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MEASURING the Moisture percentage and the Energy Content of Biomass

For many applications it is very important to be able to know the Moisture Content of the processed Biomass.

One of those applications is measuring in the feed to the Boiler of a Biomass fed Power Plant. Many of those Power Plants are fed with a variety of feedstock. And even if the feed is always the same type of (woody) biomass, there can be big variations in the size of the chips and their moisture content.

Changing Moisture percentages lead to a changing Calorific Values. And thus lead those changes to a change in the combustion properties of that Biomass. If the Moisture Percentage goes up, the ratio between Solid mass - which attributes to the Energy content - and the water - which consumes the available energy for its heating and evaporation will change.

Moisturemeter II for measuring Biomass on a Conveyor Belt

So if the Fuel Value is changing, you have to adjust the combustion process in the boiler.

Nowadays, in almost all installations, the Fuel value is determined in an on-site laboratory. For doing that, samples are taken from the storage, and they are pre-processed and measured.

This time consuming procedure has several disadvantages: It takes man-hours to do it, the result will only be available after a too long time and of course the result of that measurement do not necessarily represent the Calorific value of the product actual fed to the boiler.

The last two disadvantages make that the results of the measurements acquired through such an offline method cannot be used for Process Control.

The only remaining way to react to changes in combustion properties is by evaluating the exhaust gases. But in that way you only can react to changes which took place in the "past", which probably already caused elevated levels of NOx en CO in the exhaust gases.

Apart from that, once you measure those changed emission levels, that is the result of a less than optimal combustion. And hence    A LOWER EFFICIENCY.

Most types of biomass are inhomogeneous and very compressible products. The distribution of the water in the product is far from even. That makes it very hard and time consuming to prepare a representative sample of the product.

For getting high quality measuring results, you have to fulfill the following requirements:

1. Making sure that the product that you measure is representative for the stream of the product.
2. Having a measuring Volume that is much bigger than the biggest structure in your product.
3. Having a very good Bulk Density compensation to compensate for changing compression and flow properties.

   We developed two types of solutions which can meet those requirements:

Our measuring equipment, the Moisturemeter I , fulfills all of these requirements.

The output of this sampling and measuring system is a moisture percentage. Together with a beltweigher that percentage can be used to "separate" the stream of product in a "stream of solids" and a "stream of water".

Moisturemeter I: Sampling and measuring system
Ad1: For getting a good representative sample out of the product stream, we use our Densimeter. This apparatus was developed back in the year 2000, and since then widely spread over Western Europe for measuring the Bulk Density of peat, according to the then new developed standard EN12580 for measuring that Bulk Density for trading purposes.

Ad2: Our measuring cylinder has a volume of 20 liters, required by that previous mentioned standard.

Ad3: Because the Moisturemeter I is built as an option on top of our Densimeter, we have perfect Bulk Density compensation for free!. Basically the machine is a Densimeter.

Back in 2005 we developed a high frequency Moisture measuring system being able to measure a thick shell of product (by then: Peat) on the inside of that cylinder. The high frequency system covers most of the wall of the cylinder, and has some 8 centimeters intrusion depth. So the covered volume is much bigger than the very fibrous and rough structure of the peat to be measured.

As a leading supplier of measuring equipment for measuring bulk density and moisture content of rough inhomogeneous bulk products, we continue developing to comply with market demands. Because we develop and produce ourselves, we are also able to adapt our equipment to your needs.

The above-mentioned solution is a fully automated system. No human involvement, no forgetting to take samples any more. Just giving you the requested results.

Our system provides you with not only the Moisture percentage, but also the absolute values of Moisture content and Bulk Density in grams per liter.
In our basic version, we need about one minute to fill the measuring cylinder. The measurement itself only takes some seconds. So basically you can have a new measuring result every one minute.

Measuring systems
There are many ways of determining the moisture content of biomass. However, all of those known methods have their own disadvantages.

Nowadays, the most common way of determining the moisture contents, is to take a sample from the product, put that in a stove, and determine the decrease in weight after having heated this sample for several hours, in some cases even 24 hours.
The main disadvantages of this method are, that you only get the result many hours after production, and that it is very labor-intensive. In most cases the product has already left the premises or is already processed by the time the results of the test becomes available. So there is less possibility to use the result for quality control, or for preventing wrong products being delivered to your customer. Because the volume of the sample used in this method is relatively small, this is in mostly cases not representative for the total batch. When producing a product which contains different sorts of raw materials, and probably are wetted also, field tests have shown that such a small sample certainly does not represent the production. 

There exists many measuring methods which can be operated on-line. The best known are the following two:

The most commonly used measuring method, Near Infra Red - shortly: NIR -, in many cases gives a very poor result. This is because this method measures only the surface of the product stream, and the measuring result are very vulnerable to changes in color, temperature and structure of the product. Furthermore the dust that is always present in excess in such processes disturbs the measurement. But the biggest issue is that the outside of the product is not representative for the internal moisture. That can happen because the product is not homogeneous, but also because the outside dries when transported on the belt, and it is more moist when it has rained.

The other known measuring system, the Microwave sensor, has the advantage compared to Near Infra Red, that is does not only measure the outside, but it measures a volume inside the product flow. However, this method cannot be used because the bulk density of the product changes in many cases from one moment to the other. Some Microwave sensors do have bulk density compensation but they determine the bulk density by analyze the high frequency signal instead of weighing the product divided by the volume. Furthermore the measured volume is way too small compared to the structure of the product.

Vochtmeten ongecomprimeerd

Situation A: result = 5

Vochtmeten gecomprimeerd

Situation B: result = 8

Blue areaMeasuring sensor (in this example a Microwave sensor)
Green areaMeasuring area
Brown areaProduct
White areaAir
H2O (blue)Water particle

Every sensor has its own measuring area (the green area).
In situation A a standard microwave sensor is measuring in an uncompressed product. The sensor determines 5 water particles.
In situation B the volume of the product used in situation A is now compressed. The bulk density of this product is increased (same weight, less volume). The same sensor with the same measuring area is now measuring 8 water particles.

All the enumerated previous problems in measuring the moisture content of your product, was for us the reason to develop a system that does not have any of those before described disadvantages.

THE INADCO MOISTUREMETER I compared to other measuring techniques

We already had a broad experience in the potting soil production, because of our Densimeter, our Belt Weighers and our Quantity Checker. Especially for inhomogeneous and fibrous products like biomass, there are many challenges to overcome when you want to measure the moisture content of these inline. When measuring woodchips, straw fibers or other compressible products with rough structures, the same issues as mentioned for measuring peat have to be solved, because they are compressible, have changing surfaces and sizes, and their colour changes from time to time.

After investigating all systems available on the market, it became clear that there was no commercial system available that could fit all of our requirements. Having ended this survey, we decided that we had to develop one ourselves!!.

The technical requirements we had by then were that our Moisturemeter should:

Moisturesensors. For determing the moisturecontent in rough and inhomogeneus and homogeneus bulkstreams.
New moisture sensors.
For use in the field

The result is the INADCO Moisturemeter I as an option built on the INADCO Densimeter. We measure a big shell of product inside the measuring cylinder. So our measured volume is very big compared to the structure of the product. And we have a perfect Bulk density compensation, because the system is built on top of the Densimeter. So our Moisturemeter is not measuring only the surface, but it measures also far inside the product. Influences from drying through the flow on the belt, or rain which wetted the outside, have no disturbing influence on the measurement. With its high frequency measuring system it measures the Moisture content of the product in grams per liter. The Densimeter measures the Bulk Density in grams per liter. So the combined results give the percentage of Moisture.

While determining the Bulk Density of the product in our cylinder, we do measure the Moisture contents of that same product at the same moment. We developed a special probe, which is able to measure that moisture content without making any contact with the sample to be measured. In this way we cannot disturb the sample while measuring. Our probe measures a very big sample, so that the inaccuracy of any small sample, does not play a role any more.
Field tests have proven that we do meet the above set of requirements.

You can view some test results in the graph below.

grafiek moisture

As you will notice, the results of our automated measurements do not always exactly correspond with the ones resulting from drying in an oven. Study of the differences showed, that in many cases these differences can be explained as a result of the "manual" sample not representing the measured soil, because of the "manual" sample simply being too small.

Moisturemeter II for measuring Biomass on a Conveyor Belt

In 2015 we introduced the MoisturemeterII on the market. Our second generation moisture meter that allows to measure inline and instantly the moisture content in a wide range of products in a "streaming" process. Due to the design of our system it can cope with big particle sizes.

Some specifications of the Moisturemeter II
Moisture percentage180%
Bulk density602,000g/l
Volume flow5500m³/h
Belt speed0.022m/s
Belt width5080cm
Belt length250400cm

So if the Fuel Value is changing, you have to adjust the combustion process in the boiler.