Recycling and valorisation of CO2 in fluidized bed process for steam gasification of biomass – RECO2

The approach was to conduct this project in three complementary phases with (i) firstly experimental and scientific work, (ii) secondly a synthesis of this experimental work allowing an assessment of the process at the industrial scale and (iii) eventually a techno-economic assessment of CO2 recycling in a process of biomass conversion by gasification in fluidized bed. The three parts included:
(i) Experimental studies at biomass particles scale to investigate the influence of carbon dioxide addition into the atmosphere during biomass pyrolysis and char gasification.
To study the same phenomena in a fluidized bed gasifier with a constant biomass flow. The size and flow increase allows an analysis of the global effect of pyrolysis and gasification at the gasifier scale.
(ii) The synthesis of experimental results from particle size studies, then at the gasifier scale in order to assess the applicable operation at industrial scale.
(iii)A techno-economic assessment of the gasification process with a partial or total CO2 recycling into the gasifier. This assessment will be done for SNG and BtL fuels.

The main results concerning the influence of CO2 are the following:
Pyrolysis :
- Changes in char characteristics: Higher surface area of the char produced in presence of CO2; for centimetric particles, influence of the atmosphere on inorganic species release.
- Gas yields: apparent yields largely influenced by water-gas shift reaction ; Higher CH4 yield.
Gasification :
- Additivity law for reactivities (Steam gasification + CO2 gasification) whatever pyrolysis conditions and particle size.
At gasifier scale :
- Increase of the carbon conversion rate into CO with CO2 injection,
- Stable char yield whatever the CO2 recycling level.
- No significant influence on the tar yield
- The decrease of H2/CO ratio is a disadvantage for the SNG and BtL chains but it should be an
advantage for DME direct synthesis which requires a H2/CO ratio of 1.
For SNG and BtL chains:
- The CO2 recycling increases the carbon yield into biofuel only when addition of external
hydrogen, to reach the appropriate H2/CO ratio for bio-fuel synthesis, is considered. However the hydrogen production by water electrolysis penalizes the production cost of SNG and biodiesel.
- The association of the biomass to biofuel chain and of the Power to Gas approach allows a reduction of energy storage cost of intermittent renewable energies.

The project results show the influence of the pyrolysis atmosphere on structure, inorganic content and reactivity of the char. The char yield is also affected. What are the mechanisms? Comprehensive studies are still needed.
The additivity law in gasification deserves to be investigated and to be correlated to the char structure.
The CO2 recycling to the gasifier with the storage of intermittent renewable energy should be investigated in a collaborative project.

During the project two publications were accepted in the 'Fuel' scientific journal on pyrolysis and gasification under CO2 atmosphere. Three oral communications in international workshops and conferences were focused on the link between scientific results and their impacts (i) at the gasification
process scale but also (ii) on techno-economic for the SNG and BtL chains. Finally, the CEA filed a patent on the improvement of the yields of the gasification with a CO2 recycling process. Its international expansion is underway.

This project deals with CO2 recycling and valorisation in a lignocellulosic biomass steam gasification process performed in a fluidised bed, with the aim of producing gaseous fuel (bioSNG: Synthetic Natural Gas) or liquid fuel (with Fischer-Tropsch synthesis).
In this kind of process, the gas directly coming from biomass thermochemical conversion contains CO2. Moreover, in order to produce fuel, it is generally necessary to perform a water-gas shift step in order to adjust the H2/CO ratio, which leads to an additional CO2 production.
This project aims at studying recycling of a part of this CO2 by reintroducing it into the gasifier, either instead of neutral gas with biomass feeding, or to replace a part of the steam in the gasifier itself, CO2 then acting as a gasifying agent.

Some experimental studies have shown that biomass pyrolysis and gasification take place differently in a mixture of H2O and CO2 and in a mixture of H2O and N2. The literature review shows that data are still missing and that mechanisms are not well understood. So, the present project aims first at studying and understanding mechanisms which govern biomass pyrolysis and gasification in a mixture H2O+CO2, and secondly at evaluating the possibilities and advantages of recycling the CO2 produced in a double fluidised bed process, from both technico-economical and environmental points of view.

Firstly, analytical studies will be led at the particle scale to isolate the pyrolysis and gasification mechanisms and so to better understand the phenomena involved. Parameters will be temperature and composition of the gaseous atmosphere around the particle. Several complementary installations implemented at the CEA and RAPSODEE will be used.
In a second stage, the same mechanisms will be studied in a fluidised bed continuously fed with biomass. In this installation, phenomena are not isolated and can thus interfere. All the results obtained at the particle scale and at the small pilot gasifier scale will be synthesized, so as to show the progress made in reaction mechanisms and key parameters comprehension.

After this important experimental step, these results will be analyzed and transposed so as to be used in technico-economical simulation tools for industrial processes evaluation. Biomass gasification in a double fluidised bed process for the production of SNG and of Fischer-Tropsch diesel will be simulated. For each of both applications, the reference case without CO2 recycling will be compared to several options of CO2 recycling.
The last step of the project consists in an environmental evaluation of the process and of its alternatives according to both studied applications. This approach will again be focused on environmental advantages and drawbacks of the different options of CO2 recycling. It will end up with a life cycle analysis of the studied processes.