The previous article in this series dealt with using flue gases from burners for CO₂-enrichment in greenhouses. Combustion gases contain a high percentage of CO₂. They are free of harmful gases provided they are produced by an appropriate burner from a ‘clean’ fuel, like pure propane, butane, LPG, natural gas, premium kerosene (paraffin) or low-sulphur oil. Flue gases from most other fuels cannot be used for CO₂-enrichment as they would damage the plants dramatically instead of boosting the production. Last time we discussed the use of (small) burners inside the greenhouse; this article deals with CO₂-enrichment from a central burner.

CENTRAL HEATING SYSTEM

Burners produce heat and CO₂ at the same time. In a greenhouse, however, heat and CO₂ are required at different periods: heat is needed when it is cold, often at night, whereas CO₂ is required only at daytime, mainly when it is sunny and warm. Burners inside the greenhouse raise the CO₂-concentration to extremely high levels in winter nights, whereas they provide no enrichment at all on sunny days when the plants need CO₂.

A central heating system has the advantage that the supply of heat and CO₂ can be separated. The burner is located outside the greenhouses and heat is transferred into the greenhouse mostly through a hot water pipe heating system. The flue gases normally go out through the stack, but alternatively they can be pressed into a duct system towards the greenhouse. Thus heat and CO₂ are pumped into the greenhouse apart.

However, using a central heater does not solve the problem that heat and CO₂ are produced at the same time, and required in different periods. This can be solved by having a heat storage facility attached to the central heater. Then fuel is burned at daytime, and the CO₂ produced is pumped directly into the greenhouse; the heat is stored and used the following night. Such systems with a hot water storage tank are in use in thousands of greenhouses in Holland, and also in other parts of the world like Canada. Theoretically there is also the option of storing CO₂ when the burner operates at night, but until now this option has been too expensive.

The CO2-unit attached to the condensor of a large boiler

CO₂ DISTRIBUTION

CO₂ containing flue gases are drawn out of the stack and pushed into a transport duct that runs to the greenhouse. This is done by a special fan, called CO₂-unit. This unit has an option to mix flue gases with air for lowering the flue gas temperature. It is necessary to have a drain pit somewhere in the transport system for removing the condens water from the flue gases. Alternatively, a condenser can be used for reducing the gas temperature and removing the moisture.

Inside the greenhouse the flue-gas-CO₂ must be spread through a distribution net. If there would be only one of a few big outlets, the plants near the outlets would suffer from extremely high CO₂ levels, while plants further away would receive hardly any CO₂. The best CO₂ distribution system is a main duct through the middle of the greenhouse with side ducts in every plant row. The side ducts are lay-flat ducts with small holes on regular distances, and closed at the end. The ducts are either laying on the ground or suspended somewhere at crop height. They should not be longer than about 40 m, to maintain enough pressure over the whole length of the duct. Also, the lay-flats at the far end of the main duct should have enough air pressure. The quality of the CO₂-distribution depends on the capacity of the fan, the resistances in the transport duct and main duct (width, curves), the number of lay-flats, the number, size and density of the holes etc. Designing and installing the distribution system therefore is work for a specialised supplier.

A double heat storage tank for optimal CO2-enrichment

HARMFUL GASES

Burners used for CO₂-enrichment must be carefully installed, adjusted and serviced. With insufficient air-supply, a burner produces ethylene and carbon monoxide (CO). When not burning properly for any reason, it also produces noxious gases like NO and NO2 (together called NOx). These gases, especially ethylene, are extremely dangerous to plants.

Small burners in the greenhouse release the flue gases directly into the greenhouse air. If there are dangerous gases formed, they accumulate to levels high enough to cause damage, but usually too low to be measurable. It is one of the advantages of a central burner that it allows monitoring the flue gases in the main duct directly after the burner. The gases are not diluted with air yet, and therefore the gas concentrations are high enough to be measured. As these measurements are done before the gases go to the greenhouse, the enrichment can be hold back in case dangerous gases are detected.

It is difficult and expensive to measure ethylene and NOx, but there are good and relatively cheap sensors available for carbon monoxide (CO). Therefore central heaters used for CO₂-enrichment are always equipped with a CO-monitor. This sensor is normally set to alarm at 30 ppm CO (measured in undiluted flue gas). An alarm indicates incomplete combustion (burning with insufficient oxygen) and it means there is probably also ethylene and NOx produced. The CO₂-enrichment is then switched off automatically and the flue gases are sent out through the stack.. A well-serviced and calibrated CO-alarm will minimise the risks associated with CO₂-enrichment, and the central heater (using a clean fuel !) then becomes the ideal source of CO₂.

CO2-enrichment form a central heater: the boiler with the condensor and the CO2 unit (from large to small)

CO₂ METER

CO₂-enrichment from a central boiler can best be controlled using a CO₂-meter. Several brands are available in NZ, mostly based on the same measuring principle (infra-red gas analysis). Generally they are good and reliable, but only if they are calibrated regularly against a gas of a known CO₂-content from a calibration bottle. Comparing a CO₂-meter against the outside air is not reliable, because the CO₂-concentration outside varies considerably. Also important is some servicing, for instance keeping the condens water jar empty. Due to incorrect calibration or lack of service, a CO₂-meter can easily indicate 100 ppm too low or too high. This results in unwanted oversupply (very expensive) or undersupply (with low effect of enrichment).

The CO₂-meter has a pump and a tube for continuously drawing air from the greenhouse to the meter. It is possible to monitor several greenhouses on one CO₂-meter, by having a multiplexer and a stronger air-pump. It can be placed in the greenhouse, in a shed or near the computer, but it will not work properly in an environment with extremely high CO₂-levels.

The CO₂-meter can control the enrichment by switching the CO₂-supply on when the CO₂-level is too low, and off when it is above a target level. So the CO₂-meter can work stand-alone with its own display, or it can be connected to the control computer.

A to Z guide for using carbon dioxide in the greenhouse:

1. CO₂ enrichment in greenhouses
2. CO₂ for growth
3. How crops respond to CO₂ enrichment
4. CO₂ enrichment in tomatoes
5. CO₂ enrichment from burning fuel
6. CO₂ from the central burner
7. Enrichment with pure CO₂