Carbon Dioxide Enrichment in Greenhouses
Sources of Carbon Dioxide for Greenhouses
Practical application of supplemental carbon dioxide to the greenhouse usually requires a relatively pure source of carbon dioxide, a distribution system, and a monitoring and control system capable of maintaining set levels. The more practical sources of carbon dioxide include; (1) pure carbon dioxide from compressed or liquid carbon dioxide tanks, (2) carbon dioxide generators from the combustion of fuels, usually propane or natural gas, or (3) boiler stack carbon dioxide recovery systems. The economics of equipment and fuel costs often dictate choices for a greenhouse size and location. In greenhouse environments, carbon dioxide delivery is generally done via inflated polyethylene tubes with small holes running the length of the tube placed below or within crop canopies.
Liquid Carbon Dioxide Supplementation
L method of obtaining carbon dioxide is onsite delivery and storage of pure food-grade carbon dioxide as either a liquid or as compressed gas. Larger operations typically use liquid carbon dioxide, while small operations may use compressed carbon dioxide. Liquid carbon dioxide is stored in pressurized tanks, usually just outside the greenhouse. The compressed carbon dioxide is in a liquid state and must be vaporized through vaporizer units before entering the greenhouse. The distribution system for liquid carbon dioxide in the greenhouse is simpler to design and install.
Carbon Dioxide Generators
Carbon dioxide generators (Figure 9.2) using hydrocarbon fuels (e.g., natural gas, propane, and kerosene) are common carbon dioxide sources in the greenhouse. Some manufacturers make burners in which either natural gas or propane can be used, as well as units with adjustable outputs. The amount of carbon dioxide produced depends on the type and purity of fuel. The carbon dioxide burner capacity ranges from 20,000 to 60,000 Btu per hour and can produce 8.2 pounds of carbon dioxide per hour by burning natural gas. The size of the unit (Btu’s produced) and the degree of horizontal air flow in the greenhouse determine the number and the location of these units.
Boiler Stack Carbon Dioxide Recovery Systems
Alternatively, a portion of the flue gas from natural gas boilers connected to hot water heating systems can be directed into the greenhouse as a means of supplementing carbon dioxide to the crop. The boiler should be equipped with a flue gas condenser to reduce the flue gas temperature and take the moisture out of the flue gases, avoiding their entrance in the greenhouse. The boilers should have monitoring systems that safe guard against flue gas introduction when the carbon monoxide level is higher than a set level. The system is designed with a small capacity ventilator with low suction resulting in a fixed volume of the flue gases.
Control of Carbon Dioxide Levels in the Greenhouse
TThe simpler forms of carbon dioxide injection control use either a time clock or a light sensor to turn the carbon dioxide generator on in the morning and off in the evening. During the day, the carbon dioxide generator is automatically turned off when the ventilating fans are on. In the event of roof ventilation, mechanical switches are installed on the ventilators to allow the carbon dioxide generator to operate only when the vents are closed. More sophisticated carbon dioxide injection systems usually consist of a carbon dioxide generator, a control system, and a feedback monitoring system for monitoring carbon dioxide levels in the greenhouse. The monitoring device is usually an infrared gas analyzer (IRGA).
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