Biodiesel-fired condensing boiler passes lab test
Brookhaven National Laboratory (BNL) and Fulton Heating Solutions recently completed laboratory testing of a 2-million-Btuh-capacity hydronic condensing boiler that uses pure biodiesel (B100) fuel. The project proved that B100 successfully can replace traditional fossil fuels, such as natural gas, when used in properly designed and operated heating equipment.
The testing was sponsored by the New York State Energy Research and Development Authority (NYSERDA), a public-benefit corporation that administers energy and environmental programs.
Boiler Testing Program
A dual-fuel boiler originally designed to operate on gas or No. 2 heating oil, the B100-fired unit incorporates a primary and secondary (condensing) heat-exchanger design (Photo A and Figure 1). When operating on No. 2 heating oil, the commercial version of the boiler has a return-water temperature above the flue-gas dew point, a control strategy that was implemented because corrosion can occur during oil-fired operation. When B100 is utilized, this is not a factor because the fuel is sulfur-free, leaving the boiler to operate efficiently with gas and liquid fuels. Able to achieve thermal-efficiency levels greater than 94 percent, the boiler is gaining attention for its high-efficiency renewable-energy applications in schools, hospitals, office buildings, and institutional applications. The boiler can be used as a stand-alone heating unit or the lead unit in a modular boiler system to maximize operating hours and annual fuel savings.
The first biodiesel fuel used in the testing was derived from tallow (rendered animal fat). Tallow-based biodiesel fuel generally is known to have less-favorable cold-temperature characteristics (e.g., high cloud-point and cold-filter-plugging-point temperatures) than biodiesel fuels derived from many plant-based oils. Testing with tallow-based biodiesel fuel enabled the quick identification and resolution of cold-temperature operating problems that could occur with a B100-fired boiler under adverse fuel-storage and boiler-room-temperature conditions.
During laboratory testing, which was performed from September 2009 to January 2010, the boiler and surrounding mechanical room were allowed to cool to the mid-50˚F range, causing the tallow-based biodiesel fuel to gel within the piping between the fuel-storage tank and the boiler. Such gelling was resolved by warming the fuel piping, and no operational difficulties were observed within the burner and boiler system.
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Additional testing was performed using biodiesel produced from soybeans, the leading source of feedstock for biodiesel production in the United States. Because the gelling temperature of soybean-derived biodiesel is much lower than that of tallow-based biodiesel, storage temperatures of 50˚F or higher generally are sufficient.
Burner performance also was examined. Ignition on B100 biodiesel, even from a cold starting condition, was smooth and identical to that observed with traditional No. 2 heating oil. Carbon-monoxide (CO) emissions and smoke-number readings essentially were zero during steady-state operation at a normal excess-air level of 25 percent. Following the test runs on the tallow- and soybean-derived biodiesels, the burner head was inspected for coke deposits. No significant deposits were found.
Boiler-emissions testing showed a measurable reduction in nitrogen-oxide (NOx), sulfur-dioxide (SO2), CO, and soot emissions during operation, while energy efficiency basically remained unchanged. SO2 emissions, in particular, were reduced to almost zero.
Figure 3 shows carbon-dioxide (CO2), NOx, and SO2 flue-gas readings during a switchover from No. 2 heating oil to B100 operation. Measurable and repeatable improvements in emissions performance were observed.
During laboratory testing, smaller volumes of condensate were produced during B100-fired operation than during natural-gas- or heating-oil-fired operation. B100 flue gas has a lower moisture-saturation temperature curve than traditional natural-gas and heating-oil fuels, resulting in lower theoretical condensate formation.
Table 1 shows pH values and acidic concentrations in condensate produced by the boiler during operation with natural gas, B100 biodiesel, and traditional No. 2 heating oil. The samples were collected at a condenser temperature of 71˚F, well below the flue-gas water dew point for all of the fuels tested. Under these conditions, the concentration of acid components and pH, both of which impact corrosivity, are favorable for B100 firing and much closer to natural-gas characteristics than No.-2-heating-oil combustion.
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The laboratory testing program also included corrosion analysis of sample heat-exchanger materials during operation with B100 (Figure 4). Test coupons of the stainless-steel material used for heat-exchanger-corrosion evaluation were incorporated into a water-cooled test rig to establish controlled material temperatures and then exposed to flue gases that resulted from condensing-boiler operation.
The test coupons of sample heat-exchanger materials were examined using scanning-electron-microscope/energy-dispersive-X-ray-spectroscopy (SEM/EDXS) analysis of surface elements. The technique allows material surfaces to be inspected and individual surface elements to be identified.
The analysis revealed carbon deposits and small-scale corrosion products on the surfaces of test coupons used for the corrosion evaluation of condensing heat-exchanger materials (Figure 5 click to open). Most deposits had a high carbon content, indicating the presence of unburned or partially burned fuel components. Deposits, including iron, chrome, and manganese, were identified on base heat-exchanger material. Traces of sulfur, chlorine, and sodium also were detected.
Predicted corrosion rates were somewhat higher for B100 firing than for natural-gas operation, reflecting lower condensation rates for B100 than for natural gas at the 107˚F heat-exchanger surface temperature used in the test.
The terms "condensing" and "non-condensing" generally refer to whether the water vapor in a boiler's flue gas is condensed. In oil-fired hydronic boilers, corrosion can occur under condensing or non-condensing conditions.
Under non-condensing conditions, corrosion can occur when flue-gas temperature is lower than acid dew point. Additionally, the corrosion rate is governed directly by the amount of sulfur in the heating fuel. Fuels with less sulfur result in cleaner boiler heat exchangers. Under acid and water-vapor condensing conditions, corrosion rates depend on sulfur content and water-condensation rate.
A low, but non-zero, water-condensation rate is the most challenging operating state because any fuel will produce higher corrosion rates under these conditions. Condensing-heat-exchanger materials must be selected carefully to ensure adequate corrosion resistance. Also, it is helpful for flue-gas and hydronic return-water flows in the secondary (condensing) heat exchanger to be in counterflow mode so the local low-condensation-rate zone can migrate back and forth along the length of the heat exchanger (depending on firing rate and return-water temperatures). The zone's migration allows corrosion effects to be spread out over heat-exchanger surfaces more evenly.
The condensing heat exchanger used in the BNL project appeared to be in excellent condition even after 800 hr of operation (Photo B). Boiler efficiency was measured using an indirect flue-loss method and a direct output/input method. As with many hydronic boilers, efficiency and condensate-collection rates were impacted by return-water temperature. At high fire, with a return-water temperature of 122˚F and an excess-air rate of 30 percent, efficiency was 88 percent. At low fire, with a return-water temperature of 90˚F and a 2-1 turndown ratio, efficiency was 93 percent.
Boiler jacket loss also was measured as part of the efficiency testing. Jacket losses were determined by dividing the jacket surface into small squares and measuring the temperature at the center of each square with a surface probe during burner high-fire operation. The measurement method followed the procedure defined in ANSI/ASHRAE Standard 103, Method of Testing for Annual Fuel Utilization Efficiency of Residential Central Furnaces and Boilers. The jacket loss was found to be 0.2 percent of steady-state energy input, a low value. As a result, the B100-fired condensing boiler for commercial heating applications will be put into limited production and demonstration.
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Properly designed and operated commercial condensing B100-fired boilers offer the opportunity to use renewable-energy sources instead of traditional fossil fuels, such as natural gas and heating oil. Alternate liquid fuels, including biodiesel, can enable building owners to achieve sustainable energy performance with low emissions, making an entire system more energy and environmentally friendly. With these systems, proper fuel handling and storage practices must be followed carefully, but these issues are well within the capability of most building-management operators. Finally, the transition to renewable energy can help create new manufacturing jobs.
Thomas A. Butcher, PhD, is a researcher and head of the energy-resources division at Brookhaven National Laboratory (BNL), where he has worked for 30 years. He received a bachelor's degree in marine engineering at the U.S. Merchant Marine Academy, a master's degree at Stevens Institute, and a doctorate at SUNY Stony Brook. A researcher at BNL, Chris Brown is a graduate of Clarkson University. The director of engineering for the new-product-development group of The Fulton Cos., James Pettiford has 15 years of experience in the design and development of advanced commercial and industrial boilers. He is a chartered engineer with the Energy Institute in the United Kingdom and holds a bachelor's degree in chemical engineering from the University of Surrey and a master's degree from the University of the West of England, Bristol. The commercial heating product manager for The Fulton Cos., Erin Sperry is responsible for modular hydronic heating products and has experience in applications engineering for the design and installation of commercial heating systems. She has a degree in mechanical engineering and management from Clarkson University.
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Biodiesel is a clean-burning fuel derived from renewable resources. It is made through a chemical process called transesterification, during which glycerin is separated from animal fat or vegetable oil (Figure 2).
Biodiesel can be used to replace or supplement petroleum-based heating oil and diesel fuel. It can be blended with petroleum fuels and stored without significant changes to traditional industry practices, except for the maintenance of certain minimum tank temperatures. In the United States, biodiesel typically is produced in accordance with the requirements of ASTM D6751, Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels.