TECHNICAL RELEASE 11-R-16
An Oregon FRA member is visiting a fellow chip supply manager and FRA member at his mill in Alabama. It is his first trip down South, and there are lots of questions to ask, such as:
• How many tons are you able to make before changing those new design chipper knives?
• How much chip inventory do you try to keep on hand?
• How much biomass is that new processor you tested able to produce per hour?
The Alabama chip buyer says: 1000 tons, 50,000 tons and about 25-30 tons. The West Coast buyer is confused. Knives don’t stay sharp in hardwood like that! Why would a mill inventory that much wood? Just how big is that new biomass machine that it can produce that much from a pile of brush?
The problem is not bragging by the Alabama buyer. The two are speaking different languages. Weights in the Southern U.S. (and in other areas of North America) are usually measured in wet (or green) tons, while Western operations and much of Canada use dry-equivalent weights. In fact, much of the world uses dry weight as the basis for measurement of wood chips.
THE BASICS OF DRY WEIGHT MENSURATION:
Confusion can also result from the nomenclature used to describe how much moisture is in wood. The % oven-dry solids or %OD is calculated by:
%OD = dry weight/wet weight *100
Moisture content can be calculated two ways:
% Moisture content dry basis = (wet weight – dry weight)/dry weight *100
This expression is the most commonly used expression of moisture content in wood. It is easily converted to %OD by subtracting the %MC dry basis from 100.
A less commonly used expression of moisture is:
% Moisture content wet basis = (wet weight – dry weight)/wet weight *100
This expression of moisture content is commonly used in wood products mills.
WHAT ARE THE BENEFITS:
In addition to expressing the amount of water in wood in different ways, the volume of solid wood is measured in full or face cords, cubic feet, cunits (100 cubic feet), and cubic meters. Why doesn’t the industry adopt and convert to a common measure of chip and wood quantity? The reasons are as complex as why the United States has not converted to the metric system—they range from “we have always done it that way” to the cost of equipment needed to make the switch to the negative reaction of the suppliers of wood and chips.
However, there have been changes in wood supply that provide economic incentives for adopting the dry-weight basis of chip purchase and use. The gradually increasing use of Southern pine thinnings has resulted in water being purchased at the price of chips. Although the wet weight of a load of chips has not changed appreciably over the years, pulp mills have been seeing a decline in pulp digester yields. Research clearly has shown that juvenile wood pulp yield is lower than that of mature wood. Mills have failed to understand the weight difference between a green cubic foot of mature pine and a green cubic foot of thinning. This difference in the wood is not only a matter of basic wood density (specific gravity), but also a matter of moisture content.
Various fiber sources have wide-ranging moisture contents.
Table 1 shows sample percentages of oven-dry solids (%OD) for commonly used fiber sources.
Buying wood on an as-is or green basis could favor either the buyer or the seller. Some mills have used historical values and trends to develop “rules-of-thumb” to approximate dry weights. For example, if the green weight is 10 tons, the dry weight is 5 tons. This would be fine if it were just a discussion between the Western and Southern chip buyers. Historically, the chips and roundwood supply of mills came from relatively few suppliers. In the last two decades the number and types of sources has increased dramatically, and as shown in the above table, the menu of suppliers could deliver a wide range of chip quality, including varying moisture contents. Applying a “rule-ofthumb” or even historically derived factors will not work for all suppliers.
The benefits of converting to a dry-weight basis are significant; establishing a dry-weight basis enables buyers to pay suppliers on an equitable basis, rather than buying water at chip prices. Pulp mills can determine yield more accurately and can provide more reliable economic assessments. Implementing a %OD-based wood accounting system requires management commitment, the right equipment, employee training in reliable methods, and periodic auditing.
The most important requirement for implementing a program successfully is working with suppliers to gain their acceptance and to modify contracts, implementing accounting systems, and possibly arranging for suppliers’ participation in developing the sampling program. Once the program has been developed and suppliers have agreed to it, it is advisable to run parallel green and dry accounting systems, to establish confidence in the system on both sides and to fine-tune the process.
Most mills already take samples for chip size and bark content classification. Mills can use these same samples for drying. However, individual truck or rail car data for size or moisture determination should not be used on a load-by-load basis. It is more reliable to use a moving average representing between 5 and 10 data points. The number of samples in the average will depend on the load-to-load variability. These data will also enable development of historical data for tracking seasonal changes and exceptional situations such as drought conditions and changes in harvest methods. Thus, one may develop a seasonal trend curve for the digester feed of a Southern pine pulp mill. Taking daily samples of the digester feed is sufficient to develop these data for tracking pulp mill yield.
It is important to conduct sampling and testing carefully and consistently. The essentials are:
– 1000 to 1500 wet grams. Since this quantity is also the recommended amount for chips used in chip size classification, a dry sample may go from the oven directly to the classifier. Sample size
Drying oven – The oven dehydrates the chip samples by forcing air circulation at a constant temperature of 105 degrees C (+/- 3 degrees). In order to dry samples from each load, the oven will be a large one (as shown in the Despatch ™ oven in Fig. 1) specifically designed for chip drying.
Drying time – Once the oven is ready for use, a series of drying tests determines the amount of time it takes for the wettest chips to dry to a constant dry weight. This process could require as few as 6 hours for an oven with high circulation rates, but most mills find that synchronizing the drying time to work shifts requires 10 to 12 hours.
Sample handling – It is essential never to put wet chips into an oven or oven compartment with nearly dry chips. The nearly dry chips will absorb the moisture from the wet chips, even at the high drying temperature. Having a separate compartment in the oven for each shift will avoid this problem. It is very important never to allow chips to cool before weighing them. Wood absorbs water quickly from the air. The oven can be turned off at the end of the drying period, but the samples should be left in the oven and weighed hot. The %OD will increase by 2-3% if cooled before weighing, and the buyer will pay chip prices for that water.
Microwave ovens can be used to dry chips. Household microwaves are not suitable, because the wood burns when the moisture content gets low, but specialized designs are available that have built-in weighing and variable microwave dosage that decreases as the wood dries. However, the sample size is only 50 to 100 grams, and only one sample can be dried at a time.
Chip moisture sensors are available and can monitor moisture content on conveyor belts to feed moisture data forward to pulping systems and, when linked to a belt scale, can provide a dry weight of wood being processed. Routine maintenance and calibration of moisture sensors is essential to obtain reliable data.
P.O. Box 4854
Federal Way, Washington 98063
Lake States and Western Region Manager