Removal of Residual Char Fines from Pyrolysis Vapors by Hot Gas Filtration

Abstract

An NREL-designed vortex reactor fast pyrolysis process development unit (PDU) has been used to investigate hot gas filtration of biomass pyrolysis vapors. Most of the experimental work employed a conventional baghouse type of filter that used NEXTEL™ ceramic cloth filter bags as the filter medium.A series of experimental runs demonstrated that hot gas filtered biocrude oils having less than 10 ppm of total alkali could be reproducibly made. Removal of the char cake from the filter elements proved to be a difficult problem. The char appears to become progressively more sintered to itself and the filter as a function of the cumulative biomass processed. Controlled oxidation does remove this dense char from the filters, but leaves residual ash on the filter cloth fibers. This ash may in turn cause subsequent biomass pyrolysis vapors that pass through the filter to produce additional char (coke) in the interstices of the filter cloth. Data are presented that suggest this char formation may contribute to a more rapid rise in the rate of filter blinding as measured by the increase in recovered filter pressure drop.

A review of the chemical and physical mechanisms of the storage stability of fast pyrolysis bio-oils

“…Table 2 shows that the chlorine content of the hot-gas-filtered hardwood bio-oils varied considerably, from 0.3 to 2 equivalents of chlorine per equivalent of potassium, sodium, and calcium (Diebold et al 1996;Scahill et al 1997). The chlorine content of switchgrass oils did not change significantly during filtration of the bio-oil and was in the range of 1200 ppm to 1600 ppm; the metal ion content decreased (Agblevor et al 1994).…”

Large-Scale Pyrolysis Oil Production: A Technology Assessment and Economic Analysis

“…There was also evidence of chemical reactions as indicated by a measured temperature rise across the filter elements. Both cracking and polymerization reactions among the various compounds in bio-oil are likely to have occurred as the pyrolysis vapors passed through the char cake on the filter elements [19]. It is suspected that polymerization reactions were responsible for bonding char particles together in the cake, making it difficult to remove from the filter elements.…”

“…Also during the 1990s, other researchers at NREL were developing hot vapor techniques to filter biomass pyrolysis vapors prior to cooling and condensation [19]. This work resulted in producing bio-oils that had very low levels of char and consequently alkali metal content, on the order of only a few ppm total alkalis.…”

Biofuels Production through Biomass Pyrolysis —A Technological Review

“…Conventionally, chars are separated from reactors using the cyclone method, but this method has the limitation of passing fine particles through the cyclone which then collect in the liquid product where they accelerate aging and create instability problems. Although a number of alternative approaches have been used to overcome this limitation including in-bed vapour filtration [102,103] and rotary particle separation [104], these techniques also have difficulties due to complex interaction between the char and pyrolytic liquid which appears to form a gel-like phase that rapidly blocks the filter. Attempts have been made to solve this problem by using solvents like methanol or ethanol to modify the liquid microstructure [5].…”