CO2 Best practice guide: Possible pollutants

Do not allow excessive CO₂ levels in greenhouses – levels of 5,000 ppm can cause dizziness or lack of coordination in humans.

Higher-than-recommended levels can cause necrosis of old tomato and cucumber leaves. African violet leaves become very hard and brittle, show a very dark greenish-grey colour and often malformed flower petals, which do not fully expand. A similar symptom with freesia flowers has been observed where the CO₂ burner was used to provide the majority of the heat requirements of the greenhouse, thereby generating excessive amounts of CO₂. Except in emergencies, do not use CO₂ burners as the prime heating system.

Ethylene at 0.05 ppm and propylene at higher levels can cause premature senescence on tomato and cucumber plants, induce sleepiness in carnations, create flower shatter of geraniums, promote excessive side-shoot development, prevent normal flower initiation and lead to flower bud abortion in chrysanthemums and poinsettia. Ethylene is often produced as a result of incomplete combustion, while propylene is usually associated with the use of propane. Leaky propane supply lines have created serious financial damage to growers in the past. Carbon monoxide (CO), which usually does not create any problems by itself, is often used as the indicator for incomplete combustion. Levels exceeding 50 ppm CO in the flue gases are an indication of the presence of ethylene at levels capable of causing crop damage.        

The concentration of ethylene measured in greenhouses on commercial nurseries in PC 287 showed that it was unlikely to cause any plant-related problems. However, anecdotal evidence suggests that ethylene damage does occur, especially on nurseries with reciprocating engine CHP. The problems normally occur early in the year when there is little venting and should not be ignored.

Burners with a high flame temperature can cause the formation of nitrous oxides (NO_x and NO₂). Excessive amounts of nitrous oxide can cause diminished growth or even necrosis. Boilers equipped with low NO_x burners must be used for flue gas utilisation as a CO₂ source.

Although highly dependent on the stage of growth and even variety, there is evidence to show reductions in the growth of tomatoes, ranging from 22 to 32%, at NO_x levels as low as 250 ppb. Typically, this was exceeded in all the greenhouses monitored in PC 287, especially when there was no venting and a CO₂ level of 1,000 ppm or more was achieved. In comparison, it was rarely exceeded on the new burners and micro-turbine CHP sites at CO₂ concentrations of less than 800 ppm.

Since sulphur dioxide can cause acute necrosis (0.2 ppm in the air), the sulphur content of the fuels should be less than 0.02%. Heating fuels are not suitable for CO₂ supplementation. SO_x is only present at the levels required to affect plant performance when kerosene is used. To minimise the risk of SO_x damage, the CO₂ level in the greenhouse should not exceed 450 ppm. However, PC 287 concludes that it is likely the yield benefit of allowing 600 ppm of CO₂ will outweigh the SO_x damage.

A low level of both SO₂ and NO_x in combination can cause greater phytotoxicity damage or be more detrimental to the plants than a high level of either. 

PC 110a suggests ways to reduce the risks of overdosing:

• Ensure that carbon monoxide (CO) monitors are working correctly and alarms are set to no more than 20 vpm
• Calibrate CO₂ analysers regularly against standard gases
• Ensure sampling lines are not blocked, otherwise incorrect measurement will occur. This most commonly arises through water collecting in sampling tubes through condensation, especially between greenhouses
• Ensure there are no leaks in sampling lines
• With a number of blocks or zones connected to an analyser through a multiplexor arrangement, there can be a long delay before any area is re-sampled. During this time, overdosing can occur. It would be better to have less zones or more analysers. A system of continuous pumping of samples to the analyser from each zone, with sequential analyses, is also an advantage
• Ensure that burners have an adequate supply of fresh air
• Do not use a CO₂ setpoint of more than 1,000 vpm in the winter
• If sampling pumps fail, by a fuse on the circuit failing for instance, the system must alarm, otherwise the computer will continue to dose if the last sample analysed was below the control set point
• Ensure that delivery pipework, especially any underground, is not blocked. The most common cause of such problems is the accumulation of water in the pipes. These should be carefully levelled to avoid undulations and be bedded on to well-drained material in the trenches
• In the winter especially, arrange for a ventilation purge of a few minutes at the end of the day if there has been little or no ventilation in the greenhouse during the day. Pollution damage depends not only on the actual concentration of pollutants but for how long this level is maintained (the concentration time product or CTP). So a lower level maintained for longer can result in damage, as well as a higher level for a shorter time