Layman’s Guide to Calculating Fuel Usage in Drying

The most often asked question we hear from our customers is “How much gas or fuel does it take to dry my product?”  The reason the question is important to any dryer user is that the cost of fuel is typically the highest expenditure they make in the drying process.

In general, the following amounts of fuel generate 1 MMbtu of heat

1,000 cubic feet of natural gas (Mcf)

11 gallons of liquid propane (or 400 cubic feet)

7.3 gallons of #2 diesel fuel

 

The first step in the process is calculating the amount of moisture in the product.  As a general rule, it takes 1500 btu to remove one pound of water from a product.  As with general rules there are exceptions and variables such as elevation, humidity, and material properties such as whether the moisture is bound to the product or is surface moisture but for simplicity, we are not considering them in this situation.  You can add a safety factor percentage to your calculations to help cover the variables you know.

Knowing moisture content by mass is crucial in determining how much water needs to be vaporized.  As an example, let’s say you have a paper pulp residual that is 50% moisture by mass, and you want to dry it down to 10% moisture by mass.  That means that you want to remove 800 pounds of water for every ton of product input in the dryer.  In this example 800 pounds at 1500 btu per pound equals 1,200,000 btu or 1.2 MMBtu.

Once you calculate the amount of moisture needed to be removed and the btu required, you simply use established heat values of gases required to generate btu’s of heat.  Some countries calculate in Therms.  10 therms equal 1MMBtu.  Another measurement is a gigajoule (GJ).  1 MMBtu equals 1.055 GJ.

According to the US Energy Information Administration figures for 2016, the average heat content for natural gas in the U.S. is 1,037 btu per cubic foot of natural gas.  In larger industrial quantities that means that one thousand cubic feet (Mcf) of natural gas equals 1.0237 million btu (MMBtu).

According to an amerigas.com blog in 2012, liquid propane at 60 degrees F equals 91,502 btu.  As a general rule then, 11 gallons of LP equals 1 MMBtu.

Another fuel source is diesel or fuel oil.  #2 diesel used in residential heating or available at the pump is as low as 137,000 btu per US Gallon, which means that it takes 7.3 gallons of diesel to generate 1MMBtu.  As the grade of diesel/fuel oil gets higher in number/viscosity the heat value goes up.  For example, a #6 bunker fuel can have a heat value of nearly 156,000 btu.  However, the thicker the oil the more preheating equipment is needed to bring it to proper viscosity for the burner, so you have to compare the cost of the added equipment to the cost of oil and heat value to see which is most economical for the system.

rotary dryer with fuel source
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