Synthesis Gas

Production of synthetic natural gas in a fluidized bed reactor

Jan Kopyscinski, April 2010

Hello everybody,

This is my first message here in this forum. My former colleague (Johannes) sent you already the link to my PhD-Thesis (see below). I just want to give some more information about my work as a Doctoral Student at the Swiss Federal Institute of Technology (ETH Zurich) and at the Paul Scherrer Institute (www.psi.ch).

In the last 4 years I conducted research in the project Bio-SNG and Methane-from-wood, so-called synthetic or substitute natural gas (SNG) from wood. Detailed information about the process can be found in my recent published review article in Fuel 2010 (Title: Production of synthetic natural gas (SNG) from coal and dry biomass - A technology review from 1950 to 2009) (http://dx.doi.org/10.1016/j.fuel.2010.01.027).

The aim of my work has been to understand the different processes within the fluidized bed methanation reactor by both experiments and modeling.

The outline is as follows:

  1. Theoretical background: Methanation (kinetic, reaction mechanism, carbon deposition, technology review from 1950 until today)
  2. Investigation of the kinetic parameters of the methanation for our catalyst and conditions.
  3. Spatially-resolved gas concentration and temperatur measurment in a fluidized bed methanation reactor. --> Hydrodynamics, mass transfer and kinetic effects.
  4. Modeling of the fluidized bed methanation reactor.

Title: Production of synthetic natural gas in a fluidized bed reactor - Understanding the hydrodynamic, mass transfer and kinetic effects.

http://e-collection.ethbib.ethz.ch/view/eth:1059

If you have any questions or comments please do not hesitate and contact me:

Dr. sc. Jan Kopyscinski
General Energy Research Department
Paul Scherrer Institute
5232 Villigen PSI
Switzerland

Email Jan.Kopyscinski@psi.ch or Jan.Kopy@web.de

Best regards

Jan

ABSTRACT

As the demand for energy is increasing world wide, not only the security of energy
supply and the stability of prices, but also climate change has become an important
issue. The production of synthetic natural gas (SNG) via thermochemical
conversion of biomass and subsequent methanation could be one route to address
these issues. The advantages are the high conversion efficiency, the already existing
gas distribution infrastructure, the well-established and efficient end-use
technologies and the recovery of a concentrated CO2 stream without any additional
cost and thus the possibility for an easy carbon capture and sequestration.
Already in the 1960s, the need for the production of synthetic natural gas arose to
fulfill the increasing demand of natural gas. In the following 20 years, different
methanation processes were developed from coal to synthetic natural gas.
However, only one commercial plant was erected in 1984 and has been producing
SNG ever since. Today, biomass is the feedstock of choice to produce SNG. At the
end of the year 2008, the first pre-commercial plant was completed and the first
wood was converted into methane-rich gas within the European project Bio-SNG.
Paul Scherrer Instiute (PSI) and its project partners developed a 1 MWSNG fluidized
bed methanation reactor based on the knowledge gained by bench-scale
experiments and first model approaches.

The advantages of a fluidized bed methanation reactor compared to fixed bed
methanation reactors are the isothermal operation, the easy controlling, the
possibility for the in-situ water gas shift reaction to adjust the H2/CO ratio, and the
lower risk of catalyst deactivation due to recirculation of the catalyst particles
through the bed.

This thesis aims to increase the understanding of the different processes within a
fluidized bed methanation reactor. A deeper inside was gained by dedicated
experiments using spatially resolved measurement techniques not only in a benchscale
fluidized bed reactor, but also in a catalytic plate reactor. In the latter, a large
number of kinetic data were collected and the influence of the temperature as well
as of the partial pressure of reactants and products were studied under technically
relevant conditions. A one-dimensional model of the catalytic plate reactor was
developed and the kinetic model parameters of the proposed Langmuir-
Hinshelwood rate expressions were estimated by comparing simulated and
measured gas composition profiles. The predicted model results are in excellent
agreement with the experimental data.

The purpose of the fluidized bed experiments was to investigate experimentally the
influences of hydrodynamics, mass transfer, and chemical effects in the
methanation reactor. In these experiments the catalyst mass, the gas velocity, and
the degree of dilution were varied. It was found out that the main conversion
occurs within the first 20 mm of the bed (CO-rich part) and that the mass transfer
between bubble and dense phase is dominating in the upper part of the bed (COlean
part). Further, it was shown that the hydrodynamics (especially the gas
velocity) seem to have a stronger influence than the chemical space velocity. With
higher gas velocity, the catalyst particles are moving faster through the CO-rich
part of the bed, which leads to a better heat transfer and no temperature hotspots
in the entrance region. However, higher gas velocity goes along with higher gas
bypassing through the bed in forms of bubbles. That means that not all gas from
the bubble phase is transferred to the dense phase, where the reactions are taking
place. The consequence is a lower conversion. To assure high CO and H2
conversion, the bed height can be increased. After each fluidized bed experiment,
catalyst samples were taken and analyzed with respect to carbon deposition using
temperature-programmed oxidation (TPO). For that purpose, a TPO method was
developed to distinguish between different carbon depositions on the catalyst
surface. The analysis of the catalyst samples showed no evidence of carbon
depositions in form of polymeric carbon.

A simple homogeneous two-phase fluidized bed model was developed using
hydrodynamic parameters from the literature and kinetic parameters determined
within this work. Modeling of the bench-scale unit showed that the initial slope of
the gas composition profiles and the outlet composition could be reproduced. The
calculated and measured gas compositions in the middle part of the reactor are not
in good agreement. It is still not clear how to describe correctly the effect of
volume contraction due to the methanation reaction on the mass transfer, the
bubble size, and the bubble gas hold-up. In addition, all the hydrodynamic
correlations used are based on measurements at ambient temperature and pressure
in larger fluidized beds without reaction. Thus, the correlations may not be valid
for small bench-scale units and may not consider the influence of volume
contraction.

PearsonTechnologies

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