March 2, 1998
Rosa Flora Limited, a greenhouse in Dunnville, Ontario, Canada, is dedicated to growing roses, alstromeria, stephanotis, and gardenias. Established in the spring of 1978 under the sole management and ownership of Otto and Corine Bulk, additions to the greenhouse have been built every year, to where the facility now has a total of 449,462 sq.ft. under cover. Due to cool ambient temperatures, Rosa Flora had used natural gas burners to heat and provide carbon dioxide (CO2) to the greenhouses. In 1992, Rosa Flora replaced the burners with a power generation station driven by two natural gas-fueled Caterpillar G3516LE engines supplied by Toromont Caterpillar. The station produces electricity, heat and CO2. The engines run at 1200 rpm, generate about 770 kW, and have a heat rate of less than 7300 BTU/bhp-hr. Power from the generators is utilized to run pumps and lights, with excess power sold to the local utility, Ontario Hydro. The heat from the engines is used to keep a warm climate inside the greenhouses. Furthermore, the exhaust gases are catalytically purified and the engine exhaust, with its high (about 7.2 percent) CO2 content, is plumbed directly into the greenhouses. Higher CO2 levels promote plant growth by enhancing the kinetics of the plants’ photosynthesis. By raising the greenhouse atmosphere level of CO2 from 350 ppm to 900-1000 ppm, while also adding water vapor and heat, plant growth rates can be increased by up to 20 percent. In this manner, about 107,526 sq.ft. of flowers can be fertilized with CO2 by combusting 1236 cu.ft. of natural gas per hour, while 107,526 sq.ft. of vegetables can be fertilized by combusting 2130 cu.ft. per hour. With these two Caterpillar G3516LE engines, 268,817 sq.ft. of flowers benefit from this system. The catalytic treatment of the exhaust gases is necessary to reduce unwanted exhaust components like oxides of nitrogen, carbon monoxide (CO), ethylene and other uncombusted hydrocarbons which would otherwise be harmful to the plants and operating personnel. The catalyst system for this application is a novel design incorporating Selective Catalytic Reduction (SCR) and oxidation catalysts to reduce oxides of nitrogen (NOx), CO, ethylene (C2H4), and other hydrocarbon (HC) emissions. NOx reduction is accomplished through the reaction of ammonia with NOx on the surface of the SCR catalysts. Ammonia is generated through thermal hydrolysis of urea, which upon decomposition inside a specially designed mixing duct, yields ammonia (NH3) and CO2. The catalytic reduction of NOx with ammonia on the SCR catalyst surface forms nitrogen (N2) and water. The catalytic oxidation of CO, ethylene, and other hydrocarbons on the oxidation catalyst forms CO2 and water. The emissions are carefully monitored and controlled, so that the exhaust gas quality comes close to the quality of breathable air. NOx emissions with this system are less than 15 g/GJ or 0.054 g/kW-hr. CO and ethylene are oxidized with excess oxygen in the exhaust across a proprietary oxidation catalyst to form CO2 and water. CO emissions are less than 5 g/GJ or 0.018 g/kW-hr and ethylene (C2H4) emissions are less that 0.35 g/GJ or 0.001 g/kW-hr. Constant monitoring of NOx and CO levels with a self calibrating analyzer package is necessary to control the quality of the exhaust gas to be injected in the greenhouses, as well as to fine tune the amount of urea to be injected upstream of the catalysts. The capital cost for this system was $ 1 7 5 , 0 0 0 . Operating 10 hours per day, the annual urea costs are $3100. The control system requires maintenance on filters and pumps which is scheduled every 1500 hours of operation. When compared to the CO2 burners that were used prior to the installation of the engines and SCR catalyst, operation of the system was extended to 350 days versus 250 days per year, operating costs were reduced by $63,000, resulting in a simple payback of less than two years. Net thermal efficiency of the plant is 96 to 97 percent, and greenhouse gas emissions are significantly reduced through the catalysts and subsequent absorption of the CO2 by the plants. The CO2 fertilizing system is designed and packaged by Hug Engineering of Switzerland, a leader in urea-based, small-scale catalytic offgas treatment plants. To date, Hug has supplied over 400 units throughout the world for soot filtering, NOx reduction, and catalytic oxidation of exhaust from engines fueled with natural gas; propane; landfill and digester gas; No. 2, No. 6, and bunker C oil. Applications include power generation, cogeneration, propulsion engines for ships and ferries, greenhouses, railroad engines, and construction equipment with a wide variety of engine sizes and manufacturers. In 1986, Hug Engineering developed the urea process for use with engine exhaust gases, and works mainly with urea as the reactant material. Urea offers considerable advantages over ammonia in terms of transport, storage and handling procedures. Urea is not subject to the dangerous substance regulations, is not poisonous and can be used without any special protective measures. Urea costs are typically about $0.33 per kg of pure urea in pellet form, and approximately 1 kg of urea is required to reduce 1 kg of NOx.