Grass for Power Generation

Grass for Power Generation
Johanes Judex, PSI April 2010

Hello everybody,

I have learned from this gasification network since three years and I learned a lot.
Thank you all, for sharing much information.

Most of the time I stayed in the background but now I want to post the link to my PhD thesis I did at the ETH Zuerich and Paul Scherrer Institut in Biomass Gasification.

I am not a native english writer so you will excuse the formal mistakes I made. Anyway I did my best to present:

Grass for power generation -- extending the fuel flexibility for IGCC power plants

The outline of the work is as follows:

The fuel: natural native grass (extensive), wood
The gasifiers: bubbling fluidised bed (5-10kW) & single pellet reactor
The sampling: continuous wet scrubber
The diagnostik: GC, GC/MS, ICP-OES, TC, Karl-Fischer, Draeger tubes, SID
The parameters: Lambda, temperature, bed material
The goal: Stable operation, product gas clean enough for gas turbine applications
The result: you have to read it and decide for yourself

Well, I hope there is something for everybody in this work.



The situation of the current electrical power production is characterised by a trend to use more and more alternative energies and the fact that the European power plant eet is ageing. Since the electrical consumption is expected to increase further, old power plants which are to be switched need to be replaced. There are two main candidates of large power plant types being discussed:
- natural gas combined cycle plants
- nuclear power plants

Nuclear power plants are not suited to implement alternative energies, whereas combined cycle power plants can be equipped with a biomass gasifier. In this way an IGCC power plant is designed, which can fully or partially run on biomass. There is also a need to employ additional
biomass feed stock types like annual plants. In this work grass is studied as additional energy resource for IGCC applications. Being able to use grass as supplementary feed stock would increase the fuel flexibility of IGCC power plants.

- A review of the chemical and physical properties of grass as solid fuel was carried out. Grass contains a very large number of different inorganic elements mostly in minor amounts. It revealed
that in addition to the elements causing corrosion in gas turbines at least Sr and Ba are found in grass. Both show similar corrosiveness to the known alkalis K and Na. Earth alkali elements (Mg, Ca) also being known for depositions and corrosion enhancement, are present in critical amounts in the fuel. It is unsure whether the positive effects (binding sulphur or vanadium) or the negative effects (deposition, surface damage) will prevail. The generally low energy density can be avoided by on-field densification or immediate pyrolysis. The water content can be expected to be lower than for all other biomass owing to good drying properties and the possibility of on-field drying.

- For the integration into IGCC power plants the grass must be thermally gasified. A lab-scale fluidised bed gasification reactor was designed and put into operation. Air blown gasification experiments were conducted with grass and thereby the stability of the process and emission of contaminants, relevant for gas turbine applications, was analysed (i. e. K, Na, Mg, Ca, Pb, V).

Gasification experiments were carried out between 700 and 750 C for fuel to air ratios between 0.17 and 0.35. Different bed materials (dolomite, SiO2, Al2O3) were tested: dolomite was excluded after first trials, because it appeared unsuitable for fluidized bed applications due to its mechanical weakness. Silica sand was used for 700 deg C only since the ash melting behaviour of grass together with silica indicated agglomeration above 700 deg C. The experiments at this temperature were conducted without bed agglomeration or defluidisation. Alumina bed material was used for 700 and 750 C, since the ash melting behaviour of grass together with alumina proved to be less critical at higher temperatures. A 10 h run was conducted to see if unstable situations in the gasification process can occur. All experiments succeeded without any de-fluidisation or agglomeration. Axial temperature profile measurements proved an isothermal behaviour. The axial devolution of the superficial gas velocity is evident but still moderate.
Despite the low temperature of 700 to 750 C, the tar concentration is lower than for fluidised bed wood gasification given in literature.

- The contaminants in the gas phase where analysed by means of a dedicated sampling train and an ICP-OES device. It was shown that by applying a hot gas filter at 400 C all the contaminant
concentrations aside from potassium and sodium were low enough to satisfy the gas turbine limits. Sodium was found in concentrations higher than the allowed limit for mono-fuel applications.
The concentrations are low enough if a cofiring approach with natural gas is targeted. Potassium could not be quantified at the given concentration level due to the insensitive response of the
ICP-OES to potassium. Secondary measures other than hot gas clean up to reduce contaminants were not employed. A novel single pellet gasifier was designed and commissioned in order to investigate the transient gasification of a single pellet.

The reactor showed similar characteristic as the fluidised bed with respect to gas devolution, fuel size and heating rate of the fuel. The experiments proved to be fully reproducible. By means
of a surface ionisation detector, the alkali emission for different grass pellets was monitored qualitatively. The results revealed that around 90% of the alkalis are emitted during the combustion phase of the char. The remaining 10% are released due to the volatilisation during the pyrolysis and gasificaition. Leaching and doping with potassium and sodium of the grass influenced the emission during the combustion phase much stronger than during the volatilisation.

The implementation of a hot gas filter at 400 C resulted in a heavily reduced alkali emission. The results indicated, that the passing alkali fraction is released during the pyrolysis, whereas the larger fraction emitted from the char combustion could be separated.

Grass is by any means suitable to be gasified in a fluidised bed gasifier. Hot gas filtration at 400 C is sufficient if the cofiring concept is followed. However, secondary measures are required to further reduce the sodium concentration if the mono-fuel concept is chosen.