Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
5 Models
In this section, the system model will be described along with the gasification
models and the two methods for CO2 capture that has been chosen for modeling
(water scrubbing and amine absorption using N-methyldiethanolamine (MDEA)
and piperazine (PZ)). The choice was based on availability of models, maturity of
technology and aim of the project. As the project focused on indirect gasification
the models for the MILENA gasifier was chosen to represent the 100 MW th case
and the Agnion gasifier was chosen to represent the 10 MW th case. These also
include the models for methanation and other necessary process steps. Other
means of CO2 capture, such as membranes, are becoming more commercially
available [47]. However, no model for this type of system was available within the
project and it was therefore decided not to include it.
The modeling work has been performed in an iterative fashion working with several
different models. The cases simulated will be described more in detail in the
next chapter. However, two general concepts have been investigated and those
are pre-methanation and post-methanation CO2 removal. In figure 18 the different
iterative simulation procedures are described. These two separation modes have
been investigated in a case where larger scale production has been investigated
(100 MWth input biomass); a smaller case (10 MW th input) has also been considered,
but only for post-methanation CO2 removal. The larger scale has been
based on the MILENA gasifier and the smaller one on a scaled-up version of the
Agnion gasifier.
Model
Integration
Simulation
of
Gasification
Simulation of
CO 2
separation
Figure 18. The order of simulation in the case of pre-methanation CO 2 removal
and post-methanation CO2 removal respectively.
5.1 System model
The system model is made in Aspen PLUS and describes the unit operations necessary
for conversion of biomass to SNG. No gasification model is present, instead
the gasification is calculated by mass and energy balances based on real
gas composition and energy requirement of the MILENA gasifier. The energy required
for drying and grinding is included in the model, as is the steam requirement
of the gasifier. Downstream the gasifier is a desulphurization step followed
by a pre-reformer that converts lower hydrocarbons and tars into synthesis gas
and methane. The next step is a water-gas shift reactor that sets the H 2:CO ratio
of the synthesis gas to 3:1. Depending on where the CO 2 separation is placed
there are a few different scenarios available downstream of the water-gas shift.
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