Do your wood chips vary enough to warrant a Flow Scanner?
The result from Flow Scanner installations in northern and central Europe is clear – wood chips on the belt have substantial moisture and dry density variations. These occur on short- and long-term time scales and have a real impact on the pulp process. In this article we share knowledge from a number of Flow Scanner installations and discuss how the information can be used to make considerable improvements to the fibre line.
Mixing it up
There is good cause to suspect that there are variations in the content of the wood chips. After all almost all pulp mills mix sawmill chips with their own chipped logs which have seasonal variations, forest thinning’s, and are often sourced from a wide geographic region. However chip mixing, silo and pile storage, and screening, may average out these differences. How much variation is there in the digester? Pulp mills try to measure and manage these variations.
The limitations of oven-dried samples
The traditional method for measuring moisture in wood chips is the gravimetric measurement of fresh and oven-dried samples. This is the only method accepted and trusted in the industry because it directly measures the amount of moisture. Almost all pulp mills can produce data from several years of lab tests. What does this show?
Lab tests show the water content (including other volatile components) in the sample with an error margin of around 1 %. The individual results have limited use. The small sample size simply cannot adequately represent the hundreds of tons flowing through the process at that time. Especially if there are rapid fluctuations so that a sample collected ten minuts after the first will show a sizeable difference.
The results are known some 12 – 24 hours later and applied to the material currently flowing through the process. It is hoped that there is no change in the wood chips during this delay. Measurement of wood density and bulk density of the wood chips on the belt is an even greater challenge with higher error and fewer data points available to analyse.
The results are best used to assess the average moisture content over the year, the seasonal variations, and perhaps some idea of the extent of monthly or weekly variations. Chemical pulp manufacturers would like to know if there are changes per hour and any trends over a few hours. Very short-term changes, occurring over seconds or minutes, have no meaning to the pulp process. Whilst long-term seasonal variations may be observed from the trend of several samples over an extended period, the hourly or daily changes could not be observed – until now.
The Flow Scanner provides true real-time moisture data
Mantex have been measuring and observing raw material variations for several years now with an increasing number of installed instruments.
The dual X-ray technology in the Flow Scanner measures the moisture content and wet mass flow of all the material passing on the conveyor belt. Samples taken are not used for digester control but are used to verify that the Flow scanner can accurately measure all the material – in real-time. The Flow scanner gives instantaneous data readings as well as hourly averages and can be used to know precisely what has entered the digester.
Some interesting facts we’ve learned
Flow Scanner data shows that material variations occur on all time-scales. Changes occurring over a few minutes have no significance for the digester process but an average increase of 2 % moisture over a few hours can be vital information. Daily variations of 3-4 % are normal for all sites and more than 5 % over a week is very common. Dry mass flow variations of 5-10 % over a 24-hour period are theoretically enough to account for most of the Kappa variations observed at pulp mills!
Graph 1 (click for larger image) Hourly average moisture values from the Flow Scanner with validation samples. Short term variations of the average moisture content by 3-4% over a week. Note the reference samples (black triangles) used to verify the Flow Scanner. These are collected over a short period (10 mins) and compared to the Flow Scanner over same period. Hence there may be a difference to the hourly average value shown here.
Graph 2 (click for larger image) Average hourly variations in dry mass flow. The scale shows the relative change with round 5-10% short term variations shown. Moisture is not the only factor affecting the amount of dry wood content in the digester. Changes in wood density and bulk density also contribute to the fluctuations. The average hourly dry mass flow is shown and is used to calculate the exact amount of dry wood that is transported to the digester..
Graph 3 (click for larger image) Average moisture variation over 48 hours. This plot shows the danger in using wood chip samples for digester control. During the first 8 hours the hourly average drops sharply giving very different results for samples taken after 1 hour and those taken after 8 hours. Any sample will also be subject to short term variance and have an associated error making it even less representative. Day two shows greater stability with a rising trend. Again very short term variations mean that the samples would not show this. In fact daily samples may not show any difference between day 1 and day 2 – with the pulp process much the worse for it.
Graph 4 (click for larger image) Hourly average moisture variations over extended period with trend line. Each orange dot is an average of measurement data over the previous hour. Shows that 3-4 % change in the average moisture content occurs over a short time period (1 day) and such variations are common over long periods. The trend line shows that a weekly averaging misses the short term (daily) variations.
What this means for a European chemical pulp mill
Changes in moisture and bulk density lead to an imbalance in the amount of white liquor and the wood-to-liquor ratio such that the pulp quality is in constant change. The economic value of being able to account for these variations and to improve the Kappa stability will depend on the individual mill. For almost all European mills the payback on an investment for a Flow Scanner is usually within 12 months.
If you would like to know more, and take advantage of our current no cost pre-study, please contact Mantex.