1622
Ind. Eng. Chem. Res. 2006, 45, 1622-1630
Influence of Process Variables on Gasification of Corn Silage in Supercritical Water Pedro D’Jesu´ s,*,† Nikolaos Boukis,† Bettina Kraushaar-Czarnetzki,‡ and Eckhard Dinjus† Institut fu¨r Technische Chemie, Chemisch-Physikalische Verfahren (ITC-CPV), Forschungszentrum Postfach 3640, 76021 Karlsruhe, Germany, and Institut fu¨r Chemische Verfahrenstechnik, UniVersita¨t Karlsruhe, Kaiserstr.12, Karlsruhe, 76131, Germany
Gasification of 5 wt % (DOM) corn silage in supercritical water was investigated in a continuous flow reactor. The influence of pressure, temperature, and residence time on the gas yield was determined. Changing the pressure in the range 250-400 bar did not alter the gas yield. The temperature was varied from 300 to 700 °C. At higher temperature, the conversion of biomass in supercritical water was completed. At lower temperature, the biomass is partly converted, and the gas yield is decreased. Residence time variations from 0.6 to 10 min revealed for each investigated temperature that with longer residence time, gas yield increased until a maximum was reached. Gas composition changed with residence time and temperature. At higher temperature, more hydrogen, methane, and ethane were obtained. The gas yield can be modeled in the studied conditions by assuming a zero-order kinetic. Introduction Energy consumption is increasing and is expected to rise by 54%, from 383 EJ in 2001 to 591 in the year 2025.1 To cover this demand with slowing down of the fossil fuel production, new sources of energy have to be developed. Using biomass for energy generation offers the potential to reduce the rise in atmospheric CO2 concentration. The world’s energy supply from biomass is estimated to be 10-14%.2 The energy content of the biomass can be converted through many types of technologies and is CO2-neutral. The three main routes for converting the energy contained in biomass resources into a useful kind of energy are biological conversion, physical conversion, and thermal conversion.3 In the case of biomass with a high moisture content (>40%), the supercritical water gasification process (SWG) is the most efficient, as compared to other biomass energy conversion technologies.4 Some studies refer to gases as the combustibles of the 21st century.5 The main product of the SWG process is hydrogen. Hydrogen is considered the fuel of the future, because its combustion generates only water as the product. Calzavara et al.6 evaluated the supercritical water gasification process for hydrogen production from cornstarch and a mixture cornstarch/ saw dust. They found that the energy efficiency reached 60% without including energy recovery when gasifying. With energy recovery, the overall energy yield reached 90%. The number of hydrogen bonds per water molecule decreases under supercritical conditions as the temperature is increased.7 The static dielectric constant of the supercritical water sinks in comparison to ambient conditions, and the diffusivity of water increases at under supercritical conditions.8 With this change in its properties, supercritical water becomes a more reactive medium for hydrolysis, hydration, hydrogen exchange, and freeradical reactions,9 which are needed for gas production from biomass. * To whom correspondence should be addressed. E-mail:
[email protected]. † Institut fu ¨ r Technische Chemie. ‡ Universita ¨ t Karlsruhe.
Above 600 °C, supercritical water has stronger oxidizing properties. Oxygen molecules in water combine with C atoms of the biomass to produce primarily CO2 due to the high density of the supercritical water, as compared to low-pressure steam.10 CO formation decreases at higher temperature, because CO conversion by the water/gas shift reaction is favored.11 Biomass reacts with water according to the ideal overall reaction 1.
C6H12O6 + 6H2O f 6CO2 + 12H2
(1)
Hydrogen is released from both water and biomass. Reaction 1 is not complete, and methane, CO, and some higher hydrocarbons are formed. In the literature, an extensive investigation on the gasification of biomass model compounds can be found. Antal et al.12 have found the best hydrogen yield for the supercritical water gasification of sawdust and different starches at high temperatures. Lee et al.13 investigated the influence of temperature in the range of 510-750 °C. They obtained total glucose conversion at 700 °C. The heating rate also influences the gasification of biomass in supercritical water. Higher heating rates lead to higher gasification efficiencies, because the formation of tars and chars is inhibited.14,15 Hao et al.16 studied the supercritical water gasification of glucose. They found that pressure has no great effect on the glucose gasification efficiency and the composition of the gas product. Matsumura et al.17 investigated the supercritical water gasification of coconut shell activated carbon. They found that changing the pressure from 25.5 to 34.5 MPa does not influence the gas composition. Other authors found that a decrease in pressure leads to an increase in hydrogen formation.18 The effect of the residence time on the supercritical water gasification of glucose is described in some publications. Carbon gasification efficiency does not vary at a longer residence time. The efficiency was observed to be decreased by a shorter residence time.13,16 The influence of the composition on the gasification of mixtures of model compounds in supercritical water was published in refs 19, 21, and 22. A decrease in gas production was observed for mixtures containing lignin.19 This decrease
10.1021/ie050367i CCC: $33.50 © 2006 American Chemical Society Published on Web 01/28/2006
Ind. Eng. Chem. Res., Vol. 45, No. 5, 2006 1623 Table 1. Elemental Composition (in wt %) of Biomass Educts educt
C
O
H
N
K
S
Si
Ca
P
Cl
Fe
cornstarch clover grass corn silage
44.44 44.90 43.40
49.34 43.3 46.70
6.22 6.8 6.17
2.2 1.02
1.1 0.98
0.3 0.93
0.17 0.35
0.64 0.20
0.32 0.14
0.12 0.13
0,15 0.01
was found to be dependent on the lignin type.23 Increases in water density and phenol content resulted in an improved lignin conversion.24 The type of biomass influences the operation conditions needed for the gasification in supercritical water. Xu und Antal25 investigated the gasification of digested sewage sludge in supercritical water at 700 °C. They prepared a paste with cornstarch and use activated carbon as the catalyst. After 1-2 h of operation, the reactor plugged. This means that in the case of real biomass, some hurdles have to be solved before an industrial process can be designed. An extensive investigation of natural biomass feedstock has to be performed. Two kinds of feedstock are thought to be appropriate for the supercritical water gasification process: organic waste, uch as sewage sludge, and energetic plants, such as corn. Corn is one of the most cultivated plants all around the world. It is adapted to almost any climate, and its cultivation does not require any extreme conditions. Corn has a high energy content and high organic matter yield (per hectare).26 Corn silage is chemically stable, it does not degrade, and it maintains its properties over long periods of time. For these reasons, corn silage is available throughout the year. Because of all these advantages, corn silage is a feedstock suitable for an industrial supercritical water gasification process. To model the process of supercritical water gasification of biomass, the influence of the process variables on the gasification of a real biomass feedstock, such as corn, has to be investigated. Experiments on supercritical water gasification of corn silage are reported in this paper. The effects of pressure, temperature, and residence time were investigated in a tubular flow reactor. Experimental Section Preparation of Educts. It was planned that starch acquired from Carl Roth GmbH would be applied first as an emulsifier for natural biomass products, such as corn silage. A temperature of 97 °C was needed for the preparation of a water/starch gel. The use of starch as an emulsifier was discarded, because a high concentration of starch was needed to produce a pumpable gel for processing solid biomass. Corn silage was obtained from the Univesity Kassel26 in pieces 2 cm long. Before the mixture with water was prepared, the biomass had to be crushed. A meat chopper (Bizerba) was used for this purpose. The size reduction was performed in three stages. First, the biomass from the silo was crushed to a particle diameter of
