Reforming of aqueous wood pyrolysis condensate in supercritical water

摘要

Conversion of real biomass into hydrogen by supercritical water reforming needs special preparation of the feed. Biomass in its natural state mostly prevails as wet carbohydrate solids, admixed with sand and soil in various compositions. The reforming technology requires a homogeneous fluid, without particulates and mineral components. Flash pyrolysis may be effective for this purpose as exploratory tests show with humid beech wood saw dust. The condensate from pyrolysis of this material was successfully converted into a hydrogen-rich fuel gas by the treatment in supercritical water at 650 ℃ and 28 MPa. The conversion in relation to reaction time followed a first-order rate equation, with CO and low concentrations of C_1-C_3 hydrocarbons as primary products. Hydrogen was formed as a secondary product, along with CO_2, as a result of the shift reaction. Small concentrations of soda in the feed promoted selectively the shift reaction and let the primary conversion unaffected. Tars were observed at the outlet of the preheater and this caused stop of flow after some time, especially with feed concentrations up from 4% wt. The pressure of the aqueous reaction medium has significant influence on the chemistry. It was found that a 6 MPa pressure retarded formation of gas and promoted excessive formation of coke, which plugged the tubular reactor. This coke was not formed at 28 MPa.