The development of process control for biogas

Partech instruments for monitoring pH, Redox, conductivity and temperature are performing a major function for attaining biogas process control at North Wyke Research, in Okehampton, Devon

Biogas process control development Biogas is a not a new technology, having been used in developing countries for many years.

However, the process has never been commercially optimised in order to make it more cost-effective and it remains under-used in the UK.

Biogas production on farms in the UK has had a chequered history and there is only one centralised anaerobic digestion system currently in operation, this being at Andigest, situated at Holsworthy in Devon.

Here two tanks each of 4,000m are producing biogas, which is burnt in generators with the resulting electricity being put into the national grid.

Biogas is a high quality energy source derived from manure and other organic resources that can be used for power production, as well as for heat.

As a consequence biogas production can minimise organic material entering landfill.

Biogas is produced by the anaerobic digestion of many different sources, including sewage, livestock manure, food and abattoir waste and energy crops.

The process is un-optimised because to date there has been an absence of suitable monitoring and control systems.

However, at North Wyke Research, in Okehampton, Devon, a long-term project is setting out to address the technical problems, such as poor fermentation, by better process control, and work is taking place on the development of a process control or early warning system for process failure.

It is hoped that this work will eventually result in a commercial proposition for the biogas industry.

North Wyke Research is well positioned to address the use of manure sources and other organic residues for biogas production, as well as being able to investigate the remaining digestate, having the potential to be a high quality liquid fertiliser.

Monitoring specific chemical changes in the fermentation tanks is a critical part of the process, and Partech Instruments is participating in the project through the provision of on-line monitoring instrumentation and process control systems for the pilot plant.

According to Dr Phil Hobbs at North Wyke Research, process optimisation can be achieved by maintaining a carbon to nitrogen ratio of 25:1; co-digestion or mixing feedstocks; and pH and temperature control.

When energy crops are used, the aim is to obtain maximum methane production from a minimum input.

The scenario for waste treatment is to attain a maximum organic loading rate while maintaining a stable process.

North Wyke Research has built a pilot scale anaerobic digester at its site, consisting of two tanks: a 1m hydrolysis tank and a 1.5m methanogenesis tank for producing methane.

The hydrolysis tank contains a number of Partech probes dipped in the feedstock to measure pH, Redox, conductivity and temperature.

The methanogenesis tank is fitted with a multi-parameter Partech Sonde measuring pH, Redox, conductivity and temperature.

Readings are fed into the adjacent process control cabin where a monitor continuously scrolls through the four parameters being measured by the probes and downloading data into the computer operating the LabView software program.

There are clear benefits to using a single instrument for multi-parameter analysis and the Partech WaterWatch 2610 is ideally suited to the North Wyke Research pilot plant.

The choice of either a combined flow cell or a dip type sonde makes installation and maintenance extremely easy and keeps costs down.

The flow cell used has a single connection to the sample system and the inclusion of a sample presence detector ensures that all the data provided is valid.

The flow cell configuration enables measurements to be taken where dip sensors are neither feasible nor desirable.

Partech has recognised that users' requirements will vary according to the application so the ability to customise units is a feature of the WaterWatch 2610.

Standard parameters can be combined in a wide variety of ways and additional parameters can be accommodated using either serial data communications or an interface for 4-20mA-based sensors.

The monitor will also interface with sample handling and preparation equipment such as pumps and filters to ensure reliable measurements over an extended period of time.

Using LabView software, it is possible to calculate and predict the alkalinity level, which is mostly due to carbonates or hydrogen carbonates, particularly in the methanogenesis tank.

The software can then make decisions about managing and controlling the feed rate from the slurry feed tanks into the digester tanks.

If the methanogenesis tank is overloaded, the tank can become too acidic.

Overfeeding of solids produces to much volatile fatty acid, which prevent methanogenic bacteria producing methane.

The hydraulic or liquid retention time in the tanks is around 20 days, but this depends on a steady alkalinity measurement which is used for process control.

Should this increase, then the software will automatically reduce the loading rate.

Measuring the alkalinity parameters has been determined as being central to the effectiveness of the process control strategy.

At the North Wyke plant alkalinity was measured in real-time as a softsensor, but the new work being undertaken uses alternative feedstocks so different levels of alkalinity are possible.

This means that it will be necessary to rebuild the calibration model to predict the alkalinity correctly.

There is a clear link between the alkalinity and the methane produced, so by controlling the alkalinity it is possible to control the methane percentage of the biogas.

The higher the methane percentage, the better the fuel quality and the better the energy obtained.

Partech instruments for monitoring pH, Redox, conductivity and temperature will perform a major function in attaining this goal.

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