Method to Produce Durable Pellets at Lower Energy Consumption Using High Moisture Corn Stover and a Corn Starch Binder in a Flat Die Pellet Mill

In this study, a protocol was developed to produce good quality pellets using a flat die pellet mill at reduced specific energy consumption testing high-moisture corn stover and a starch based binder. The results indicated that adding a corn starch binder improved the pellet durability, reduced percent fines and decreased specific energy consumption.

Transcript

The overall goal of this procedure is to develop a methodology for high moisture pelleting of biomass, using a binder in a flat die pellet mill. This method can help to answer key questions in the bio energy field, such as how to reduce the cost of pellet production while producing pellets that meet quality specifications necessary for different transportation scenarios. The main advantage of this technique is that it is customizable to the needs of each end user.

The information learned can then be used as the basis to develop scaled-up models and for testing the process in pilot scale and commercial scale pellet mills. Use a laboratory scale flat die pellet mill equipped with a 10 horsepower motor to carry out the pelleting tests. Wrap heating tape around the lower portion of the hopper on all four sides.

Then screw on the feeder and insulate the hopper and feeder with glass wool, to prevent heat loss. Next, connect the heating tape to a temperature controller so that the biomass can be preheated. Then, connect a VFD to the pellet mill motor.

The feeder motor is controlled with a potentiometer, so direct current is used to adjust the pellet mill feeding speed. Next, connect a power meter to the pellet mill motor to record the motor's power consumption. Then, select a pellet die with an eight millimeter diameter opening and a length to diameter ratio of two point six.

Adjust the spacing between the roller and die to zero point one eight to zero point two zero millimeters. Lastly, add a horizontal pellet cooler to cool the warm pellets as they come out of the die. Connect the cooler to an exhaust system to circulate fresh air.

Prepare three to four kilograms of corn stover, ground in a hammer mill, fitted with a four point eight millimeter screen. Also obtain 100 percent pure cornstarch binder from a local market. Measure the moisture content of both substances.

For example, weigh out three 25 to 50 gram samples of the corn stover and put them in an oven set to 105 degrees Celsius for 24 hours. The next day, weigh the dried samples and calculate their moisture content. Next, measure the bulk density of both substances.

Fill a Plexiglas cylinder until it overflows and level the top surface with a straight edge. Then, weigh the cylinder with the material. Divide the weight by the volume of the cylinder.

Repeat the measurement in triplicate and use the average. Now, add cornstarch binder to the ground corn stover according to Table two in the text protocol. Adjust the amount of water to use according to the moisture level of the ground corn stover.

Make mixtures with 33, 36 and 39 percent moisture content by blending the water into the stover starch mix with a ribbon blender. Each mix is about three to four kilograms. Store the mixtures in sealed containers and keep them at four to five degrees Celsius.

After about 24 hours, their moisture will equilibrate and they can be tested. Warm the corn stover starch mix to room temperature. Load the material into the feed hopper of the pellet mill.

Set the mill to run at 60 Hertz or 380 rpm. Then, start the mill and feed the mixture uniformly by adjusting the feeding rate to produce pellets at a steady state. Cool the pellets in the horizontal pellet cooler.

Then, separate the fines from the pellets using a six point three millimeter screen. Weigh the material that has passed through the screen to determine the percent fines from the material. Set aside two kilograms of pellets to measure their moisture content and durability, and dry the remaining pellets at 70 degrees Celsius for three to four hours, to reduce the final moisture content of the pellets to less than nine percent.

Using previously described methods, measure the moisture content and bulk density of the wet pellets and the dry pellets, which are also referred to as green or cured pellets respectively. Next, determine the pellets'durability. Place about 500 grams of pellets without fines into each compartment of the pellet durability tester and tumble the pellets at 50 revolutions per minute for 10 minutes.

Then, sieve the tumbled material using a six point three millimeter screen and weigh the pellets to calculate their durability. To calculate the expansion ratio, take a single pellet and smooth both ends with grit utility cloth. Then, measure the pellet diameter using Vernier calipers.

Do this with 10 pellets. Expansion ratio is square of extruded pellet diameter divided by square of the die diameter. Lastly, determine the pellet mill power consumption.

First, record the no-load power consumption of the pellet mill running empty at 60 Hertz. Then run the pellet mill with material, measure the power consumption and make the appropriate calculations. Pellets with variable binder and moisture content were compared.

The moisture content of the biomass was reduced by about seven to eight percent, after pelleting with no binder. Addition of four percent binder reduced the loss of moisture in the feed stock to three to five percent. Pellet diameter increased with greater moisture content.

This increase was statistically significant between 33 and 39 percent moisture, with no binder. Dried pellets had a lower expansion ratio. Increased moisture content increased the expansion ratio, but adding a starch binder had no significant effect on the expansion ratio.

Bulk density decreased with an increase in starting feed stock moisture content. In addition, there is some indication that bulk density decreased with an increase in starch content. The green and cured durability values of the pellets increased with binder content.

Peak green durability was found with a moisture content of 33 percent and four percent starch binder. Cured durability was much higher, peaking at 98 percent durability for pellets made with 33 percent moisture content and four percent binder. The percent fines generated increased with moisture content.

Adding binders resulted in lowering the percent fines for all moisture contents. The specific energy consumption was reduced with binder additions at all moisture contents. Adding two to four percent binder reduced the specific energy consumption by about 20 to 40 percent.

Once mastered, this technique can be done in two hours, if performed properly. While performing this procedure, it's important to remember the safety procedures to be followed for operating a flat die pellet mill. High temperatures and pinching hazards call for appropriate personal protection equipment.

Following this procedure, you can alter wide variety of parameters, such as feed stock type, biomass particle size, feed stock moisture content, binder type, preheating temperature, die speed, LBD ratio, length to diameter ratio of the die, After its development, this technique paved the way for researchers to develop low cost pellets in a laboratory, for bio-energy applications. We plan to use this information to extend the process to pilot scale and commercial scale mills.

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Journal (Environment)

Journal (Environment)

Method to Produce Durable Pellets at Lower Energy Consumption Using High Moisture Corn Stover and a Corn Starch Binder in a Flat Die Pellet Mill

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