Article pubs.acs.org/EF
Thermodynamic Study of the Alkali Release Behavior during Steam Gasification of Several Biomasses F. Defoort,*,† C. Dupont,† J. Durruty,† J. Guillaudeau,† L. Bedel,† S. Ravel,† M. Campargue,‡ F. Labalette,§ and D. Da Silva Perez⊥ †
CEA/DRT/LITEN, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France RAGT Energie SAS, Albi, France § GIE-Arvalis ONIDOL, Paris, France ⊥ FCBA, Saint-Martin d’Hères, France ‡
S Supporting Information *
ABSTRACT: The aim of this paper is to study the alkali volatilization of various biomasses during steam gasification both through thermodynamic calculations and gasification experiments in a fluidized bed. Thermodynamic calculations with a database of simple compounds predicted two different alkali volatilization behaviors under the typical temperature range of fluidized bed gasifier (750−1000 °C). For silica-rich biomasses, calculations predicted the gas phase to be constituted of KCl and HCl with a low K release fraction in gas. The volatilized gas was predicted to condense in the cooled part of the reactor mainly as KCl in agreement with experimental results. For silica-poor biomasses, the calculated gas phase was found to be constituted of KCl and KOH with a higher K release fraction in gas than for silica-rich biomasses. The volatilized gas was predicted to condense in the cooled part of the reactor mainly as K2CO3, which disagreed with experimental results. These discrepancies in gas phase speciation and condensation may be due to the absence of liquid solution in database used in thermodynamic calculations and to kinetic limitations in the gas and/or condensed phases.
1. INTRODUCTION In the present energy context, there is a growing interest for processes using lignocellulosic biomass. Among those processes, gasification appears as particularly promising. Even if the process is close to industrialization, some issues still need to be addressed. One major issue is related to the gasifier flexibility in terms of feedstock because of the limited availability of biomass and of its variable nature worldwide. Indeed, biomass has a highly variable composition, notably in terms of inorganic species.1 Those species, among which alkali compounds, may greatly impact the process. Indeed, either they remain in solid form and tend to agglomerate, or they volatilize as harmful species that may lead to corrosion or may affect the performance and the durability of catalysts used for syngas synthesis into Diesel Fischer−Tropsch or synthetic natural gas. Hence it is of crucial importance for process control to be able to predict their behavior during gasification. In particular, knowing the syngas content at gasifier exit enables forecasting of suitable gas cleaning equipment before applications. The thermodynamic equilibrium model is a useful tool to predict sufficiently high temperature processes involving chemical reactions. Equilibrium calculations provide a reference state for species and the maximum chemical interaction that may be reached for each species under the operating conditions of temperature and pressure. It is well-known that biomass gasification in fluidized bed, i.e., at temperatures typically below 1000 °C, is kinetically limited regarding the formation of the main gaseous species (CO, CO2, CH4, H2).2 However, for inorganic species in the gas phase, no kinetic limitation has been reported so far except for NH3.3 © 2015 American Chemical Society
Such a thermodynamic model has been used by several authors to predict the behavior of inorganic release during thermochemical conversion of various biomasses, for instance, Jensen,4 Wei,5 Sonwane,6 Kuramochi,7 Zevenhoven-Onderwater,8 Mojtahedi,9 Thy,10 Tchoffor,11 Turn,12 Johansen,13 Froment,14 and Stemmler.15 Most of these authors dealt with air/O2 gasification, pyrolysis, or combustion, and only very few of them dealt with steam gasification.11,14,15 These studies often suffer from the absence of experiments for quantitative model comparison.5,6 Kuramochi compared his modeling results on HCl with O2 gasification experiments found in the literature.16 Jensen,4 Thy,10 Johansen,13 and Tchoffor11 used qualitative thermodynamic calculations to facilitate interpretation of their experimental results. On the contrary, inorganics release experiments were performed at the lab-scale in several studies, but no comparison was made with thermodynamic equilibrium model, or the comparison was made only qualitatively. This is the case for instance in the studies of Bjorkman,17 Knudsen,18 Keown,19 Okuno,20 van Lith,21 Jiang,22 Diaz-Ramirez,23 Arvelakis,24 or Novakovic.25 When thermodynamic calculations are performed, the influence of the database used may strongly impact results and associated conclusions. In this context, the studies of Zevenhoven-Onderwater8 and Tchoffor11 are of great interest, as they were carried out with a more advanced database than Received: April 30, 2015 Revised: October 14, 2015 Published: November 6, 2015 7242
DOI: 10.1021/acs.energyfuels.5b01755 Energy Fuels 2015, 29, 7242−7253
Article
Energy & Fuels Table 1. Moisture, Ash Content, and Elemental Composition of the Samples property
unit (dry basis except moisture)
moisture ash 550 °C C H O calculated Al As B Ca Cd Cl Cr F Fe Hg K Mg Mn N Na Ni P Pb S Si Ti Zn
wt % wt % wt % wt % wt % ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm
K/Cl Ca/Si K/Si
mol/mol mol/mol mol/mol
WS
T
8.30 9.2 7.60 4.4 48.1 46.3 5.85 5.83 37.46 41.86 130 170
