HortResearch Publication - The A to Z guide for using CO2.
CO2 enrichment up to 700 - 1000 ppm boosts the production of greenhouse crops. There are various methods for elevating the CO2 concentration in greenhouses: (1) using flue gases from burning fossil fuel; (2) inserting pure CO2 gas; (3) using organic mulch that produces CO2 and (4) opening windows to let fresh air in. This article is about using flue gases for CO2 enrichment, and about requirements to fuel and burners.
SUITABLE FUELS
Burning fossil fuel produces a mixture of mainly CO2, water vapour and air. As an example, combustion of 1 m3 natural gas produces almost 2 kg CO2, and flue gases from natural gas contain around 10% CO2 (if not diluted with extra air). Flue gases therefore are an excellent source of CO2. But for CO2 enrichment the flue gas may not contain dangerous amounts of injurious components. These can originate either from the fuel or can be formed in the burning process.
The biggest problem coming from the fuel is sulphur (chemical formula S). Some fuels contain large amounts of sulphur and burning this gives rise to sulphur dioxide (SO2), which severely damages plants. So only ‘clean’ fuels, virtually free of sulphur and almost entirely consisting of hydrocarbons, can be used for CO2 production. Suitable fuels are pure propane, butane, LPG, natural gas, premium kerosene (paraffin) and low-sulphur oil. Definitely unsuitable fuels are coal and heavy oil. Most other fuels (wood products) cannot be used for CO2-enrichment because of various contaminants in the flue gases.
It is obvious that pure propane, LPG etc. are safe for CO2-enrichment, but natural gas requires some explanation. Natural gas contains a number of components and sulphur could be one of them. The New Zealand natural gas, however, is basically sulphur free, apart from a minimal spore of sulphur-containing odorization gas that is added. The potential of natural gas can be seen in Holland, where around 98 % of the glasshouse area is heated and CO2-enriched with this fuel. The CO2-enrichment markedly contributes to the high average production figures. In New Zealand too, natural gas is very suitable for CO2 enrichment, but unfortunately natural gas is not available in large parts of the country. Alternative options are the other sulphur-free fossil fuels mentioned above.
NOXIOUS GASES FROM BURNING
Even from a clean fuel, the combustion gases can contain noxious compounds. One reason is that always some nitrogen (N2) from the air is burned, resulting in nitric oxide (NO) and nitrogen dioxide (NO2), together called NOx. Plant growth is reduced dramatically by Nox in the air, often without showing symptoms or damage.
The solution to the NOx problem has to come from technical improvements of the burner, like better mixing of fuel and air, and better temperature tuning in the burner itself. In Holland, where NOx-emission is legally restricted, some manufacturers have developed special low-NOx burners. Generally it pays off to use clean burners, not only for CO2-enrichment but especially for heating, because during heating NOx (and CO2 and other gases) can accumulate to very high levels.
INCOMPLETE BURNING
Another important source of harmful components in flue gases is incomplete combustion caused by lack of oxygen. If the burner receives insufficient air relative to the amount of fuel, some fuel components will get half-burned (yielding unsaturated hydrocarbons). The most harmful to plants is ethylene (C2H4), because it acts as a plant hormone that stimulates the plant maturing and ageing. Another very dangerous gas from incomplete combustion is carbon monoxide (CO) which is fatal to humans. Also there is more nitric oxide (NO) formed under low-oxygen conditions.
Shortage of oxygen can occur very easily in winter, when heaters are in use and ventilators or windows are closed. The burners can use more oxygen than comes into the greenhouse, and the oxygen content of the greenhouse air drops by some percents. This causes incomplete combustion with production of ethylene etc. These gases (and also CO2) then accumulate to dangerous levels.
The solution to this problem is to take care of sufficient air supply to the burner, by having it installed professionally and having it checked regularly. When incomplete combustion is imminent, problems can be avoided by slightly opening the windows or activating side wall fans. The best guarantee for safe CO2-enrichment is by monitoring for incomplete combustion, but this can only be done properly with a central heating system (see next later article).
BURNERS IN THE GREENHOUSE
The most direct and simple (though not the best) method for CO2 enrichment is by clean burners in the greenhouse that release the flue gases into the glasshouse environment. Burners range from very simple small burners just for CO2 generation to large units primarily meant for hot-air heating. Most small burners are suspended, and most bigger models are standing. There are atmospheric burners (with natural draw of ambient air) and fan-assisted burners, also called ventilator burners. The latter have a fan built in for collecting air, either greenhouse air or outside air through a duct. All larger burners should be ventilator burners, and they should be tuned on a sufficiently air-to-fuel ratio to avoid incomplete combustion under all circumstances.
Another disadvantage of some burners is that they can operate on full capacity only, and have to be switched on and off frequently. However, a starting burner produces much ethylene and other noxious gases. There are burners with a low and high flame, or with a modulated flame, that can stay on continuously, resulting in a much purer greenhouse atmosphere.
A good distribution of CO2 is essential for a good effect of CO2 enrichment. Instead of releasing the flue gases freely, it is better to blow them into the greenhouse by a built-in fan. Alternatively, the distribution of heat and CO2 can be improved by large air-mixing fans in the greenhouse or by distribution tubes attached to the burners.
So CO2-enrichment with clean burners inside the greenhouse is a good option, though not ideal. The CO2 level can become too high and even injurious in winter, whereas on sunny days when plants need CO2, just a little or no CO2 at all is supplied. A better, but expensive option would be having a central burner, as will be the topic of a next article.