4842
Ind. Eng. Chem. Res. 2000, 39, 4842-4848
Gasification of Pyrocatechol in Supercritical Water in the Presence of Potassium Hydroxide Andrea Kruse,* Danny Meier, Pia Rimbrecht, and Michael Schacht Institut fu¨ r Technische Chemie CPV, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany
The results presented are fundamental investigations with regard to developing a process for hydrogen production from high moisture biomass and wastewater. Pyrocatechol is used as a model compound for lignin in biomass and for aromatic compounds in wastewaters. The experiments of pyrocatechol gasification provides knowledge of the chemical reactions occurring during gasification and their dependence on reaction conditions. The experiments were carried out in two different reactor types, a batch and a tubular reactor, to achieve long reaction times at low temperatures as well as short reaction times at high temperatures. The pressure, temperature, concentration, and reaction time dependence of the gasification is presented and compared with calculated equilibrium data. The influence of potassium hydroxide addition to improve the hydrogen yield was studied and the effect of different catalysts is discussed. Introduction Considering the efforts to reduce the carbon dioxide emission as a consequence of energy production from fossil carbon resources, biomass, and organic wastes may play an important role as renewable electric power sources. Especially the gasification of biomass and the subsequent use of the gases for electric power generation is of high interest. A special case in this area is biomass and waste of high water content, for example, waste biomass from the agriculture or food industry. The supply potential of this wet biomass is considerable and these materials often cause remarkable disposal costs. Classical gas-phase gasification of biomass with high moisture is expensive because it has to be dried prior to this process. This is not necessary if the gasification is performed in hot compressed water where the water is the reactant and solvent simultaneously. The polymeric structure of lignin and cellulose is hydrolyzed and the degradation products are mainly dissolved in water. In this way soot formation is drastically decreased or even completely suppressed. Without soot, biomass can be gasified at lower temperatures because biomass is more reactive than coke. For this reason gasification in supercritical water is carried out at 700 °C or below but gas-phase gasification is at 1000 °C. Our goal is to develop a process to produce hydrogen from high moisture biomass and wastewaters. The hydrogen may be used for electric power generation by fuel cells. The advantage of biomass gasification in supercritical water is the preferred production of hydrogen and carbon dioxide instead of synthesis gas formed in the gas-phase gasification. This is a consequence of the high water excess in the supercritical water process. A second process step converting synthesis gas to hydrogen and carbon dioxide is not necessary. Earlier experiments showed that the usage of certain catalysts increase the gasification yields.1-6 Alkali salts2-6 and charcoal1 are reliable catalysts for hydrogen formation; nickel2 and other hydrogenation catalysts2 were found to improve the methane formation. Recent * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +497247822244.
investigations on the gasification of coal6 in supercritical water also show a positive impact of alkali salts. To improve the understanding of the chemistry of gasification, experiments with biomass and model compounds for biomass are carried out. The most important experimental results of the gasification of pyrocatechol, as a model compound for aromatic compounds in wastewater and for lignin in biomass, are presented in this article. The main subjects are the comparison of experimental and calculated data and the influence of salts. For example, the influence of pressure, temperature, reaction time, and feed concentration are shown. Experimental Setup The experiments were carried out in a tumbling reactor and in a tubular reactor. The Inconel 625-lined tumbling reactor has an internal volume of 1000 mL and is constructed for a pressure of up to 500 bar and a temperature of up to 500 °C (Figure 1).3-5 The tubular reactor3-5 consists of a 500-mm long horizontal tube (outer diameter 15.4 mm, internal diameter 8 mm), made of Inconel 625 (Figure 2). Its construction is based on a former small tubular reactor.7 Three separately regulated heating coils heat 400 mm of the tube. Inside the tube a movable thermocouple records the internal temperature profile along the reactor. Other thermocouples are fixed outside the reaction tube for temperature control. The ends of the reactor tube are cooled. An HPLC pump is used for feeding up to 5 g/min of the aqueous solution. Variation of the flows leads to different reaction times. The experiments are started by feeding pure water for 1 h to attain the desired temperature set-point. Then, the pyrocatechol solution is fed. After some centimeters the temperature set-point is reached and the deviation is
