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Introduction
Supplementation of CO2 is increasingly popular as a method of increasing the agricultural production from greenhouses. The advantages are not simply restricted to more and larger fruits, but the plants will also grow more quickly. In this way it is possible to be the first to bring local produce to market, which will always ensure the best price for the farmer. Conventional supplementation systems are also popular in countries where heating of greenhouses either not necessary or not generally practiced. In our latitudes there is simply no alternative to a certain degree of heating to ensure the survival of the less hardy plants in winter. Especially now, when imported fruit and vegetables are on sale earlier and cheaper than ever before, the local producers must find a way to also bring their material to market earlier and at a lower cost.
The conventional supplementation system uses liquid carbon dioxide with a fixed dosing valve to produce an increased level of CO2 during the day. There are also systems using propane burners to produce the required CO2. butane burners would be more efficient, but butane is liquid at -1°C, which could prove a problem in winter! With these systems, the heating effect is counted as a useful byproduct, but no more. Natural gas, however, produces more water vapour and less CO2, making it less useful for pure supplementation, although it is the most environmentally option for heating purposes, for the same reason.
This technology is epecially popular in Canada, where a increase in the harvest of around 40 % is generally achieved. Due to the lower investment costs, the most popular method there is CO2 supplementation from liquid gas tanks, although the use of atmospheric propane burners is now being actively propagated and used.
It is only partly the investment costs that cause this development. From early childhood on, people are told that exhaust gases are poisonous and therefore dangerous. Nobody is prepared to die to achieve a possible increase in the harvest, so such technology is viewed in a somewhat old-fashioned way. And this attitude is not completely to be discounted, since the present systems are mostly based on rough approximations and similar generalisations. The system is switched on shortly after dawn and switched off again after an estimated time. There are tables giving values stating roughly how much fuel or CO2 is needed for a specified area of greenhouse, and the burner or nozzle are simply switched on at intervals during the day until the stated amount of CO2 or fuel has been consumed.
In our latitudes, where most areas are supplied with natural gas by pipeline and it is generally necessary to heat greenhouses, it would naturally be fairly obvious that the stack gases from the burner can be used for this purpose. Due to the already mentioned mistrust of breathing stack gases, which is certainly not merely simple superstition, it is not really desirable that this type of system should be allowed to simply evolve and be used without any form of control. The modern natural gas burners are relatively efficient, but no burner can turn air and fuel into carbon dioxide and water with one hundred percent efficiency.
This type of burner will also have a longer duty cycle, and it is never possible to completely protect against accidents or defects. Hence it is essential to continuously monitor the stack gases to ensure that the agricultural working area also remains free of dangerous materials. The old rules of thumb are hardly state of the art for a society that has signed the Kyoto Protocol, and more accurate methods are well overdue in this area.
Here we see a system which not only measures the carbon monoxide concentration in the stack gases, but also monitors the carbon dioxide level in the greenhouse. With this equipment there is a guarantee that stack gases that can be considered dangerous are transported safely into the environment as wel as ensuring that an paoopriate level of carbon dioxide is present at all times in the greenhouse, even when the burner is not in use. The measurement of CO2 can naturally also be used to avoid extreme levels of carbon dioxide, which are not only a possible danger to the personnel, but can also be damaging to the plant life. Above certain concentrations of carbon dioxide there is a number of possible disturbances to growth that can occur due to accelerated growth in certain parts of the plant structure.
Naturally, this type of technology is fully compliant with all modern ecological practices. Under normal circumstances the carbon dioxide produced by the burner would be simply vented to atmosphere, but in this way it is partly bound up in the carbon structure of the plants, leading to a total reduction in the CO2 production. The upper level for carbon monoxide concentration in the flue gases is chosen so that the switch from passing the flue gases into the greenhouse to exhausting them to atmosphere occurs at a level that is still well within the legal limit for this type of burner. Thus there is the added security margin of a burner problem being recognised in time when it can be solved easily without causing unnecessary ecological stress.
Monitoring of the gases becomes even more important when alternative fuels are used to heat the greenhouse. It is more than possible that a greenhouse could be heated using biogas. Biogas can be produced very easily by an anaerobic digester fed with waste from the greenhouses. Naturally, this will work better with the addition of animal waste to include some extra nitrogen, but it is quite possible ot produce good quality biogas from green waste alone. It is nt really possible, however, to predict in advance what the composition of biogas will be, hence the products of combustion cannot be calculated, but must be measured. A good biogas plant will produce gas containing roughly 80 % methane, but the most important word here is "roughly"!
For economic reasons all the assumptions here will be based on a system consisting of a main burner fed with natural gas or biogas (or a mixture of both) and a secondary burner fed mainly with biogas. It is a regrettable fact that less biogas is available in the colder seasons, exactly at the time when extra heating is needed, so it is unavoidable in most cases to use at least partly natural gas as fuel. Naturally, an admixture of biogas is possible and desirable at any time, not just for economic reasons, but also to use the carbon dioxide present in the biogas and to avoid the uncontrolled release of CO2 during normal composting. A greenhouse, naturally, has to be heated in the hours of darkness just as much as during the day, if not more so, but there is no need to supplement the CO2 levels at this time. Since photosynthesis is only possible when light is available, the plants will absorb oxygen during the night and produce their own carbon dioxide. Hence there will generally be relatively high levels of CO2 in the greenhouse in the early morning. Since this system is designed to only produce a slight increase in the CO2 levels, this would not cause a real problem, even if the system worked through the night, but the use of a secondary source of carbon dioxide would be wasteful at this time. Outside air contains roughly 350 ppm CO2 on average, dependent on area etc. In a greenhouse at ground level this can easily drop to 200 ppm or lower, which will certainly cause a noticeable slowing of growth. The planned increase is limited to a maximum value of 4000 ppm. This increase should bring a noticeable acceleration of growth, without any undesirable effects or causing risk to the personnel. The flue gases from the burner are monitored to ensure that no possibility of poisoning occurs, even after long periods spent in hte greenhouse. Although the upper limit of 60 ppm in the flue gases would seem to be well above the danger threshold, there is in practice no risk off ill-effects, since the gas will be thoroughly diluted in the greenhouse, hence the true level of carbon monoxide can never exceed single digit ppm values.