Methods applicable to a great variety of solid biofuels
In recent years, systems have sprung up which use near infrared X-ray (NIR) spectroscopy, radio frequency (RF) and microwave spectroscopy as alternatives to the traditional method of measuring humidity by weight difference in the fuel burner.
What these methods have in common is that they can be used for a great variety of solid biofuels, even in very varied mixtures, and that they are based on the measurement of the amount of radiation absorbed by a sample during a wavelength interval.
The NIR method uses wavelengths between visible and infrared, and this provides a great deal of information about organic material. The RF method and microwave spectroscopy use radiophonic waves at a frequency similar to that of mobile phones and measure any delay or diminishing of the signal. X-rays use a higher-energy radiation to work out the atomic composition of the material.
Improving sampling precision
With these methods, on-line analysis can be carried out on the flow of material passing along the conveyor belt or by extracting a sample.
The method of taking samples to assess the humidity of a load using stove-drying carries with it significant errors in measurement as it is difficult to select samples which are really representative of large loads. For example, if a 0.5kg sample is taken from a load of 20 – 30 tonnes, the margin of error can exceed 10%, especially if samples are taken from the surface or near the surface.
Fluctuations in humidity can greatly alter the heat-generating capacity of biomass. For this reason, knowing as accurately as possible the real humidity content of biomass material is of the utmost importance. The Swedish Organisation for Energy Industry Research, Varmesforsk, along with the Swedish Energy Agency and the private company, Bestwood AB, have developed a prototype of the BAS-700 system: using a probe mounted on a crane connected to a spectrometer, biomass humidity is measured simultaneously at different points on a load, thus allowing a representative value to be calculated in real time and the data to be fed into the plant’s computer system. The system achieves a precision of 3% of units of humidity, a better result than manual sampling or the fuel-burner method. It also operates automatically, subject to supervision by the truck drivers, which allows measurements to be taken 24 hours a day.
The system feeds data into the boiler control system, and this regulates the supply of biomass in such a way that the net energy content of the biomass going into the boiler remains stable. It also makes it easier to control costs as this method avoids systematic differences between the measurement value and the real humidity value.
These systems are still very expensive to run and, for this reason, are used mostly in big generating plants. In the future, however, small plants will be able to acquire similar systems with a satisfactory performance. Automatic measuring systems will undoubtedly be part of the standard equipment of co-generation plants of a certain size - performance efficiency will become increasingly important and, in the longer term, it will be unthinkable not to have access to real-time fuel analysis.