FUEL ENERGY FROM PROCESS WASTE BIOMASS

By Dr. E.A. Richards, P.E.

__________

Economic conditions continue to indicate that the already high energy costs paid by brewers to run their plant process systems are continuing on an upward spiral. Natural gas prices are almost five times what they were a decade ago and show no signs of leveling off, even though oil prices have stalled momentarily.

In view of these increasing costs, the possibility that a company could be able to burn its own process generated waste to produce process steam can be very attractive if the concept used is technically and economically viable and the payout period is reasonable.

For example, some large breweries generate significant wastes in the form of spent grains, hops, and the residue from malting and milling operations. Depending on quantity, this biomass waste can provide a relatively constant source of energy which will not only lower fuel costs but will also relieve the cost for disposal.

Biomass Study Report

To elaborate on the potential for biomass usage, the results of a study recently completed for a major midwest-based corn packing company will be discussed. The study was initiated due to the fact that during a two-month season, the packing company generated extremely high volumes of cob and shuck waste which required costly disposal by a hauling service company. This paper will describe the direction taken and the approach recommended for taking maximum advantage of the biomass waste material.

For the study program, the following parameters were established:

Total season weight of biomass: 56,000,000 pounds

Moisture content as processed: 81%

Btu per pound at 81% moisture: 1482 btu

Weight after pressing to 60% moisture: 26,000,000 pounds

Btu per pound at 60% moisture: 3120 btu

Weight after drying to 10% moisture: 11,822,222 pounds

Btu per pound at 10% moisture: 7020 btu

Btu per pound bone dry basis: 7800 btu

Season length: 60 days

Steam used during season: 19,545,848 pounds

Natural gas cost: $.50 per therm

Power cost: $.06 per kilowatt hour

Hauling cost: $10 per ton

The initial step was to determine the potential fuel value of the material. Samples of the waste material were weight before and after drying to establish moisture content, pulverized for testing, and proximate an ultimate analyses carried out. Btu values were determined by the bomb calorimeter method and by calculation. For purposes of the mass flow calculation, the value of 7800 Btu per pound dry basis was selected and used.

Moisture Content

In any waste fuel application based on the use of biomass, the critical factor is one of moisture content. For cobs and shucks, the as-processed moisture content limited energy values to about 1482 Btu per pound. To fire the waste material at this moisture level would be practically a break-even proposition, useful only for waste incineration in an area where laws make compliance mandatory.

In the past, the technical inability to achieve a really effective removal of moisture on a large scale by low cost mechanical means has always had a negative effect on the economics of waste fuel firing. Compounding this, direct-fired or steam-tube type drying of the biomass added to the cost to the degree that it became cheaper to simply dispose of the material that to overcome the deficiencies of the process.

Equipment

In order to best deal with the corn waste problem, it was decided to utilize heavy duty pressing and drying equipment which had established a reputation for in the brewing, sugar beat, and fish meal industries. These units were the Stord-Bartz twin screw press and the Stord Bartz Rotadisc dryer, both of which have become mainstays in major breweries who have modernized their spent grains processing and drying operations to a highly sophisticated level.

Touching briefly on the qualities which render them unique, the large twin-screw press can handle up to 90 tons per hour at low operating speed, with material moisture content lowered to levels of 50% to 60% depending on the type of material, and the steam disc dryer which can evaporate one pound of water using about 1.2 pounds of steam.

Existing Solid Fuel Boilers

The situation at the canning factory was simplified by the fact that two coal-fired boilers had been taken off-line because a more efficient 100,000 pounds per hour natural gas boiler had been installed. The direction of the project thus turned to using the waste fuel to fire the existing coal boilers and running the gas-fired boiler as a standby.

System Options The Btu tests had shown that the inherent heat value of the biomass waste was high enough to justify further project analysis. At that point, three different firing system options were chosen for investigation:

Option 1. A moving grate system to burn large size waste of up to 60% moisture content.

Option 2. An injection burner to fire the dried and pulverized waste.

Option 3. Pelletizing the dried waste into fuel to be burned like pulverized coal.

Option 1. The cob and shuck waste from the corn factory building would go to an existing coarse chopper which would break down the cob material to a 3” - 6” size. After coarse chopping, the waste would enter the system and flow to a Fox “fine chopper” where all of the waste would be cut to 1” - 2” pieces. From the fine chopper the waste would go to a Sprout Waldron attrition mill where the cellular structure of the waste would be further degraded to maximize the effect of the pressing operation.

From the mill, the waste would enter the press where its moisture content would be lowered to a level of about 58% - 60%. Leaving the press, the dewatered waste would be lifted by bucket elevator to a belt conveyor and travel to the boiler house. In the boiler house area the conveyed material would pass through a flue-gas heated, air-to-air heat exchanger section to remove more of the moisture in preparation for firing.

In this option, one of the boilers would be retrofit with a Solid Fuels, Inc./Combustion Systems four-section traveling grate burner unit which is specifically designed for biomass firing. At the point where it enters the burner area, the Btu content of the waste fuel would be about 3,120 Btu per pound or more, depending on the effectiveness of the flue-gas drying system.

It was calculated that at an estimated efficiency of 64% for the boiler, there would be no problem in firing enough waste fuel for the corn pack season. The irony of the situation was that out of the 56,000,000 pounds of biomass, only about 10,278,015 pounds would be used for fuel, with 45,721,985 pounds still to be hauled away at a cost of about $228,000.

The green material cannot be stored for future use because of spoilage. However, in other applications where steam requirements parallel waste generation, this particular waste fuel firing choice could be viable.

Option 2. This option would employ the same equipment and process line as that of Option 1 except that the fine chopper would be set to produce 1/8” - 1/4” particles. A steam disc dryer would be added to the line along with a hammermill which would pulverize the dried waste fuel for ease of injection firing.

After drying and pulverizing, the waste fuel would leave the mill and be pneumatically conveyed to a pair of AO Smith 31’ diameter x 90’ high straight-side glass-lined storage silos. From the silos the fuel would feed the injection burner installed in one of the boilers. This option would also require additional furnace refractory.

Out of 11, 822, 222 pounds of dried fuel created, 3,683,919 pounds would be used for drying, leaving 8,138,303 pounds for use. The steam required for the corn pack would require 4,060,450 pounds of fuel, leaving 4,077,853 pounds (2039 Tons) of dried and stored fuel for other uses. The total net of 8,138,303 pounds of fuel at 7020 Btu per pound is equivalent in value to $277,934 in terms of natural gas fired at 74% efficiency.

Added to the above value the savings of $288,000 for hauling, minus $53,000 in power cost to operate, results in a net figure of $504,934 for the system. If the system costs are $2 million, then payout would be 238 days at $8,416 total value per day.

The main drawback in Option 2 is that only one of the two existing boilers would be retrofitted for the price. Another area of concern is the possibility that hot spots might develop in the stored fuel, although the hot spot problem could be alleviated by recirculation techniques and equipment which would add to project cost.

Option 3. This option would utilize the same equipment used in Options 1 and 2, with the addition of a Sprout Waldron pelletizing system. From the hammermill the fuel would pneumatically conveyed to the pellet mill hopper and travel through the pelletizing process. From the pellet mill the 1/2” diameter x 2” long pellets would be loaded into a pair of straight-side glass-lined silos, 25’ diameter by 90 feet high. On demand, the fuel pellets would be conveyed to the boiler house for firing.

This option was judged to be the best choice of the three for this application and was recommended. It allowed for either one or both of the existing non-gas boilers to be on line. No retrofit to the non-gas units was required - other than some modification to the ash system which was included in the cost.

Because of the somewhat lower efficiency of pellet firing, 68% versus 72% for the Option 2, injection burner firing, more of the available fuel was used for drying purposes. After deduction the costs for power and operating, the resulting net value of the pelletized fuel came to a total of $481,504. For the project cost of $2.3 million this meant a payout of 287 operating days at $8,026 per operating day.

The Option 3 system had other advantages. For example, each acre of stalks and leaves left over in the cornfield after the ears were picked air-dries to an average level of 58% moisture. Test samples of stalks and leaves showed the weight of this material to be over two tons per acre. When this material is collected from the field by the farmers, using their mobile chopping units for pre-processing, the finished pellets should net after delivery and processing costs about $35 per ton as animal feed.

The dried corn plant pellets will not command the price that spent grains will because the protein level is about 7% and the total digestible nutrient (TDN) is near 60% compared to 26% protein and 66% TDN for spent grains. But the protein, fiber, mineral, and vitamin content of the corn plant pellets will still make them well-suited for use in most feed mixture applications.

Spent Grains Use

The economics of choosing to use brewers spent grains as animal feed or as fuel can be illustrated as follows:

For a 5 million barrel brewery, the estimated spent grain production can be set at 216,500 tons per year at a grains moisture content of 85%. In ideal situations, selling this amount wet at $8 a ton would gross about $1,732,000. Drying the spent grain and selling it as animal food would be a return of $2,621,310 after the cost for natural gas drying, power, and operations. Drying, pelletizing, and storing the spent grain for use as fuel would offer a value of $1,364,362 in terms of 74% efficiency natural gas after drying, power, and operating costs are deducted.

Obviously, drying the spent grains for sale as animal feed is the more lucrative option. Only if there were no convenient market for either wet or dry spent grains would using them as fuel be of merit.

Conclusions

Any brewery, or food processing plant, that has a source of biomass available in substantial quantity should consider processing the material for recovery of its inherent energy value. In the face of ever escalating fuel costs, any method whereby these costs could be lowered or, perhaps eliminated, deserves attention. If the cost for hauling and disposal of waste is significant, then the economic advantages of waste fuel firing become even more pronounce.

It should be noted that due to the short two-month corn processing period, the payout time was given in terms of operating days. For a brewery with a year-round operation, the project payout period for its particular biomass would be reported in real-time years.

And, because the coal-fired boilers were existing units, the decision to fire with pellets in the same manner as coal was a simple one due to the excellent heat content of the processed waste, its combustion characteristics, which are similar to that of coal.

For applications where no such boilers already exist, there are a number of efficient steam generation units available for solid fuel firing which can use injection, suspension, or grate type burners to fire waste fuel in its many forms. Among the units available are those supplied by Cleaver-Brooks, Guaranty Performance, Gordon-Piatt, and others.

ADDENDUM

I. Simplified Derivation of BTU/Ib for Biomass Materials

A reasonably accurate heat value in BTU/Ib for a given type of biomass can easily be determined with a minimum of lab equipment and a derived, factored equation. The procedure is as follows:

Proximate Analysis

1. Heat the material in a 2250C oven until dry and grind or pulverize to small, easily handled particles.

2. Weigh out 1.0 g of material and place it in a small container sealed from outside air except for a small escape port for gases.

3. Heat the container over a 3.5 inch Bunsen flame for 2.5 minutes. This will drive off the volatiles which can be burned as they escape.

4. Let cool and weigh the devolatilized residue which is fixed carbon and ash. The difference will be the weight of the volatile matter.

5. Heat the residue in the container with the cover removed so that the fixed carbon oxidizes leaving pure ash.

6. Weigh the ash. The difference between the devolatilized material and the ash will be the weight of the fixed carbon.

7. Compute the percentages for volatiles, fixed carbon and ash based on the recorded weights. To derive the values for a general ultimate analysis (total carbon, hydrogen and oxygen) and a Btu/lb value, it will be necessary to use the volatiles and fixed carbon numbers only.

Factors and Equation

V = Volatiles Cf = Fixed Carbon Ct = Total Carbon H = Hydrogen 0 = Oxygen

Factors: Ct = .4005V + Cf H = .0768V O = .5199V

Equation:

BTU/Ib = [{(H - ((0/8)) x 61,000)} + {((.4005 V) + Cf) x 14,100}] /100

TYPICAL CALCULATION:

Volatiles, V = 76%; Fixed Carbon, Cf = 21.7%

Ct = .4005 x 76 + 21.7 = 52.14% Total Carbon

H = .0768 x 76 = 5.84% Hydrogen

O = .5199 x 78 = 39.51% Oxygen

BTU/Ib = [ { ((5.84 x (39.51/8)) x 61,000) } + { ((.4005 x 76) + 21.7) x 14,100} ] / 100 = 7901 Btu/lb of biomass material

Plant engineers will recognize the equation to be a modified version of Du long's formula with biomass factors incorporated. Sulfur and nitrogen content are not critical to this calculation.

II. Fuel Analysis - Actual Waste From The Plant

Tests were run on corn husk and cob samples obtained from the corn plant shortly after corn pack. The material was weighed, then dried and reweighed. Results indicated an average of 80.750/0 moisture and 19.25% dry solids. The Btu values of the material were established both by the bomb calorimeter test and calculation check. All figures are given as percent except Btu/lb.

Volatiles

Test Number: (1) 77.5 (2) 76.2 (3) 76.0 (4) 78.7 (5) 76.1 (6) 77.4 (7) 77.3 (Average) 77.0

Fixed carbon

Test Number: (1) 21.2 (2) 21.1 (3) 21.7 (4) 19.8 (5) 21.6 (6) 21.2 (7) 21.4 (Average) 21.1

Ash

Test Number: (1) 1.30 (2) 2.70 (3) 2.30 (4) 1.50 (5) 2.30 (6) 1.40 (7) 1.30 (Average) 1.83

Total carbon

Test Number: (1) 52.2 (2) 51.6 (3) 52.1 (4) 51.3 (5) 52.0 (6) 52.2 (7) 52.4 (Average) 52.0

Hydrogen

Test Number: (1) 5.95 (2) 5.85 (3) 5.84 (4) 6.04 (5) 5.84 (6) 5.94 (7) 5.93 (Average) 5.91

Oxygen

Test Number: (1) 40.3 (2) 39.6 (3) 39.5 (4) 40.9 (5) 39.5 (6) 40.2 (7) 40.2 (Average) 40.0

Btu/lb

Test Number: (1) 7929 (2) 7832 (3) 7904 (4) 7808 (5) 7890 (6) 7923 (7) 7945 (Average)7890 Btu/lb

The above sample runs shown each reflect the averages of the five tests which made up each run, a total of 35 separate tests. Runs which contained the high Btu value of 8720 and the low Btu value of 7349 were not used.

III. Analysis Of Florida Sweet Corn

The results following were from tests run on Florida sweet corn for purposes of comparison because the sweet corn season in the Midwest was over and no fresh material was available. The corn was fresh-picked in Florida, packed in ice and trucked to our lab. It was picked up from the vendor as it was uncrated and brought to the lab for test. There was a four day interval between fresh picking and testing.

After weighing, the kernels were sliced from the ear to simulate pack processing and reweighing took place before drying and pulverizing. The results shown are a result of four tests and an average of the four, with all following values are in milligrams except where percentages (%) are noted.

Test number: (1) 435 (2) 300 (3) 315 (4) 405 (Average) 364

Test number: (1) 120 (2) 075 (3) 077 (4) 113 (Average) 0.96

Test number: (1) 158 (2) 113 (3) 097 (4) 148 (Average) 129

Test number: (1) 150 (2) 105 (3) 130 (4) 135 (Average) 130

Test number: Test number: (1) 278 (2) 188 (3) 185 (4) 265 (Average) 229

Test number: (1) 308 (2) 218 (3) 233 (4) 285 (Average) 261

Test number: (1) 065 (2) 050 (3) 042 (4) 054 (Average) 054

Test number: (1) 7.0 (2) 7.0 (3) 11. (4) 9.0 (Average) 9.0

Test number: (1) 72.4 (2) 75.0 (3) 75.6 (4) 72.1 (Average) 75.0

Test number: (1) 79 (2) 77 (3) 82 (4) 79 (Average) 79

Test number: (1) 21 (2) 23 (3) 18 (4) 21 (Average) 21

Test number: (1) 73.4 (2) 75.0 (3) 71.8 (4) 76.0 (Average) 74.0

Test number: (1) 3.3 (2) 4.0 (3) 4.2 (4) 3.7 (Average) 3.8

Test number: (1) 23.3 (2) 21.0 (3) 24.0 (4) 20.0 (Average) 22.2

Test number: (1) 52.7 (2) 51.0 (3) 52.8 (4) 50.7 (Average) 51.8

Test number: (1) 5.6 (2) 5.8 (3) 5.5 (4) 5.8 (Average) 5.7

Test number: (1) 38.2 (2) 39.9 (3) 37.3 (4) 39.5 (Average) 38.8

Test number: (1) 7964 (2) 7742 (3) 7960 (4) 7706 (Average) 7847 Btu/lb

IV. Analysis Of Field Dried Stalks And Leaves

Field dried stalks and leaves from sweet corn plants, less the ears which had been picked, were gathered from a standing patch, cut off about 8 inches from the field surface. Tests determined that the average weight of a single stalk and leaves as picked was 283.49 g and the dry weight was 119.07 g, a moisture content of 58%. All figures are given as percent except Btu/lb.

Test number: (1) 74.0 (2) 73.0 (3) 74.6 (4) 74.5 (5) 75.8 Average 74.4

Test number: (1) 21.8 (2) 21.2 (3) 22.4 (4) 21.2 (5) 21.2 Average 21.5

Test number: (1) 4.2 (2) 5.9 (3) 3.0 (4) 4.4 (5) 3.0 Average 4.1

Test number: (1) 51.4 (2) 51.0 (3) 52.3 (4) 50.9 (5) 51.6 Average 51.4

Test number: (1) 5.70 (2) 5.70 (3) 5.70 (4) 5.70 (5) 5.80 Average 5.70

Test number: (1) 38.5 (2) 38.8 (3) 38.8 (4) 38.7 (5) 39.4 Average 38.8

Test number: (1) 7780 (2) 7730 (3) 7913 (4) 7723 (5) 7821 Average 7795 Btu/lb

If the stalk and leaf material is pelletized for use as animal feed, then the following analysis describes the nutritional value:

If you are involved with a food processing plant that generates large quantities of biomass, and you would like to know how much that biomass could be worth to you in terms of fuel energy, contact:

Dr. E.A. Richards, P.E.

email: drer@execpc.com

Note: Since its publication, this article has been updated to reflect current economic conditions.

Email Address: drer@execpc.com

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