Introduction
Process
Fixed-bed
Fluidized-bed
Status of the technology
Economics
Links
Leaflet
Movie
Contact

Introduction
Generally, biomass gasification is a thermal conversion technology where a solid fuel is converted into a combustible gas. A limited supply of oxygen, air, steam or a combination serves as the oxidizing agent. The product gas mainly consists of carbon monoxide, carbon dioxide, hydrogen, methane, water, nitrogen, but also contaminants like e.g. small char particles, ash and tars. After cleaning the gas makes is suitable for boiler, engine use, and turbine use to produce heat and power (CHP).

Short process description
Gasifiers are already investigated for more than a century, and many different types have been developed. The figure below summarizes the main types and their typical operating window.




For CHP applications in the small to medium size BTG applies the co-current fixed bed, and the fluidized bed gasifier. In both cases air is used as the oxidant, and the gasifier is operated at ambient pressure. The next table presents typical gas composition data as obtained from wood and charcoal co-current gasifiers operated on low to medium moisture content fuels (wood 20%, charcoal 7%).

Component	Wood gas (vol%)	Charcoal gas (vol%)
Nitrogen	50 - 54	55 - 65
Carbon monoxide	17 - 22	28 - 32
Carbon dioxide	9 - 15	1 - 3
Hydrogen	12 - 20	4 - 10
Methane	2 - 3	0 - 2
Heating value (MJ/Nm3)	5 - 5.9	4.5 - 5.6

For synthesis gas production BTG considers the pressurized, oxygen-blown entrained flow gasifier fed with bio-oil (see bio-oil application ).

Gas conditioning
Dependent on the application, type of gasifier and contaminants in the fuel, a certain level of gas conditioning (cleaning/cooling) is required. The most frequent impurities are hydrocarbons (tar), dust (particulates), ammonia, sulphur, chloride, alkalies, etc. which need to be removed or converted. Dust is usually removed by cyclones and fabric filters. Ammonia, sulphur and chloride can be removed by scrubbers or by using additives. The most critical component to be handled however is tar (see tar removal ).Cooling is required for (i) combustion in gas engines, (ii) when filters are applied with a maximum allowable temperature or (iii) when compressors are incorporated like with atmospheric IGCC

Electricity production
For biomass-CHP gas engines, (micro-) turbines, and in the future possibly fuel cells can be applied as the prime mover. So far, BTG mainly possess operational experience with gas engine application.

Fixed-bed
The different fixed-bed reactor types are often characterised by the direction of the gasflow through the reactor (upward, downward or horizontal) or by the direction of respectively the solid flow and the gas stream (co-current, counter-current or cross-current). For specific feedstocks a co-current gasifier is used with the advantage that the tar content in the producer gas is low. Additional gas cleaning -prior to fuelling a prime mover- is avoided. Obviously this will reduce the investment and operational costs.A .. KWel demonstration plant fuelled with waste is in preparation.

Fluidized-bed
In a fluidized bed gasifier air and biomass are mixed up in a hot bed of solid material (e.g. sand). Due to the intense mixing the different zones -drying, pyrolysis, oxidation, reduction- can not be distinguished; the temperature is uniform throughout the bed. Contrary to fixed bed gasifiers the air-biomass ratio can be changed, and as a result the bed temperature can be controlled. The producer gas will always contain certain amounts of tar, which need to be removed (see tar removal ).

In BTG's laboratory a complete "biomass-to-electricity" chain is available for testing. This chain includes the biomass feeding, a fluidized bed gasifier, catalytic tar removal, gas cooling and gas engine+generator. Maximum capacity is about 25 kg of biomass per hour (100 - 150 kWth). A large number of different feedstocks have been tested in this installation as e.g. wood, energy crops and dried chickens manure.







Currently, a demonstration plant is erected based on this technology. In that particular case dried chicken manure is used as the feedstock. The capacity is about 60 kWel.

Status of the technology
The technology is close to commercialization and therefore BTG has informed the international community in detail about the status for many years, i.e. about the current installations and the current manufacturers. Details can be found on www.gasifiers.org . Over 90 installations and over 60 manufacturers are listed now indicating the large interest in biomass gasification.
Despite many R&D efforts for the last decades, commercial status is still not achieved for several technical and non-technical reasons. To promote the technology in general and to contribute to the Kyoto protocol, BTG initiated a European wide Network on Gasification, GasNet, in which 20 members from all European countries participate. Information on different aspects are exchanged and distributed through internet at www.gasnet.uk.net and bi-annual newsletters. GasNet co-operates with PyNe (Pyrolysis Network) and the IEA Bioenergy Agreement, Task 33 on Thermal Conversion.

Applications
The fixed bed and fluidized bed gasifiers applied by BTG are mainly meant for the small and medium size. Fixed bed gasifiers are used in case of a well defined feedstock; fluidized beds are more tolerant with respect to the feedstock.
The simplest application is the single production of heat for e.g. district heating. For CHP applications BTG used -so far- gas engines, but on the short-to medium term the use of (micro-) turbines might become interesting, and in the long term also fuel cells.
A further interesting addition to the CHP will be cooling e.g.the application of ad- or absorption coolers.

Economics
Several economic studies have been made on biomass gasification regarding the feasibility and long-term prospects. The first demonstration projects are mostly far too expensive to become profitable. Investment figures of more than 5,000 €/kW electric are not exceptional. However, it is expected that due to the learning curve, the investment costs can be reduced to approximately 2,000 €/kW electric within the coming decade. Operational experience and value engineering is needed to achieve this goal.
Another aspect is the operational costs, in particularly the price of the feedstock. These can be expensive like short rotation coppice (SRC) or cheap (negative) like waste residues. Transportation, fuel handling and processing adds to the cost of the feedstock. Furthermore, labour costs must be minimised through process control and automation. Practical experience is needed to determine the maintenance costs. Remuneration of electricity and heat can also be decisive in the overall economics.
For the short to medium term, biomass gasification can not compete with fossil fuel produced power. Therefore, comparison must be made to alternative renewable energy sources. Studies showed that biomass gasification can compete with other RES when capital costs can be reduced and favourable conditions are created. Both conditions are likely to happen.

Links
www.gasifiers.org
www.gasnet.uk.net
www.tarweb.net
www.woodgas.com
www.thermalnet.co.uk

Leaflet
Leaflet chicken litter gasification

Movie
Introduction
movie

Contact
Ir. H.A.M. Knoef
Tel +31 53 486 11 90

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