinated PVC (solid circles) at 600°C (NaOH/C = 3.2–3.9)
In the steam gasification of PVC using a batch reactor, the yield of hydrogen was low because the physical contact between the molten sodium hydroxide and the sample was insufficient; this contact problem resulted from deposition of volatile material derived from the decomposition of PVC on the inner surface of the reactor. Alkaline additives increased the rate of steam gasification of activated carbon, which was determined with a semi-batch reactor, in the following order: KOH > NaOH > Ca(OH)2 > Na2CO3. In the presence of KOH or NaOH, hydrogen was produced by steam gasification of activated carbon or dehydrochlorinated PVC under relatively mild conditions. The yield of hydrogen increased with temperature, eventually reaching a constant value that was almost twice the amount of carbon contained in the original sample. The rate of hydrogen production increased with partial steam pressure and NaOH/C molar ratio; however, the rate approached a constant value at ratios greater than 2.0.
115 Acknowledgments The authors sincerely thank Mr. Tomohumi Sato and Mr. Masakazu Inaba for assistance in operating the experimental equipment.
7.
8.
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