Production of bio-methane by gasification: cost reduction options
Bram van der Drift, Luc Rabou, Berend Vreugdenhil, Christiaan van der Meijden
ECN, P.O. Box 1, 1755 ZG Petten, The Netherlands
Background
Bio-methane, also called green gas and renewable natural gas and substitute/synthetic natural
gas (SNG), can be made by gasification of woody feedstock. The process consists of a thermal
gasification step at temperatures of 700°C or higher. The raw gas subsequently is cleaned and
upgraded to essentially pure methane. Contrary to coal-based processes that produce SNG,
biomass-based processes produce a renewable SNG (bioSNG). Not only the CO2 footprint is
very low (or even better than “low”), biomass-based processes also offer several other specific
benefits.
Since biomass is much more reactive than coal, gasification processes can be operated at
temperatures as low as 700°C, whereas coal needs much higher temperatures to be converted.
This results in a direct efficiency gain. Additionally, the raw gas already contains high amounts
of methane due to the low gasification temperature when operating on biomass. This gives an
additional efficiency gain. The total energy efficiency from biomass-to-SNG can therefore reach
values of as high as 70%, whereas coal-to-SNG do not go beyond 60%.
Another benefit of low temperatures of biomass gasification, is the interesting spectrum of
valuable hydrocarbon molecules. Next to methane, high quantities of ethylene and benzene are
produced. Ethylene and benzene together may comprise as much as 25% of the energy value
of the raw gas. Simultaneously, these compounds needs special reactors and high amounts of
steam to be converted into methane without coking issues in the nickel-based catalysts.
Separation of ethylene and benzene in a biomass-to-SNG process therefore makes much
sense: more valuable products are produced and the overall process becomes more efficient.
Aims
ECN has been developing concepts to produce bioSNG through gasification. A complete
process is operating on small-scale in the ECN labs. On the basis of this, a demonstration plant
will be constructed near ECN. The business case of this plant relies on a feed-in tariff for the
bioSNG produced. The aim of further ECN innovations is to reduce production costs of bioSNG
to make it competitive to the alternatives. This certainly involves up-scaling, but innovations are
in the area of co-producing chemical components of high value.
Methods
ECN performed experimental studies to find ways to co-produce olefins (mainly ethylene) and
aromatics (e.g. benzene) in a biomass-to-SNG process as a way to reduce the production costs
of bioSNG. The economic benefits of the co-production schemes have been quantified.
Results
The lab-scale biomass-to-SNG facility at ECN involves a sequence of unit operations to clean
and upgrade the gas to bioSNG. Downstream gasification, this essentially comes down to
cleaning the gas from contaminants (particles, sulphur, chlorine, tars, …) and converting the
rest to methane. This certainly involves the conversion of H2 and CO to methane, but also all
hydrocarbons (ethylene, benzene, …) are converted into methane.
In our quest to find ways to reduce bioSNG production costs, the facility has been equipped with
an extra scrubbing system to remove one-ring aromatics from the gas. First tests prove to be so
successful, that additional developments were started to increase the yield of aromatics to
increase the economic benefits. This involved ways to convert olefins to aromatics in the
downstream process. Also operational conditions in the gasifier were varied to increase the
initial yield. Up to a three-fold increase can be reported on the yield of BTX (Benzene, Toluene,
Xylene). The overall benefit could be translated to approx. 3 euro/GJ reduction of the bioSNG
production costs.
Gasifier
Tar
removal
HDS
S
removal
Prereformer
CO2
removal
MethaMethanation
Methanation
nation
Upgrading
Simplified biomass-to-SNG process base case including the removal of organic sulphur (HDS:
HydroDeSulphurization) and aromatics (pre-reformer); everything is converted to methane
Gasifier
Tar
removal
BTX
Hydrogenation
S and CO2
removal
MethaMethanation
Methanation
nation
Upgrading
The example of a biomass-to-SNG process where Benzene/Toluene/Xylene (BTX) is separated
as co-products to improve the business case
Further
Methanation
The complete biomass-to-SNG facility at ECN in the Netherlands
thatgas
is used to develop
and
MILENA
gasifier
OLGA
tar
removal
HDS
reactor
cleaning
reactors
prove the process and to test the options to reduce costs
1 bar
1 bar
6 bar
6 bar
6 bar
Apart from the co-production options to reduce the production costs of bioSNG, the paper will
shortly touch upon several other methods to reduce costs. This includes the use of the CO2 byproduct, the combination with Power-to-Gas, and bioCCS.