The Suitability of Flexible Fuel Biomass Heating for the Greenhouse Crop Environment and the Effect of Crop Profitability When Compared to Propane Heating

Final Report for FNC06-629

Project Type: Farmer/Rancher
Funds awarded in 2006: $5,560.00
Projected End Date: 12/31/2008
Region: North Central
State: Nebraska
Project Coordinator:
Expand All

Project Information


This project is to identify a flexible fuel biomass furnace that would have the heating capability needed for a hoop style, double polyethylene covered greenhouse and its capability to maintain a stable environment for crop production. Crop profitability would also be evaluated based upon fuel expense, handling costs for the fuel and by-products, and return on equipment investment.


Greenhouse crop production during the winter results in large energy demands to keep the greenhouse environment suitable for plant production. For the smaller grower, the greenhouse typically is heated by forced air furnaces, or unit heaters, utilizing either propane or natural gas as the energy source. On average, the price of propane has increased 16% each of the previous 5 years. Natural gas has had an average increase of 17%, however, the prices have been extremely volatile on a monthly basis whereas propane prices are more predictable.

In an effort to conserve on the increasing fossil fuel costs, many growers have lowered the production temperatures to the lowest they can and still grow plant material. These lower temperatures may result in more plant mortality and diminished plant quality due to higher fungus incidence in cool and highly humid environments. The grower needs to eliminate the concern over heating costs and redirect their attention back to growing quality and profitable plant material. To do this, a more stable and predictable priced energy source needs to be identified.

The most common and least expensive type of heating is by the use of forced air unit heaters using gas (natural or propane) as the energy source. Gas is a fuel which is convenient and requires no labor for use in the unit heater. If a grower was to convert fuels, say from gas to biomass, the simplest conversion would be to select a biomass forced air unit heater. Biomass heating has become more popular in recent years to the homeowner as a form of supplemental heating. Corn and wood pellets prices have remained inexpensive and fairly constant over recent years. Given the fuel price stability, I felt biomass would make an excellent alternative fuel for the large heat demands of a greenhouse. One consideration with biomass is, it requires handling in order to load the furnace and the heating unit requires occasional cleaning.

The purpose of this project was to evaluate the feasibility of using a biomass heating unit for greenhouses. Of particular interest is, whether the biomass unit heater will be able to recover quickly enough for the unique demands within the greenhouse. The amount of labor required to operate this heating method will be monitored and a return on investment calculation will determine if a cost savings can be realized. Finally, the growing environment will be evaluated for temperature stability and humidity or condensate control, as well as, identification of production issues that materialize during the study.

Equipment Investigation
In order to implement the use of a biomass heating unit, I needed to explore the various units currently on the market, learn their construction and how each of them work. With this information I could better select a unit suitable for greenhouse applications.

In doing an internet search, I was able to find several biomass furnace vendors at a Penn State website ( called Energy Strategies, Burning Shelled Corn as Fuel. I received more detailed information by phone and mail from several of these vendors and narrowed it down to four distinctly different models. I had the opportunity to make a personal visit with two manufacturers, Eagle Manufacturing and Year-A-Round, and saw the other two units at greenhouses I visited. The following is a brief summary of what I learned.

The first unit was a 170,000 btu Golden Grain Stove Model 3101 at a retail price of $3995. This unit was portable, had a 3 bushel hopper, double auger, two speed blower and didn’t require a chimney. The second unit was a 165,000 btu A-Maize-Ing Heat furnace by LDJ Manufacturing at a retail cost of $4,000. It had a 14 bushel hopper, single auger, three speed blower and required a chimney. Third, was a 200,000 btu Eagle Manufacturing Model 200 biomass furnace at a retail price of $4,200. This unit is put in a fixed location and uses a double walled chimney. It has a 7 bushel hopper, double feed augers and a single speed blower. Eagle Manufacturing also offered a model 300 which is a 300,000 btu furnace for $5,100 with a 10 bushel hopper and a larger heat exchanger. At the top of my prices for a suitable unit heater for my greenhouse, was from Year-A-Round Corporation. They had a model 500-FA, or 50,000 – 500,000 btu furnace, for $9,395. This unit had a 17 bushel hopper, manual Glow Plug ignition, burn pot with agitator, separate ash pan and solid state control panel.

Generally, all of the units have a storage bin of various capacities for the biomass; a single or double screw type auger that feeds the biomass from the bin to the burn chamber; some type of “burn pot” for the biomass; a heat exchanger; and a combustion blower. The combustion blower is a critical component of the unit as it is what keeps the corn, or biomass, burning. A fine balance of the fuel feeding system and the combustion fan has been established by each of the manufacturers. Too much, or too little, of either can put out the fire. The more expensive the system, the more detailed the unit becomes. In the more expensive systems, the burn pot (where the actual fire occurs) may have an agitator and a separate ash pan so the fire doesn’t have to be put out in order to clean out the unit. What I did notice with all of the units I investigated was, the initial firing cannot be automated. Whether you use lighter fluid on fire starter blocks or you purchase an expensive system that has electronic ignition rods, some sort of manual labor must occur to fire up the system.

Biomass Equipment Selection
Expense, durability and fuel flexibility were the primary considerations when selecting which biomass heating unit to test in my greenhouse project. The greenhouse environment is harsh due to the frequent incidence of water, constant high humidity and extreme summer temperatures. Of major consideration in this study was to have flexibility in the biomass and not be limited to burning corn. Corn is a commodity which is often in high demand due to its multiple uses as feed for livestock, cooking oils, syrups and the currently booming ethanol fuel market which could greatly affect its per bushel cost. By having fuel flexibility, our greenhouse operation will potentially stabilize energy costs which will allow for more crop profitability. Affordability of the unit is another major concern on selection. Careful consideration must be given for the biomass heaters initial cost and the added labor for operation, as these additional costs could counteract any savings in fuel expense.

Given its fuel flexibility and the apparent durability for harsh environments, I visited with Eagle Manufacturing Incorporated about using their shop model 200 for the study. In discussions with Joe Engle, owner, on my concerns of temperature stability within the greenhouse, we decided to error on the side of caution and use their model 300. Joe supplied us with a unit and also helped with the installation and make any performance adjustments necessary to evaluate the use within greenhouses.

With gas heating units, there is no labor during their normal operation. With biomass heating, one would expect an increase in labor due to fuel handling and unit cleaning. In an effort to reduce handling labor, I chose to purchase a used 235 bushel bulk bin from a local farming operation that no longer was feeding hogs. I attempted to find a used utility auger, however, quickly learned most used augers are in poor shape. I was able to order an inexpensive 4” x 12’ auger through a regional farm store. After installation, I found that loading the furnace is quick and easy.

Fuel Comparison
The easiest way to compare greenhouse energy use between fuels would be to do a side by side comparison. Since we have only one greenhouse at the time, I received information from Penn State’s Agricultural and Engineering website on Equivalent Heating Values. From their equivalent chart, I could estimate the amount of an alternative fuel that would have been used to heat the greenhouse space. As part of the explanation for fuel btu (British Thermal Unit), I found that each of the fuels have a thermal efficiency. Most of the salesman from each of the furnace companies incorrectly compared the btu’s of the fuels and did not adjust them for the thermal efficiency (or transfer of energy from fuel to air) of the individual fuel.

For example, shelled corn has a heat content of 6,800 BTU/lb or 380,800 BTU/bushel. The thermal efficiency of corn is 75% so you have a net of 5,100 BTU/lb or 185,600 BTU/bushel. Propane has a heat content of 91,600 BTU/gallon with an 85% thermal efficiency resulting in a net value of 77,860 BTU/gallon. The net BTU from each of the fuel type make a more realistic comparison.

77,860 BTU(gal Propane)/5,100 BTU(lb corn)
15 lbs corn = 1 gallon propane or
1 bushel of corn (56lbs)=3.7 gallons propane or
1 bushel of corn = 3.4 therms natural gas

Propane and corn would usually be purchased prior to the heating season. However, refueling during the peak would be necessary but a pre-season “lock-in” price could be determined. Natural gas prices fluctuate monthly depending upon demand. If available, the natural gas supplier usually offers a limited number of customers lock in prices prior to the heating season. The history above is assuming an individual was able to lock in prices during September for all three fuels.


Materials and methods:

This biomass heating project was conducted in a 21’ x 96’ Poly-Tex Expansion Mansion greenhouse. It is constructed of 2” tubular steel, has 6’ side walls and a round roof shape with a center ridge height of 10’. The greenhouse was covered with two layers of Klerks K50, 6 mil polyethylene film. The greenhouse is situated in a rural area on elevated land. There are no windbreaks, resulting in full exposure to the sun and wind. Inside of the greenhouse, there are four 16” horizontal air flow fans creating uniformity of the greenhouse environment. The greenhouse utilizes intake vents and two, 24” two-stage exhaust fans operated from a simple thermostat for cooling needs.

The Eagle Manufacturing Incorporated shop model 300 biomass furnace was selected as the heating unit and installed inside of the greenhouse on the north end. A 235 bushel bulk bin was placed immediately north of the greenhouse and a 4” utility auger was installed to make the transfer of the biomass from the bulk bin to the furnace easy. The furnace had a 6” double walled chimney installed which penetrated the north wall and elbowed up and terminated above the 10’ ridge line of the greenhouse.

The greenhouse was heated during the growing season utilizing corn as the primary energy source. The previous propane unit heater was utilized as back-up heating in the event of temperature problems. Careful records were maintained to record time of labor and production issues. Labor time will be recorded in minutes utilizing $10.00 per hour as the pay rate. University of Nebraska researchers installed monitoring equipment to evaluate the temperature stability within the greenhouse and to track changes in humidity and free water on the glazing or leaf surfaces

Research results and discussion:

Biomass Evaluation
UNL Biological Systems Engineering studied the energy content within various potential biomass for potential energy sources. Fuels included shelled corn, hazelnut shells, pecan shells, walnut shells, distiller grain pellets, and wood pellets, shown in Table 1.

Table 1. Summary of Bomb Calorimetric Tests
Fuel Type; Average Gross Heat of Combustion
(BTU per lbm)
Hazelnut Shells; 8,159+624
Pecan Shells; 8,983+527
Shelled Corn; 7,857+349
Walnut Shells; 8,951+680
DDG Pellets; 8,364+257
Wood Pellets; 8,217+27

Data was acquired with an adiabatic oxygen bomb calorimeter using the American Society Testing and Materials (ASTM) procedure designation D2015. The bomb calorimeter is located in the Industrial Agricultural Products Center (IAPC lab), Chase Hall on East Campus. An isothermal jacket for calorimeter water was used. Small samples of biofuel were burned under 30 atmospheres of pure oxygen.

A typical bomb calorimetric test on a single biomass sample (approximately 1 gram) requires 30-40 minutes. Gross heats of combustion values are shown in Table 1. Most biomass contain small amounts of nitrogen and sulfur. The D2015 procedure accounts for energy tied up in nitrogen oxides and sulfur dioxides. These energy amounts are adjusted by pH titration. A small amount of energy is also lost with the fuse wire during the test.

Greenhouse Equipment Evaluation
The grower purchased 190 bushels of shelled corn at $3.05 per bushel during Fall 2007. Propane during Fall 2007 was also purchased at $1.89 per gallon. The fall purchased corn was burned during a three week period during February 2008 and met most of the night time heat loss needs, except for a couple of nights where the propane unit heater came on automatically to make up the difference. This probably meant that the biomass burner was slightly undersized or could not produce the heat needed. Table 2 shows the volume amount of corn versus propane and projected savings with both Fall 2007 and March 2008 prices. The projected savings if the corn had been purchased in March 2008 would have been less because of rising corn and propane prices.

Table 2. Preliminary fuel cost analysis and comparison of shelled corn with propane.

Fuel Type Shelled Corn
Density 1 62 lbm per bushel
Energy Content1 7,800 Btu per lbm
Furnace Efficiency1 50%
Unit Price $3.05 per bushel
Amt Used 190 bushels
11,780 lbm
Total Energy 91,884,000 Btu
Total Heat 45,942,000 Btu
Total Cost $580

Fall Purchased
Fuel Type Propane
Energy Content 91,600 Btu per gal
Furnace Efficiency 81%
Unit Price $1.89 per gallon
Amt Used 619 gallons

Total Energy 56,718,519 Btu
Total Heat 45,942,000 Btu
Total Cost $1,170
Total Savings $591

Spring Purchased
Fuel Type Shelled Corn
Density1 62 lbm per bushel
Energy Content1 7,800 Btu per lbm
Furnace Efficiency1 50%
Unit Price $5.35 per bushel
Amt Used 190 bushels
11,780 lbm
Total Energy 91,884,000 Btu
Total Heat 45,942,000 Btu
Total Cost $1,017

Spring Purchased
Fuel Type Propane
Energy Content 91,600 Btu per gal
Furnace Efficiency 81%
Unit Price $1.95 per gallon
Amt Used 619 gallons

Total Energy 56,718,519 Btu
Total Heat 45,942,000 Btu
Total Cost $1,207
Total Potential Savings $191

Equipment Efficiency
Because the grower was well engaged in a marketable crop, only a couple furnace efficiency tests were performed during this period shown in Figure 2. The auger speed settings and fan velocity in early tests were as such apparently to give only an approximate 50% furnace efficiency. The unit has a fixed sprocket feed twin auger system. Much higher efficiencies would seem possible with some auger speed adjustments.

To maximize biomass burn efficiency and to increase heat output by adjusting auger speeds and hot side heat exchanger air flow. The biofuel pellet feed rate can not exceed what can be burned in the fire box and delivered efficiently through the heat exchanger. Ideally, a counter flow heat exchanger with sufficient contact area would be used between the hot and cold sides for maximum efficiency.

Greenhouse Environment
The greenhouse environment and surroundings was monitored on 10-minute interval, 24-hours per day by a set of data loggers. The house was monitored internally in three measurement zones for air temperature and humidity, total and photosynthetically active radiation (PAR), plant temperature, floor temperature, potting soil temperature, and inside roof glazing temperature. The latter were used to calculate sensible and latent heat exchange rates of the crop with their surroundings and moisture condensation potential on the leaves, floor, and inside glazing throughout each day from early February to late April.

Outside air temperature, total solar radiation, and wind speed were also measured, but backed up with hourly data from Lincoln supplied by the High Plains Automated Weather Data Network. Ventilation fan, unit heater, and biomass burner operations were monitored with non-intrusive, split core current sensors, placed on the appropriate electric supply and control wires. A continuous record and calculation of greenhouse nighttime heat loss and daytime heat gain were calculated using these data and the formulae of the ANSI/American Society of Agricultural and Biological Engineering Standard EP404.3 “Heating, Ventilating and Cooling Greenhouses”. Sample results are shown in Figure 1.

Night time heat loss over the growing period ranged from 25,000 to 160,000 BTU per hour. The loggers communicated with a central computer located in the North end of the house using wireless technology. It was noted that there was a smooth biomass heating temperature maintenance and apparent 10% reduction in nighttime relative humidity.


Much data was generated during the project, which gave the engineers much delight. As a grower, I saw some interesting results which other growers may find of value and some results which environmentally conscious consumers may want to learn.

First of all, furnace efficiency is important. Prior to selection of the Eagle Manufacturing Model 300 Shop Furnace, I investigated several corn burning furnaces available throughout the United States which were affordable to smaller greenhouse operations. All of the units can create heat but can they capture it? When giving a talk to some growers in St. Joseph, MO this spring, one simple grower who had a corn furnace said it all in one sentence, “I wish as much heat would go in the greenhouse as goes up the chimney”. From my experience, Eagle Manufacturing is making great strides to capture as much heat as they can when utilizing a force air heat exchanger.

Second, greenhouse temperatures tend to modulate very gradually when using a biomass furnace. Unlike the typical start and stop operation of standard gas furnaces, the biomass furnace runs continually. The fire temperature is modulated by adjusting the auger speed from continuous drive (high fire) to intermittent (low fire). This ramping up and down makes a more uniform temperature within the greenhouse environment. Gas furnaces using “start-stop” controls allows for intermittent hot and cold periods which can affect plant growth.

The low fire condition of the furnace was of concern to the grower as a waste of fuel, but this was quickly discounted given the benefits of maintaining the continued burn. As the plants grow and fill the greenhouse, humidity becomes a major issue within the environment. With the continual burning of the furnace, tests conducted showed a significant reduction in the humidity within the greenhouse. Typical growing practices require many preventative fungicide chemical applications to maintain the superior quality of plants. With the reduction of humidity within the greenhouse using the biomass furnace, we eliminated all foliage fungicide applications given the drier environment. This is good for the environmentally conscious consumer.

Third, we did save money. Not as much as you would hope given there is increased labor with the biomass furnace and there is a certain amount of cost fluctuation in these alternative fuels. We tested corn as our primary fuel as it maintained its integrity better than the other fuels and was easily available from local farmers. We did see corn prices double in the last year but with improved efficiencies of biomass furnaces, burning of $9 corn may still be feasible if fossil fuel prices continue to be volatile.

The increased manufacturing of ethanol has made dried distillers grain (DDS) an attractive fuel possibility given its wide spread availability within the Midwest. The Adams’ greenhouse was especially interested in this product as a new ethanol plant recently began operation within 3 miles of their greenhouse. It was found, however, even though there was much energy available within the pellets, both the DDS and the wood pellets had problems maintaining their granular structure within the humid greenhouse environment. Either of these fuels could become valuable in the future if the binding agent could prevent degradation of the pellet in humid environments and when handled in automated equipment.

Several nut shells were tested and were found to be suitable for use within the biomass furnace. Nut processing plants are fairly regional so this would be a great energy source if available. When discussing the nut shell energy content with Larry Martin of Heartland Nuts and More (Valparaiso, NE), Stacy found out that that their nut processing plant has the ability to run their shell by-products through a roller mill to make uniform pellets for use within corn stoves. With the high energy content within the shells, if the nuts are processed appropriately and are available, this would be the best energy producing biomass for heat generation.

Though this is not a permanent fix to our energy needs of the future, it is a viable one during the times of change. One must be aware that burning fuels is not the cleanest alternative, whether its corn or other types of biomass. The truth also is, no energy source that involves burning anything is truly renewable as energy is spent to create the biomass. Our future goal is, as technology improvements and affordability are made in capturing the wind and sun, we will move in that direction.

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

Presentation of project research and development to 28 guests at the National Small Farm Trade Show & Conference in Columbia, MO, November 3, 2007.

Presentation of project research, development and preliminary biomass test results to 35 guests at the Great Plains Vegetable Growers Conference, St. Joseph, MO, January 25, 2008.

Poster display at Great Plains Vegetable Growers Conference in St. Joseph, MO, January 24-26, 2008, with 300 guests.

Traveling poster display for conferences, with Eagle Manufacturing Inc., Webster City, IA. November 2007 to June 2008.

Poster display and personal interaction with consumers at Haymarket Farmers Market and Old Cheney Farmers Market, Lincoln, NE. May 2008.

Sustainable energy program comes to Firth Greenhouse. Press release news article in Voice News, 108 Locust St. - PO Box 148 - Hickman, NE 68372 - 402-792-2255. October 30, 2008

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.