High-Temperature Steam Gasification of Municipal Solid Waste

Article pubs.acs.org/EF

High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood Uisung Lee, J. N. Chung,* and Herbert A. Ingley Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611, United States ABSTRACT: The main objective of this study is to investigate the feasibility of clean gaseous fuels made from plastics, automobile tire rubber, municipal solid waste (MSW), and woody biomass feedstock using a high-temperature pure-steam gasification process. Super-high-temperature steam at 1000 °C was used as the gasifying agent to generate the syngas which mainly contains hydrogen and carbon monoxide and can be used as a gaseous fuel. Since the process does not involve air, the syngas is free of nitrogen and its oxides which usually dilutes the syngas and lowers its heating value in the case of an air-blown partial-oxidation/partial gasification process. A lab-scale experimental apparatus was used to produce the high-quality syngas, and the gas was analyzed using a gas chromatography. The results showed that the syngas contained very high H2 concentrations, and the heating values of the syngas for all four types of feedstock reached 8−10 MJ/m3 which were approximately 2.5 times higher by weight and 1.6 times by volume as compared to those from the previous air-blown gasification system. In addition, a thermodynamic equilibrium model was developed and successfully verified by the experimental results.

1. INTRODUCTION It has been a long-time goal for mankind to convert its wastes into useful energy. However, about 135 million tons (53.8% of municipal solid waste (MSW)) were discarded in landfills in the United States in 2012.1 The energy content of the discarded MSW in 2012 was estimated to be approximately 1.9 × 1018 J. Putting the waste energy aside, it is certain that the use of landfills is not a long-term solution due to insufficient landfill area and the environmental impact such as groundwater pollution, air pollution, and vegetation damage.2 Incineration has been an option for replacing the landfill from the 1970s since it can recover some portion of the energy from the MSW. This process can generate electricity at an efficiency of around 20% for thermal production from the wastes.3 However, it causes several serious problems such as air pollution. Dioxin, namely polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs), is one of those toxic chemical air pollutants which can cause cancer, immune system damage, and interference with regulatory hormones.4 Gasification is one of the processes for converting MSW into a gaseous fuel which is called syngas. Unlike the incineration which burns the MSW directly to obtain the thermal energy and subsequently the electrical energy, gasification generates syngas which can then be combusted at high temperature, leading to higher conversion efficiencies. The gasification process can generate the electricity with an efficiency of around 34% which is higher than those possible with incineration.3 Existing common gasification technologies use reactors that operate in the temperature range of 400−850 °C. They do not need an external heat source as they rely on the air-breathing combustion of a portion of the feedstock to produce heat to facilitate the gasification. Thus, they are really partial combustors and partial gasifiers. As a result, these air-blown gasifiers produce low-quality syngas that contains undesirable char, tar, and soot due to the lower reactor temperatures. Furthermore, their harmful emissions are similar to those emitted from industrial furnaces and burners due to the © 2014 American Chemical Society

involvement of oxygen from the air-breathing combustion. The lower operating temperatures cannot break down all the materials in the feedstock so these gasifiers are highly selective in the types of feedstock they can process. In addition to that, the syngas is diluted by nitrogen because of the air intake. Theoretically, nitrogen forms around half of the syngas by volume for air-breathing gasification which results in the syngas having a low heating value. High-temperature steam gasification utilizes an external heat source. This is a technologically advanced and environmentally friendly process that uses extremely high-temperature steam as its gasifying medium in an oxygen-free environment to completely decompose the feedstock including char, tar, soot and solid waste into pure synthesis gas and inert vitreous slag. The solid vitreous slag can be easily separated and disposed of. The syngas produced from the steam gasification usually has a much higher heating value than those from the air-blown gasification as the air free environment makes it possible to eliminate the presence of nitrogen and its oxides in the syngas. Since many applications such as internal combustion engines or gas turbines require the fuel to have a certain minimum heating value to get over the idle condition, it is very useful to have the higher heating values resulting from the steam gasification. This syngas is also a better gaseous fuel because it can be combusted at higher temperatures, which leads a higher thermal efficiency. Some researchers have studied the gasification of various types of feedstock with high-temperature steam to obtain higher hydrogen concentrations. Chang et al. investigated the steam gasification of agriculture waste at temperatures between 600 and 1000 °C for the production of biohydrogen and syngas in a fluidized bed reactor.5 They also developed a kinetic model to determine the order of the reaction and activation energy. Received: April 3, 2014 Revised: June 1, 2014 Published: June 2, 2014 4573

dx.doi.org/10.1021/ef500713j | Energy Fuels 2014, 28, 4573−4587

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Figure 1. Schematic of the steam gasification system.

40% by volume at 1050 °C, and tar production was decreased at this high temperature due to thermal cracking and steam reforming. In another effort, a study of tar removal from the steam gasification of MSW was conducted using dolomite as the catalyst during the reaction by Guan et al.12 The results show that there is an optimum point where the tar could be reduced from 5.1% to 3.6% with the two-region MSW catalytic gasifier. He et al.13 also studied steam gasification of MSW using calcined dolomite as the catalyst. They investigated the effect of the catalyst and the temperature on the syngas generation and the species concentrations. The MSW sample was mainly composed of kitchen garbage and small amounts of paper, textile, wood and plastic. They found that high temperature is favorable for generating hydrogen-rich gas, and the calcined dolomite played an important role for reducing tar. On the basis of the review of the existing literature on the steam gasification, researchers generally used electrical furnaces as their reactor since it is easier to control the reaction temperature instead of supplying the high-temperature steam directly. However, there have been inconsistencies in determining the temperature effect for those who used electrical furnaces. Even though they studied the effect of the reaction temperature for the syngas composition and generation, they have not specified whether the temperature is the steam temperature, furnace temperature, or some calibrated steam temperature. The current work involves the high-temperature steam gasification for various feedstocks to produce high-quality syngas products which have high heating values, and contain high concentrations of hydrogen and carbon monoxide. Instead of using an electrical furnace as a reactor, a well-insulated reactor was used to verify the steam temperature effect on the syngas generation and its concentration. The high temperature air-free steam gasification process used 1000 °C steam to gasify the feedstock so that most of the carbon was converted to pure fuel gas.

Their results suggested that at the equivalent ratio of 0.2 and at 1000 °C the maximum yield of biohydrogen (29.5%) and CO (23.6%) was achieved and the CO2 concentration at this condition is 10.9% only. Nipattummakul et al.6 used sewage sludge as the feedstock and high temperature steam which was controlled with an electrical furnace after steam was produced from hydrogen and oxygen combustion. The authors showed the results such as syngas production, hydrogen concentration, and the energy conversion efficiency with respect to the reactor temperature. Hydrogen yield increased to 3 times when compared to that of the air gasification. However, it was not mentioned how high the temperature of steam was supplied to the reactor. Since the steam temperature may not be the same as the reactor temperature, the actual steam temperature which is introduced to the reactor may affect the results. Ahmed and Gupta7 used paper as their feedstock to compare pyrolysis and steam gasification. They increased reactor temperature from 400 to 700 °C for the pyrolysis and from 600 to 1000 °C for the steam gasification process, respectively. Syngas generation rate, gas composition, and energy yield were compared, and the gasification process showed better results for hydrogen generation and material destruction due to the char gasification. They also studied the kinetics of the gasification process for woodchips using CO2 as the gasifying agent.8 They found that the steam gasification has around twice the average reaction rate when compared to that of CO2 gasification. Umeki et al.9 studied the steam gasification for woody biomass with high-temperature steam exceeding 1200 K. Hightemperature steam played an important role as the gasifying agent while it carried the heat to the reactor. The hydrogen concentration was found at 35−55 vol % which is higher compared to that of air gasification. The cold-gas efficiency was as high as 60.4%; the gross cold-gas efficiency would decrease to 35% when the energy supplied to the steam was considered. They also developed a numerical model to simulate an updraft biomass gasifier with high-temperature steam and successfully compared it to the experimental data.10 Skoulou et al.11 used olive kernel as the feedstock to investigate the characteristics of the steam gasification as a function of the residence time and reactor temperatures. They showed that the production changes for each gas species with respect to the residence time. Hydrogen yield reached around

2. EXPERIMENTAL SETUP 2.1. System Setup. The experimental system consisted of five major components: a high-temperature steam generator, feedstock infusion unit, gasification reactor, data acquisition unit, and gas analysis unit. Figure 1 shows the schematic of the overall system, and Figure 2 4574


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Figure 2. Photograph of the overall steam gasification system. provides a photograph of the system. The gasification reactor is fed near the bottom with the high-temperature steam generated from the steam generator and superheated by three furnaces, and the feedstock is introduced to the reactor on the top by the screw feeder. The hightemperature steam and the feedstock meet in the reactor where the thermal−chemical reactions take place. The syngas produced in the reactor exits from the top and then is cooled and cleaned in a copper coil in order to get rid of the residual tar and moisture. Each component is described in detail in the following sections. 2.2. System Components. 2.2.1. Steam Generator and Superheater. The steam generation unit provides the intermediate temperature steam to the furnaces for superhating. A peristaltic pump, PP-2, supplies deionized water from a reservoir at a rate of 1.2 ± 0.2 kg/h. After leaving the pump, the water then enters the steam generator, HGA-S/H, which turns the water into 250 °C steam at the steam generator exit. Once the steam leaves the steam generator, it enters the furnace and is further superheated by three high-intensity electric tube furnaces placed in-series. The steam can reach in excess of 1000 °C when entering the gasification reactor. 2.2.2. Feeder. The feedstock is introduced into the reactor with a screw feeder. Before the experimentation, the feeder was calibrated for each feedstock material, so that the feeder provides a consistent amount of feedstock for every revolution of the screw. Once the steam temperature reaches a steady-state condition, the entire feeder and feedstock storage container are purged with argon gas to remove any air from the feedstock before it is introduced to the reactor to make sure that there is no air entering the reactor. 2.2.3. Gasification Reactor. The gasification reactor is a fixed-bed type. A stainless steel vertical tube (nominal 1 in. diameter) is used to house the reactor bed that is made of a mesh screen. The size of the reactor is limited by the relatively small amount of high-temperature steam generation. The mesh screen that fills the entire cross section forms a bed at the center of the reactor to hold the feedstock, while this screen also allows the steam to flow through from the bottom to

the top of the reactor. There are two thermocouples located right before the steam enters the screen. They measure the center and the wall temperatures, respectively, which are used to estimate the true steam temperature by compensating for the heat transfer to the wall by thermal radiation. Before runs, insulation is applied to the reactor to minimize the heat loss to the environment. 2.2.4. Syngas Cooling, Cleaning, and Sampling. The syngas produced in the reactor is cooled down as it passes through the coil cooler to condense out any excess moisture and tars. The resulting condensate is collected in a vessel at the bottom of the coil. Undesirable contaminants produced during the initial gasification reactions such as tar and char are also collected with the condensed water. There are three valves for the exhaust and sampling. Syngas samples are collected through the gas sampling port with gas sampling bags at timed intervals. 2.2.5. Data Acquisition Unit. Steam temperature going into the reactor is the most important parameter in the test. In order to control and record the temperatures of the gasification system, several thermocouples were installed. They are connected to the data acquisition system, and the measured temperatures are recorded by the computer that also controls the settings of the superheaters. 2.2.6. Gas Chromatograph. A gas chromatograph (GC), model 910 from Buck Scientific, and the software, PeakSimple, are used to analyze the composition of the syngas samples. Argon is used as the carrier gas for the GC so that hydrogen could be easily detected. The GC is equipped with two columns, Molecular Sieve and Hayesep-D, and it has a thermal conductivity detector (TCD). The molecular sieve column is able to separate hydrogen, oxygen, nitrogen, methane, and carbon monoxide, and the Hayesep-D column separates carbon monoxide and carbon dioxide. The TCD consists of four tungsten− rhenium filaments, and an electric current is used to heat the filaments. TCD compares the temperature unbalance between the carrier gas, which is the reference, and the unknown gas once separated in the 4575


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Figure 3. Composition of the synthetic MSW based on the EPA data.1

Figure 4. Photographs of the synthetic MSW and its compositions. columns. In order to calibrate the GC, four calibration gases which represent a typical synthetic gas composition are used. 2.2.7. Feedstock Characterization. The synthetic MSW used in this research is based on the MSW generation data published by the U.S. Environmental Protection Agency as shown in Figure 3.1 Recycled materials (glass and metal) are not considered for this feedstock sample since they are not favorable for gasification. Each MSW component is collected and ground as shown in Figure 4, and then all the components are blended and mixed on the basis of the data given in Figure 3. There are seven major components: paper, wood, yard trimmings, food scrap, plastics, rubber, and textile. Unlike other materials, food scrap is hard to define and collect due to its nonhomogenous nature. To avoid this, ground dog food is utilized to represent food scraps. Dog food has been frequently used as a typical artificial food waste containing solid organic matter, which is a common type of biomass waste containing proteins, fats, vitamins, fiber, and inorganic minerals.14−17 Since the moisture content of the dog food and typical food scraps differ from each other, water is added to mimic the real MSW food scrap. All the components are ground to increase the surface area for reaction and to avoid congestion in the feeder that also enhances the homogeneity of the resulting feedstock. The above process makes the

test results more consistent for the same conditions. Proximate and ultimate analyses of the synthetic MSW are performed by Keystone Materials Testing, Inc. as shown in Table 1. It shows results similar to those of other proximate and ultimate analysis results which were found in the literature, such as the synthetic MSW based on the typical MSW distribution in Hong Kong, the MSW collected from several different cities in South Korea, and the refuse-derived fuel (RDF) pellet in United Kingdom.18−20 Some of these components were also used in our separate experiments to evaluate the syngas production from specific feedstock streams such as plastic water bottles (PET, polyethylene terephthalate), rubber powder from used automobile tires obtained from Global Tire Recycling, and forest waste.

3. EXPERIMENTAL PROCEDURE Water and feedstock were provided to the water reservoir and the feeder hopper, respectively. Due to the capacity of the steam generator, the maximum mass flow rate of the superheated steam was measured at 1.2 ± 0.2 kg/h. The steam to feedstock mass ratio and the operation time determine the required quantities of water and feedstock. The water pump was primed before the test while it was disconnected from the system. Also, the feeder was calibrated to ensure that the feeder would supply the designated amount of feedstock into the gasifier since each feedstock has a different density. 4576


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Table 1. Proximate and Ultimate Analysis of the Synthetic MSW results as-is moisture ash carbon hydrogen nitrogen oxygen by difference fixed carbon sulfur volatile matter low heat value high heat value results dry-weight corrected ash carbon hydrogen nitrogen oxygen by difference fixed carbon sulfur volatile matter low heat value high heat value

Table 2. Experimental Uncertainties flow rate mole fraction

wt % 14.93 8.48 36.91 6.17 1.13 47.18 13.52 0.12 63.06 11.89 MJ/kg (5115 BTU/lb) 13.22 MJ/kg (5687 BTU/lb) wt %

H2 CO CO2 CH4

LHV temperature measurement steam temperature

±0.2 kg/h ±2.0% ±2.5% ±2.0% ±1.0% ±0.52 MJ/m3 ±1.1 °C ±3.4 °C

necessary to measure the temperature of the steam that enters the reactor accurately. However, it is almost impossible to measure the true steam temperature precisely with a single thermocouple due to the thermal radiation effects when there is a temperature difference between the steam and the walls of the reactor. Since the temperature registered by the thermocouple is based on the energy balance between convection and thermal radiation on the thermocouple lead, the error would be significant when the radiation factor is large. If the temperature difference is huge between the steam and the wall, the radiation to the wall is too large to neglect, and the error which is the difference between the actual steam temperature and that measured by the thermocouple could be several hundred degrees centigrade in magnitude. Therefore, the radiation losses used to estimate the true steam temperature cannot be neglected.21 Thermocouples were placed at the wall and at the center of the tube just before the steam meets the feedstock in order to estimate the true steam temperature as shown in Figure 5. The

10.00 43.40 5.29 1.30 39.88 15.90 0.10 74.10 13.98 MJ/kg (6012 BTU/lb) 15.55 MJ/kg (6685 BTU/lb)

Then all the system components were connected and checked as the system schematic illustrates. The reactor, steam generator, and the superheater were insulated to minimize heat loss to the environment. Once the system was ready to run, the steam generator and three tube furnaces were powered to preheat the reactor with high temperature steam. It took approximately 3 h for the steam to reach the desired steady-state temperature of 1000 °C at this condition. All the air in the system was purged properly. The feeder was also purged with argon gas in order not to leave any air in the feeder which could contaminate the syngas and affect the results. The feedstock was then fed into the reactor. A semibatch-type feeding mechanism was used since the syngas could escape through the feeder when the syngas samples are collected. Thus, once the valve between the feeder and the reactor was opened, a specific amount of feedstock was supplied for the designated reaction time based on the steam to biomass ratio (STBR) in 10 to 20 s. Then, the valve was closed, and several samples were taken using sampling bags to determine the trends on how the gas composition changed. After the designated reaction time has passed and the syngas generation rate slowed down, another batch of feedstock was then provided, and the same procedures were repeated. There is one exhaust valve and two sampling valves located at the end of the syngas cooling section. During preheating and the feedstock feeding period, only the exhaust valve was open, and the sampling bags were connected to the sampling ports with the sampling valves closed. Right after the feedstock was introduced, the exhaust valve was closed, and the valves to the sampling port were opened. To remove any air trapped in the tube between the sampling valve and the sampling bag, the exhaust valve and the sampling valve were opened together for a while to purge this line. Samples were taken at an interval of one to one and a half minutes until the syngas generation rate subsided. Then syngas samples were analyzed by the gas chromatograph (GC) which had already been calibrated for analyzing the syngas composition. The uncertainties involved in the experimentation are listed in Table 2.

Figure 5. Schematic diagram for the energy balance at the thermocouple.

energy balance at the tip of the thermocouple that is located in the center of the tube is given in eq 1. The heat going into the thermocouple from the steam should be equal to the radiation loss to the colder wall at the thermocouple lead when it is at steady-state conditions. Therefore, the actual steam temperature could be estimated using the wall temperature and the temperature reading of the thermocouple at the center of the tube once the heat transfer coefficient and the emissivity are known.

4. SYSTEM ANALYSIS 4.1. Thermocouple Calibration Model. Since the steam temperature affects the syngas composition directly, it was 4577


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LHV(MJ/m 3) = (CO × 126.36 + H 2 × 107.98 + CH4 × 358.18)/1000

where CO, H2, and CH4 are the volume percentages for these gas components in the syngas. 4.3. System Efficiency. 4.3.1. Cold Gas Efficiency. The cold gas efficiency is determined as the ratio of the chemical energy of the product gas to the total input energy including the chemical energy of the feedstock and the high-temperature steam energy as defined below.

Ts is the actual steam temperature to be estimated, while TTC and Tw are the measured temperatures by the thermocouples at the center and the wall, respectively, and the Stefan−Boltzmann constant, σ is 5.67 × 10−8 (W/m2 K4). Two parameters, heat transfer coefficient, h, and emissivity, ε, should be estimated properly to calculate the true steam temperature. The heat transfer coefficient could be derived from eq 2,

h = Nu

ks DTC

ηcold gas = (2)

chemical energy of the product gas total energy input(feedstock and steam)

(5)

4.3.2. Hot Gas Efficiency. The produced hot syngas also contains abundant thermal energy in it, and this energy can be recovered while the syngas is cooled down. However, the cold gas efficiency neglects the product gas thermal energy entirely. However, the hot gas efficiency includes all the thermal energy of the hot syngas. The thermal energy can be recovered to support the generation of steam or to preheat the feedstock to increase the system efficiency. Hot gas efficiency is the maximum efficiency that a gasification system can reach, and the hot gas efficiency can be defined as below.

where ks is the thermal conductivity of the steam which is a thermodynamic property, and DTC is the thermocouple bead diameter. Nu is the Nusselt number, which is the ratio of convective to conductive heat transfer under the same temperature difference, which could be calculated from an empirical formula. Since the thermocouple tip could be assumed as a cylinder, the Churchill and Bernstein formula could be used to obtain Nu.22 ⎛ Re ⎞5/8⎤ 0.62(ReD)1/2 Pr1/3 ⎡⎢ Nu = 0.3 + 1 + ⎜ ⎟ ⎥ 1/4 ⎢ ⎝ 282, 000 ⎠ ⎥⎦ ⎡ ⎣ 0.4 2/3 ⎤ 1 + Pr ⎥⎦ ⎣⎢

(4)

4/5

ηhot gas =

( )

chemical and thermal energy of the product gas total energy input (feedstock and steam) (6)

(3)

where, Pr is Prandtl number (Pr = cp,sμs/ks) and Re is Reynolds number (Re = ρsUDTC/μs) for the steam. The properties, μs,cp,s, ρs and ks are dynamic viscosity, specific heat, density, and thermal conductivity of the steam, respectively. In order to evaluate the Re and Pr numbers for eq 3, the thermodynamic properties for the steam from 105 to 1000 °C were collected from National Institute of Standards and Technology (NIST) webbook,23 and the Re and Pr numbers were calculated for each temperature condition with the steam mass flow rate and the reactor dimensions where the temperatures were measured. Then, the Nu and the convective heat transfer coefficient can be calculated from eq 3 and eq 2, respectively.24,25 Though the emissivity in eq 1 depends on the thermocouple and the wall, it is assumed that the emissivity is simply reduced to the thermocouple emissivity since the surface area of the thermocouple is much smaller than the wall surface.26 The emissivity value was chosen as 0.9 that was taken from Brundage et al.27 as the current experimental conditions matched those in Brundage et al.27 that also used type K thermocouple with Inconel 600 sheath material. With the help of a MATLAB code to solve eq 1, it is possible to estimate the steam temperature when the two thermocouple temperatures from the center and the wall are given. When the center thermocouple temperature is 880 ± 1.1 °C and the wall temperature is 802 ± 1.1 °C, the heat transfer coefficient at the thermocouple bead is calculated to be at 178 W/m·K using eq 2 and eq 3, and the steam temperature was then estimated as 1000 °C ± 3.4 °C, and the test was performed with these temperature conditions. 4.2. Heating Value. Since syngas is the mixture of various types of gases, the heating value of the syngas could be calculated from eq 4 below once the volumetric gas compositions were found from the gas analysis.28

5. SYSTEM MODELING 5.1. Simulation Procedures. A thermochemical equilibrium model was developed to simulate the pure steam gasification process. The assumptions adopted for the model are listed below. • The gasification reactor is in the thermodynamic equilibrium condition. • In the energy equation, there is no heat loss term, assuming that the system is perfectly insulated and there is no parasitic influence on the system i.e. the process is completely adiabatic. • Only C, H, O contents of the feedstock were considered as in other literatures to validate the model, so other mineral contents were not considered because of the negligible amounts present. • C(s) (solid carbon) is accounted for and assumed to be converted to syngas species, and the exit syngas is composed of H2, CO, CO2, H2O, and CH4. • Other higher hydrocarbons are neglected. The composition gases are modeled to exhibit ideal gas behavior irrespective of the high operating temperatures in the gasifier. On the basis of the above assumptions, the overall equation used to represent the chemical reactions during the steam gasification process is given below. A detailed explanation of the methodology can be found in Balu and Chung.29

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the solid carbon is zero. The steam gasification process starts from the four marked points toward the H2O while STBR increases. The solid carbon exists where the reactor temperature (equilibrium temperature), Tg, is relatively low due to the low STBR. The amount of the solid carbon formed decreases, while the STBR and the reactor temperature both increase, and finally, once the processes cross the carbon decomposition boundaries for a certain temperature, the solid carbon would be totally converted into gases.33 There are three parameters, the reactor temperature (equilibrium temperature), the STBR, and the steam temperature, which drive the gasification process and control the product gas composition when the feedstock is given. If two of those parameters are fixed, the third one can be calculated from the energy balance of the system. The reactor temperature and the STBR are commonly used for the equilibrium model, and the steam temperature is not usually specified in that case. However, the steam temperature was fixed in this study as in the experiment, and the reactor temperature was calculated using the energy equation with a given STBR. For the case where the solid carbon is formed in the gasification, then there are six unknown coefficients (a1 to a6) to solve for according to eq 7, and there is one more undetermined parameter, the reactor temperature or the STBR for the given steam temperature. Seven equations are required to solve for the seven unknowns, and three equations can be derived from carbon, hydrogen, and oxygen balances using eq 7 as follows:

Feedstock can be expressed by the general chemical formula, CHXOY where the values for X and Y were determined from the ultimate analysis of the feedstock. The biomass feedstock can be assumed to be composed of the dry biomass portion and its moisture content. The equivalent number of moles of the moisture in the feedstock was expressed in w which can be calculated from the equation below. w=

MWbiomass × MC MWH2O × (1 − MC)

(8)

where MWbiomass and MWH2O are the molecular weights of the biomass feedstock and water, respectively, and MC is the moisture content of the feedstock. The amount of steam was defined as m which is the steam to biomass molar ratio for one mole of biomass feedstock. In this paper, steam to biomass mass ratio (STBR) as a function of m and w is shown in eq 9. MWH2O × m STBR = (MWbiomass + MWH2O × w) (9) Four types of feedstock, wood, rubber, plastic and MSW, were chosen for the simulation study for the purpose of comparison with the experimental results. The compositions of the four feedstock are provided in Table 3 using the ultimate Table 3. Chemical Properties for Four Types of Feedstock Materials wood30 C (wt %) H (wt %) O (wt %) moisture content (wt %) LHV (MJ/kg) HHV (MJ/kg) chemical composition heat of formation (J/mol)

plastic31

47.13 5.63 38.54 8.18

62.20 4.20 32.87 0

rubber32 81.15 7.10 3.32 1.48

MSW

carbon balance: a 2 + a4 + a5 + a6 − 1 = 0

43.40 5.29 39.88 14.93

hydrogen balance: 2a1 + 2a3 + 4a5 − X − 2w − 2m = 0 (12)

oxygen balance: a 2 + a3 + 2a4 − Y − w − m = 0 19.14 20.48 CH1.43O0.61

21.85 22.77 CH0.8O0.4

32.06 33.61 CH1.05O0.03

13.98 15.55 CH1.46O0.69

−122 273

−72 254

−88 320

−221 579

o

C(s) + CO2 ↔ 2CO first reaction

(14)

C(s) + H 2O ↔ H 2 + CO second reaction

(15)

C(s) + 2H 2 ↔ CH4 third reaction

(16)

For each reaction the equilibrium constant, K i (i th independent chemical reaction), which is a function of temperature only, can be expressed in terms of the partial pressure and the number of moles of each component using a1 to a6.

o

h ̅ f = HHVBM + (nCO2h ̅ f (CO2 ) + n H2O(S)h ̅ f (H 2O(S))

K1 =

o

− nO2h ̅ f (O2 ))

(13)

There are three independent chemical reactions which include six product components as illustrated below,

analysis data for dry feedstock, and the heat of formation for o each feedstock (h ̅ f ) was calculated using the higher heating value of the biomass (HHVBM), since the HHVBM comes from the perfect combustion for the biomass feedstock at the standard condition (298.15 K, 1 atm). o

(11)

(10)

where ni is the number of moles for the complete combustion o and h ̅ f (i) is the heat of formation for species, i, respectively. The subscript S represents the liquid phase. Each biomass feedstock can be identified in a molar triangular diagram which illustrates the compositions of the biomass, the gasification conversion paths, and the existence of the solid carbon, C(s). Four types of feedstock and their steam gasification processes are plotted in Figure 6. The solid carbon deposition boundaries for various reactor temperature conditions at 1 atm are shown by curves as well. These curves indicate the carbon, hydrogen, and oxygen compositions where

K2 =

K3 =

[PCO]2 a 22 ·P = PCO2 a5(a1 + a 2 + a3 + a4 + a5)

PH2·PCO PH2O

PCH4 2

[PH2]

=

=

a1·a 2 ·P a4(a1 + a 2 + a3 + a4 + a5)

(17)

(18)

a6(a1 + a 2 + a3 + a4 + a5) a12 ·P

(19)

where P denotes the system pressure and Pi is the partial pressure for each gas constituent i. The equilibrium constants depend only on the reactor temperature and can be calculated and tabulated using MATLAB with the thermodynamic equations from NASA 4579


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Figure 6. Molar triangular diagram for four types of feedstock (wood, plastic, rubber, and MSW) and the carbon deposition boundaries for various reactor temperature conditions at 1 atm.

with respect to the reactor temperature.34 In order to relate the reactor temperature with the steam temperature an energy balance equation for the gasifier reactor is required. It was assumed that the reactor is fully insulated, and therefore there is no heat exchange with the surroundings. The energy contents of the biomass and its moisture and the energy associated with the high temperature steam are the total energy input that should be balanced by the sum of enthalpies of all gas species in the syngas at the equilibrium temperature (reactor temperature) as shown below.

Similar steps can be followed for two reactions, and then there would be six equations with six unknowns, which can be solved by the same method. The simulation model was compared and validated with Detournay et al.’s results.35 A good agreement was found on both cases, with and without solid carbon. 5.2. Model Simulation Results. By applying the gasification equilibrium model developed above, simulations were performed for the four types of feedstock with three different steam inlet temperatures of 800, 1000, and 1200 °C. As indicated in Figure 7, the number of moles of each species produced in the syngas per mole of dry feedstock consumed versus the STBR can be generally grouped into two different regions, one with the carbon deposit and the other without carbon deposit. First, let us address the region with the solid carbon deposit and its effects on the syngas compositions during steam gasification. In general, the solid carbon is only formed at a very narrow range of the lower STBR and the relationship between the carbon deposit quantity and the STBR is linear with the quantity of solid carbon formation decreasing with increasing STBR. The main reason for the solid carbon formation is that, since the heat of gasification is solely supplied by the steam, there is not enough heat to completely covert all the carbon elements to CO, CO2, and CH4 at the lower STBR. Therefore, the carbon formation does affect the trends and characteristics of the syngas compositions as a function of the STBR. As a result, this scenario explains the increase of CO, CO2, and CH4 production as STBR is increased due to more available heat. However, it is noted that for the wood and plastic the level of the steam temperature does not make much difference on the quantity of the carbon deposit as the extra thermal energy carried by the steam at a higher temperature is used to generate more hydrogen due to the higher equilibrium

o

where h ̅ f (BM) is the enthalpy of formation of biomass feedstock, hf̅ (H 2O(S)) is the heat of formation of liquid water, h̅vap is the heat of vaporization of water, h̅steam is the enthalpy of the steam, and h̅j denotes the enthalpy for each species at the equilibrium temperature. The heat of formation for each biomass feedstock can be calculated from the C, H, O compositions and its heat of combustion as shown in eq 10. Seven nonlinear simultaneous equations are set and ready to be solved for the six unknown coefficients, from a1 to a6. As the reactor temperature goes up, i.e. the STBR increases, the solid carbon, C(s), decreases and finally reaches zero. Then there are only five unknown coefficients in eq 7, from a1 to a5, and only two independent chemical reactions without solid carbon are sufficient to solve the equations instead of three above. CH4 + H 2O ↔ CO + 3H 2

(21)

CO + H 2O ↔ CO2 + H 2

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Figure 7. Number of moles of each species produced in the syngas per mole of dry feedstock consumed versus the STBR with three different steam temperatures, 800, 1000, and 1200 °C. (a) wood, (b) plastic, (c) rubber, and (d) MSW.

constant, K2, at higher temperature for hydrogen production (eq 15). However, the three carbon lines are clearly separated from one another in the cases of rubber and MSW that can be explained with the carbon deposition boundaries shown in Figure 6. The carbon deposition boundary lines vary with respect to the equilibrium temperatures, and the separation spacing between lines are quite different even with the same reactor temperature increments. The separation spacing between carbon deposition boundary lines on the steam gasification paths are relatively large for the low reactor temperature region, and they become smaller when the reactor temperature increases. Once the energy balance, eq 20, is applied for the four types of feedstock, the equilibrium reactor temperatures are determined, and they are relatively high for the wood and the plastic, and quite low for the rubber and the MSW. Large gaps of the carbon deposition boundaries lead to a clear distinction of the solid carbon lines for the rubber and the MSW, when the steam inlet temperature is changed which causes the corresponding change of the reactor temperature. With the same reason, there is no solution when the STBR is too low for the rubber and the MSW since the equilibrium

temperatures drop below the atmospheric temperature in order to meet the energy balance requirement. It is seen that the trends and slopes take different approaches or totally reverse after no more solid carbon can be formed (STBR > 1.1 for wood and STBR > 1.7 for plastic) except only for the CO production. This outcome can be explained by the following. When there is no more solid carbon deposition, the total number of available carbon elements for the production of gaseous species is fixed, so the production of CO, CO2 and CH4 must share all the fixed amount of carbon elements. Since we also have more oxygen elements brought in by the steam due to the higher STBR, the production of CO and CO2 must increase to accommodate the extra oxygen atoms that forces the drop in the production of CH4 as a result of the mass balance for the carbon elements. On the basis of the same concept, the slope of H2 production increases as a result of the more hydrogen atoms becoming available due to the reduction of CH4 production. In general, the trends of the rest of the gas constituents are very similar among the four feedstock materials. The hydrogen mole production always increases where the reactor temperature increases due to increasing STBR. Therefore, the higher 4581


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Figure 8. Simulated mole fractions of the product gas species in dry condition, the reactor temperature and the lower heating value with respect to the STBR with 1000 °C of steam. (a) wood, (b) plastic, (c) rubber, and (d) MSW.

of the equilibrium constants. A higher steam temperature leads to higher hydrogen and lower methane mole fractions. However, the amount of steam should be considered as well with the equilibrium constants to see the STBR effect because the equations for the equilibrium constants and the unknowns are functions of the partial pressure or the mole fractions. Figure 8 shows the mole fraction of each syngas species in the dry condition, reactor temperature, and the estimated lower heating value of the syngas when 1000 °C of steam is introduced for four feedstock materials. Very similar trends are found for the four feedstock materials. Except for the reactor equilibrium temperatures that increase monotonically with increasing STBR, the gas mole fractions and lower heating values are all asymptotically approaching respective saturation values after STBR reaches values between 4 and 5. However, for the rubber the STBR needs to reach 14 for those quantities to approach saturation values due the fact that rubber has a much higher intrinsic heat of formation and the weight of CHXOY is quite smaller than those for the other three. At a given gasifying steam temperature, the STBR is the single dominant parameter. The lower heating value starts at a very large value and drops to reach the saturation value around 8 MJ/m3 for all feedstock materials. The reason why the hydrogen concentrations level off for the wood and the plastic at the STBR around 2.5, is not because of the reactor

steam temperature and higher STBR lead to higher hydrogen production. The equilibrium constants are the major parameters which affect the gas concentration trend the most since the equilibrium constants themselves vary significantly when the temperature changes. Whenever there is solid carbon deposit, K3 is dominant if the reactor temperature is low, and the solid carbon would be consumed to generate the methane. That explains why the reactions at low steam temperatures have higher methane concentration with the same STBR conditions. K2 increases while the steam temperature goes up; in contrast K3 decreases. The solid carbon and the steam would react and start to generate hydrogen. As mentioned above, the equilibrium temperatures for the wood and the plastic are high, and they show slightly different trends with respect to the STBR only. However, when the reactor temperatures are considered, basically the trends are similar. K1 is relatively much lower compared to the other two, and that is why we have high CO2 and low CO concentration for all cases. The solid carbon can be used up through the reactions identified by eq 15 and eq 16, but the CO2 remains. For the higher STBR and no solid carbon region, K4 is relatively high and decreases while the temperature increases, and K5 is totally the opposite. Therefore, the trends when the steam temperature changes could be explained by the variations 4582


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Figure 9. Measured syngas component mole fractions and lower heating values at different elapsed times after the feedstock is introduced with 1000 °C steam temperature, (a) wood, (b) plastic, (c) rubber, and (d) MSW (there is no CO2 data for the rubber due to the GC problem).

feedstock, and the samples were evaluated using a GC to determine the gas compositions. Figure 9 shows the mole fractions of syngas components and the corresponding lower heating values in MJ/m3 with respect to the elapsed time after the feedstock was introduced to the reactor. Feedstock was supplied intermittently two to three times on the basis of the syngas generation rates since the feedstock was introduced using a semibatch-type screw feeder. In Figure 9 the downward pointing arrow indicates the time when the feedstock was introduced. As guided by the theoretical modeling results, we can conclude that the hydrogen concentration would go up when the STBR and the reaction temperature are both increased as shown in Figure 8. Therefore, the hydrogen mole fraction can be used as an indicator for the reactor operating conditions. A higher hydrogen mole fraction means that the reaction temperature and the STBR are both higher. As we can see, the experimental results shown in Figure 9 generally demonstrate this trend. After the introduction of the feedstock, for all four feedstock materials, the hydrogen mole fractions first take a drop and then continue to rise for the rest of this cycle as the reactor

temperature but because of the higher H2O concentration due to higher STBR. At larger values of STBR, the gasification reactor temperature reaches between 600 to 800 °C. That is because the steam is the only heat source for the steam gasification process, and the reactor temperature keeps rising when there is more steam to break down the biomass. However, the mole fractions would become saturated when the STBR is high enough. Even though the gas mole fraction curves are different from one another they all become saturated to certain values. These curves show similar trends for all the feedstock materials.

6. RESULTS AND DISCUSSION 6.1. Gas Composition and Heating Value. The steam gasification experiments were performed using four different types of feedstock−wood, automobile tire rubber, water bottle plastic, and synthetic MSW. The temperature of the steam when entering the reactor is around 1000 °C. As mentioned in the Experimental Procedure section, the syngas produced was collected in sample bags at different times after introducing the 4583


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temperature and the STBR are both low initially due to the transient effect of adding a large amount of room-temperature feedstock mass into to the reactor. Therefore, the STBR would decrease to a very low level right after the feedstock is supplied. This is why the hydrogen concentration decreases and carbon dioxide concentration increases when the feedstock is supplied. However, once the feeder valve is closed, the STBR increases as time goes by because the steam is being supplied continuously. Since the feedstock is close to the ambient temperature, the fresh feedstock decreases the reaction temperature as it enters the reactor. As expected when using steam at the super high temperature of 1000 °C, the syngas composition is dominated by hydrogen with mole fractions ranging from 50% to 60%, and CO, CO2 ,and CH4 combined make the balance for the rest. The main reason is due to the water gas shift reaction that converts the CO to hydrogen in a high-temperature steam environment that is another huge benefit for using steam as the gasifying agent. Relatively speaking, the time-dependent lower heating values of the syngas show a rather constant trend that is also as expected due to the fact that the reactor is supplied by a constant steam flow rate. Among the syngas composition data for the rubber given in Figure 9, CO2 data is missing due to a GC problem. However, it does not affect the lower heating value calculation according to eq 4 since the heating value of carbon dioxide is zero. The accuracy of the GC was within ±2.5% for CO, ± 2.0% for H2 and CO2, and ±1.0% for CH4 based on the volume concentrations. This was checked repeatedly by the gas mixtures with known concentrations. The reaction temperature would also have an effect on the gas composition results. Since the feedstock is close to ambient temperature, the fresh feedstock decreases the reaction temperature as it enters the reactor. However, soon after that, the steam regenerates the reactor temperature, which leads to higher hydrogen concentration. CO2 is the next highest species with mole fractions around 20%, and the next is CO that has about 10% in mole fraction. CH4 is the lowest except for the rubber as it has the highest weight percent in hydrogen. Figure 10 shows the averaged syngas composition data for the entire experimental period (10−12 min). It is apparent that there are only minor differences among the different types of feedstock and the steam gasification system can convert any material which has carbon, hydrogen, and oxygen elements in it into a gaseous fuel with a relatively much higher volume concentration of hydrogen that is solely due to the water gas shift reaction as a result of using steam. In general, the hydrogen mole fraction that is around 50−60% dwarfs the other three gas species. The volume mole fractions of CO and CO2 are around 10%, and the volume mole fraction for CH4 is around 3%, which is the lowest. Comparing among the four syngas species, the plastic gives the highest hydrogen volume concentration, while it produces the lowest volume concentrations in CO, CO2, and CH4. The wood feedstock has the lowest hydrogen volume concentration and the highest CO volume concentration. Since only H2, CO, CH4, and CO2 were analyzed, the lumped volume concentration of the residual gases is labeled as Res. in Figure 10, and the contents of the gases were not determined in detail. Further investigations are required in order to determine these components. A higher hydrogen concentration for a syngas does not directly mean higher heating value on a volume basis since the density of hydrogen is very low even though it has a quite high heating value per unit mass. Nevertheless, the lower heating values for

Figure 10. Averaged syngas compositions with respect to types of feedstock using 1000 °C steam condition (there is no CO2 data for the rubber due to the GC problem).

the syngas generated from steam gasification are quite high. The lower heating values based on the measured mole fraction data in Figure 9 for the four types of feedstock are illustrated in Table 4. These LHV results ranging from 7.8 to 10.8 MJ/m3 are Table 4. Averaged Syngas LHV from Four Types of Feedstock Materials feedstock

wood

plastic

rubber

MSW

averaged LHV (MJ/m3) ± 0.52 MJ/m3

9.7

7.8

10.8

8.2

encouraging when these are compared to the ones ranging from 4 to 5 MJ/m3 documented for using similar feedstock in the airblown partial combustion partial gasification system.29 This is mainly due to the large amount of hydrogen generated solely due to the use of steam as the gasifying agent. The current experimental results were also compared with other researchers’ data which were obtained using a steam gasification process. The comparison is given in Figure 11. As can be seen the results from the other three papers all showed quite similar trends and magnitudes even with different types of feedstock. Umeki et al.9 used wood, He et al. used MSW with calcined dolomite catalyst,13 and Nipattummakul et al.,6 gasified sewage sludge using steam. Due to similarities among the four feedstock materials, the experimental results were compared only to the model simulation results for the synthetic MSW, and the comparison is plotted in Figure 12. The difference between the experimental and simulation results might be due to the assumptions made for the numerical model. The simulation does consider only four types of gas components in the syngas, and there is no methane for the numerical simulation since most of methane is converted into hydrogen and carbon monoxide as shown in eq 21 at high-temperature conditions. In general, the agreement is quite good. The modeling results could be improved by considering the effect of diffusion rate. Sharma compared an equilibrium model and a kinetic model, and this study shows that the equilibrium model overpredicts for hydrogen and carbon dioxide, and underestimates carbon monoxide and methane compared to the kinetic model.36 4584


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Figure 11. Averaged syngas compositions with respect to types of feedstock using 1000 °C steam condition (there is no CO2 data for the rubber due to the GC problem).

Figure 13. Comparison between air gasification and steam gasification for woody biomass simulation. The reactor temperature of the air gasification is 900 °C,29 and 700 °C for the steam gasification using 1000 °C of steam.

are due to extra water gas shift reaction in steam gasification and N2 dilution in air-blown gasification. 6.2. System Efficiency. The cold and hot gas efficiencies can be calculated using eq 5 and eq 6, respectively. Here, the steam gasification model simulation results for the MSW were used to calculate these parameters, and the results are plotted with respect to STBR in Figure 14. Initially when the STBR is

Figure 12. Comparison between the experimental results and the simulation results for synthetic MSW gasification.

Considering this comparison the results would be closer to the experimental results. The air-blown partial combustion and partial gasification system is known to generate the syngas that is diluted by nitrogen which is inert and cannot be combusted. It is interesting to compare the syngas compositions between the two gasification methods. As the experimental data are limited, and it is hard to find similar experimental conditions and feedstock used for comparison, we decided to compare the numerical model simulation of the current high-temperature steam gasification with that of air-blown partial combustion and partial gasification by Balu and Chung.29 The simulated syngas compositions and the lower heating values for wood gasification are compared between the air-blown gasification and the steam gasification in Figure 13. As expected, the main difference is the hydrogen to carbon monoxide molar ratio. In steam gasification that ratio is about 12 whereas in air-blown gasification that ratio is 1. Following the same scenario, the LHV of the syngas from steam gasification is more than twice that in air-blown gasification. Again, the differences between the two methods

Figure 14. Cold and hot gas efficiencies for the MSW gasification using 1000 °C of steam with respect to the steam to biomass mass ratio.

increased, the hot gas efficiency goes up sharply. Once the STBR reaches around 1.3, that is the point after which there is no more solid carbon formation; the hot gas efficiency curve takes a drastic turn to become almost flat at about 94% with very slow increases for the rest of the STBR range. For the cold gas efficiency, first it quickly rises to reach a peak where the STBR is around 1.3, and after that it begins to monotonically decrease for the rest of the STBR range since the amount of energy input for the steam generation increases with increasing 4585


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to 60%, and CO, CO2 and CH4 combined makes the balance for the rest. It was found that the main difference between steam gasification and air-blown partial combustion and partial gasification is the hydrogen to carbon monoxide molar ratio. In steam gasification that ratio is about 12 in the current superhigh-temperature steam gasification, whereas in air-blown gasification that ratio is 1. Following the same scenario, the LHV of the syngas from steam gasification is more than twice that from air-blown gasification. Again, the differences between the two methods are due to the extra water gas shift reaction in steam gasification and N2 dilution in air-blown gasification. For MSW, the cold gas efficiency was found to peak at 73% with the STBR equal to 1.3, and after that it monotonically decreases to 34% when the STBR is increased to 8. While the hot gas efficiency stays almost at the constant value of 94% when the STBR is greater than 1.3, other feedstock materials all have similar efficiency values.

STBR. As mentioned previously, the hot gas efficiency is the maximum possible system efficiency because the syngas thermal energy is recovered totally. We have also plotted three more curves that represent 40%, 60%, and 80% recovery of the available syngas thermal energy, respectively. The curve becomes flatter when the recovery percentage gets higher. 6.3. High-Temperature Steam Economic Feasibility. As high-temperature steam was used in the current allothermal process, the economic feasibility can be justified as follows. (1) For the steam entering the reactor at 1000 °C, the syngas produced is about 700 °C that still contains high-quality thermal energy. Since downstream processing of the syngas does not require a high-temperature condition, we can recover most of the thermal energy to heat the steam. As Figure 13 shows, if there is a 100% recovery, the overall system thermal efficiency can reach 94%. (2) When high-temperature steam is used as the gasifying agent, as shown in Figure 12 the syngas produced contains much more hydrogen (molar ratio H2/CO = 12) as compared to traditional air-blown partial oxidation, partial gasification method (molar ratio H2/ CO = 1). As a result, the syngas in high-temperature steam gasification is more energy dense with a higher LHV (Table 4). (3) As indicated in the paper, the main driving force for choosing high-temperature steam is its ability to gasify solid waste and produce no negative effects to the environment. Therefore, the most tangible economic benefit of the high-temperature steam gasification is the fact that the feedstock is free (MSW, rubber from used tires, and plastic from waste containers). (4) The high-temperature steam can also be produced using concentrated solar power the technology of which has been under intense research and development for reducing the cost.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +1 352-392-9607. Fax: +1 352-392-1071. E-mail: jnchung@ufl.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the Hinkley Center for Solid and Hazardous Waste Management. Mr. John Anderson of Quantera Energy, LLC donated a $26k High-Temperature Steam Generator. Partial support from the Florida Energy Systems Consortium (FESC) is also acknowledged.

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REFERENCES

(1) United States Environmental Protection Agency (U.S. EPA), http://www.epa.gov/osw/nonhaz/municipal/. (2) ElFadel, M.; Findikakis, A.; Leckie, J. J. Environ. Manage. 1997, 50, 1−25. (3) Murphy, J.; McKeogh, E. Renewable Energy 2004, 29, 1043−1057. (4) McKay, G. Chem. Eng. J. 2002, 86, 343−368. (5) Chang, A. C.; Chang, H.; Lin, F.; Lin, K.; Chen, C. Int. J. Hydrogen Energy 2011, 36, 14252−14260. (6) Nipattummakul, N.; Ahmed, I. I.; Kerdsuwan, S.; Gupta, A. K. Int. J. Hydrogen Energy 2010, 35, 11738−11745. (7) Ahmed, I.; Gupta, A. K. Appl. Energy 2009, 86, 1813−1821. (8) Ahmed, I. I.; Gupta, A. K. Appl. Energy 2011, 88, 1613−1619. (9) Umeki, K.; Yamamoto, K.; Namioka, T.; Yoshikawa, K. Appl. Energy 2010, 87, 791−798. (10) Umeki, K.; Namioka, T.; Yoshikawa, K. Appl. Energy 2012, 90, 38−45. (11) Skoulou, V.; Swiderski, A.; Yang, W.; Zabaniotou, A. Bioresour. Technol. 2009, 100, 2444−2451. (12) Guan, Y.; Luo, S.; Liu, S.; Xiao, B.; Cai, L. Int. J. Hydrogen Energy 2009, 34, 9341−9346. (13) He, M.; Hu, Z.; Xiao, B.; Li, J.; Guo, X.; Luo, S.; Yang, F.; Feng, Y.; Yang, G.; Liu, S. Int. J. Hydrogen Energy 2009, 34, 195−203. (14) Mizuno, T.; Goto, M.; Kodama, A.; Hirose, T. Ind. Eng. Chem. Res. 2000, 39, 2807−2810. (15) Chang, J.; Tsai, J.; Wu, K. Waste Manage. 2005, 25, 1037−1045. (16) Chang, J.; Tsai, J.; Wu, K. Bioresour. Technol. 2006, 97, 116− 122. (17) Hwang, I.; Aoyama, H.; Matsuto, T.; Nakagishi, T.; Matsuo, T. Waste Manage. 2012, 32, 410−416. (18) Onwudili, J. A.; Williams, P. T. Energy Fuels 2007, 21, 3676− 3683.

7. CONCLUSION A theoretical equilibrium model and laboratory-scale experimental results have been documented in this paper for superhigh-temperature (∼1000 °C) steam gasification of four feedstock materials. In general, the equilibrium model and the experimental results agree with each other. First, it can be concluded that the feasibility of the high-temperature pure steam gasification of wood, rubber, plastic, and municipal solid waste was demonstrated successfully. By using various types of feedstock the system was able to generate syngas of high energy quality that has a much higher heating value than that produced from an air-blown gasification system. The lower heating values from the four types of feedstock were found to have the range of 7.8 MJ/m3 for the plastic to 10.8 MJ/m3 for the rubber when 1000 °C steam was used as the gasifying agent. The compositions of the syngas varied with the different feedstock materials, but the trends of mole fractions and lower heating values as a function of the STBR are very similar to one another. In general, the trends of syngas composition characteristics are closely related to the solid carbon deposit. The trends are totally different after there is no more solid carbon formation. As expected when using steam at the superhigh temperature of 1000 °C, the syngas composition is dominated by hydrogen with mole fractions ranging from 50% 4586


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