Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
1 Introduction
The production of substitute natural gas (SNG) from biomass is an interesting
route for CO2 emissions reduction and substitution of fossil natural gas. SNG can
be produced by gasification of biomass and subsequent upgrading of the producer
gas. In the gasification of biomass a producer gas, which mainly contains hydrogen,
carbon monoxide, carbon dioxide, methane and water, is produced. This gas
can be upgraded via for example methanation and CO2-removal to produce SNG.
Several gas cleaning steps are also necessary.
The produced methane may be used as vehicle fuel or in any other application
where fossil-based natural gas is used. To be able to use the gas as vehicle gas
or for export to the gas grid the Wobbe-index has to be adjusted to the correct interval.
If the gas is to be used as vehicle fuel the motor octane number also has to
be sufficiently high. There are two ways of adjusting the Wobbe-index and that is
to either add a hydrocarbon with a higher energy density than methane or to remove
carbon dioxide. This specific project focuses on the second way of adjusting
the Wobbe-index.
1.1 Aim
The purpose of the study was to investigate whether any of the existing carbon
dioxide separation methods already used for biogas upgrading applications (water
scrubbing, amine scrubbing or PSA) is suitable for use in biomass gasification and
subsequent SNG production.
The gasification system simulations were performed using available operational
data, models and literature information and focused on indirect gasification technology.
Indirect gasification was chosen, as it is the biomass gasification technology
with the most demonstration activities, the best data availability and the highest
inherent methane formation. The technology is however limited with respect to
scale-of-operation and thus the conclusions from this study will be relevant only up
to circa 80-100 MWth.
1.2 Report structure
The structure of the report is as follows. A brief description on how SNG is produced
from biomass through gasification will be given in section 2 together with a
short summary of the existing plants. In section 3 different gasification technologies
will be described as well as the most common gas impurities encountered in
gasification and the existing gas cleaning technologies to address these issues. A
review of the most common CO2-removal techniques will be presented in section
4, which is divided into physical absorption, chemical absorption and physical separation
methods.
To investigate which parameters that are of critical importance for the CO2removal
process a case study has been performed for small scale (10 MWth) and
large scale (100 MWth) SNG-process. Two CO2-removal techniques have been
included in the case study namely water scrubbing (physical absorption) and
amine scrubbing with MDEA/PZ (chemical absorption). These are described in
section 5 with a detailed discussions on the specific problems related to a gasifier
product gas and the results of the case study are reported in section 6. Section 7
summarizes the results for the entire project and section 8 is a summary of the
main conclusions drawn within the project.
12 Svenskt Gastekniskt Center AB, Malmö – www.sgc.se