The EBIPREP project is being carried out by an interdisciplinary research group consisting of chemists, process engineers, bioprocess engineers and physicists specialized in sensors and process control. The aim is to develop new solutions for the usage of woodchips and the wood juice obtained during the mechanical drying process. Besides the woodchip gasification and the catalytic cleaning of the wood gas, the use of the wood juice in biogas plants and in biotechnological production of valuable substances, e.g. in enzyme production, plays the dominant role.

In the following paragraphs, the individual sub processes are presented and the innovative character of the overall process (Fig. 1) is explained.

Central elements of the project are the products of mechanical wood drying process, the wood chips and the wood juice. During the pressing process, 200 to 250 liters of wood juice per ton of fresh wood chips are produced, which contains valuable minerals and nutrients. Until now, the wood juice had to be disposed with costs in a sewage plant.

In this project, the wood juice in the bio fermenter is to be used as a mineral-rich substrate and converted into yeast or enzymes as exemplary organic products.

Biofermentation

The biotechnological production of valuable substances is often without alternative regarding the complicated chemical structure of organic products. This is particularly true for the large-scale production of enzymes or for the yeast production.

In the laboratory and pilot plant scale, well-defined mineral salt media are used to cultivate microorganisms in order to achieve reproducible results. On a production scale, this is too expensive. Mostly so-called complex media are used here. These types of media are composed of various natural products such as malt extract, yeast extract, peptone, molasses or wood juice.

These media components usually have a sufficient amount of essential nutrients that are necessary for the growth of microorganisms. However, the increasing utilization of the complex media components in industrial scale is also associated with an increase in costs of these media components.

For this reason, the use of wood juice as a cultural media component could represent a meaningful utilization and improve the biotechnological production process technologically and economically.

Biogas process

Alternatively, the wood juice can be converted into biogas by using a waste material from the food industry in a biogas reactor. The reutilisation of sugar-containing food residues in biogas plants can lead to various problems. On the one hand, even at low substrate volume flows the high concentration of monosaccharide might lead to rapid acidification in the biogas reactor, so that the biogas process is irreversibly damaged. It is known that the blending of biologically more Cosubstrates, which are less biodegradable, delays a rapid acid formation and stabilizes the biogas process. This has to be checked for the wood juice as a Cosubstrate.

On the other hand, waste water often lacks important nutrients to achieve a complete methanisation of the carbon. The addition of the mineral-rich wood juice could also raise the biogas yield in this case.

In order to achieve a complete degradation of the food residues, very long residence times (usually 45 to 70 days) of the substrate in the fermenter should be expected, and thus also with appropriate biogas reactor sizes. Our own tests have shown that in most biogas substrates, approx. 80% of the biogas is produced in the first 3-4 days in batch tests. The complete turnover of the available carbon takes a considerably longer period of time (usually more than 30 days). in this context, it is of great interest to check within this project when it makes economic and technical sense to interrupt the biogas process prematurely. Non-converted substances that are difficult to degrade could then be converted into synthesis gas as biogas residues much more economically in the wood gasifier. As a result, the biogas containers could be dimensioned much smaller (with the same amount of food residues).

Gasification process

During gasification, by adding a gasification agent (usually air) an inhomogeneous solid fuel is converted into a combustible synthesis gas which contains the useful components carbon monoxide, hydrogen, methane and the inert gases carbon dioxide and nitrogen. In order to ensure trouble-free operation of the gasifier, the fuel must meet these requirements:

- The free spaces of the fuel bulk must fit to the surface of the fuel pieces, good results are achieved with edge lengths between 20 and 80 mm.

- The charcoal produced in the process must be mechanically stable so that the free spaces between the fuel pieces remain open for gas flow.

- The fuel humidity must be limited so that the heating value of the humid fuel reaches the optimum gasification temperature (at least 1200 °C).

The gasifier planned for this project is a co-current gasifier, which achieves high gas and biomass flow rates and therefore high energy densities even at small dimensions. In the test reactor at Offenburg University of Applied Sciences, a fuel input power of up to 60 kW is achieved with a reactor volume of approx. 200 litres.

The wood gasifier plays a central role in the overall process. All residual materials that are either no longer biologically usable (fermentation residues) or that are not economically viable (biogas residues) are thermally converted into synthesis gas together with the wood chips. This intelligent use of biomass leads to an additional, energetic use of existing waste materials.

Catalytic purification of the wood gas

The catalytic synthesis gas cleaning process represents an important sub-process for the low-pollutant utilization of biomass. In wood gasifiers, condensing tar in the subsequent energetic use of syngas causes problems such as clogged pipes, sensors or damaged internal combustion engines.

Although there excist various catalysts for tar reforming, but the high requirements such as effective tar removal by methane reforming, low deactivation of the catalyst (deposition / sintering processes), possibility of active phase regeneration, high mechanical strength and good thermal conductivity in combination with a marketable price currently is not realized.

For this reason, the development and testing of new catalyst materials is the subject of this project.

The importance of sensor based process analysis for the project idea

The overall process of biomass conversation to produce resources and biogas can only be realized by combination of the partial processes in an optimized way, if changes of the process parameter values are reliably monitored in time and immediate reaction on those changes is possible. In case of biogas production by fermentation it is important to know the actual status of dissolved acids. Knowing the actual kind and amount of acid (acetic, butanoic or  propionic acid) allows to model the current microbial state of the fermentation process. Those informations can be used to adapt the feeding procedure of the bio gas reactor and help to optimize the fermentation process.

For this purpose, for the first time it is intended to develop a membrane based gas carrier probe which uses a thermocyclically operated metal oxide sensor chip as the sensing element. This sensor system [11, 12] might enable in-situ analysis of different organic acids. For combination of the most suitable gas sensitive layers, different highly sensitive metal oxides have to be prepared and  sensor test-chips have to be fabricated with technologies available at Inst. of Sensor and Information Systems, Karlsruhe University of Applied Sciences.

In case of wood gasifying temporarily changing composition of the biomass fuel is known to be the reason for variations of the process quality. Such uncontrolled variations of the gasifying process may be responsible for increased tar content of the synthesis gas. If a continuous estimation of the tar content is available, this value could be used for feedback control of the gasifying process. This motivates development of a novel sensor system for online monitoring of tar in presence of high concentrations of synthesis gases, mainly methane and carbon monoxide.

Die Analyse der Umweltbelastung

Ein dauerhafter Einsatz und die gleichbleibende Funktionalität dieser neu zu entwickelnden Katalysatoren setzen die Kenntnisse der Inhaltsstoffe im Rohsynthesegas voraus. Die enthaltenen Partikel und Gaskomponenten müssen in Qualität und Quantität erfasst werden, um die Katalysatoren vor Überlastung und Vergiftung zu schützen. Zur Verstromung der beiden Produktgase in BHKW's müssen die Gase Reinheits- und Qualitätsanforderungen erfüllen, welche durch eine katalytische Behandlung erreicht werden können. Auch an dieser Stelle ist eine entsprechende Partikel- und Gasmesstechnik zu etablieren.