Berkel, N., van

English

van Berkel, N.

NARCIS 

Some technical aspects of CO2 enrichment

Name not available

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'fi T 
us 
Some Technical Aspects of 
C02 Enrichment 
By N. van Berkel, Glasshouse Crops Research and Experiment 
Station, Naaldwijk, The Netherlands 
INTRODUCTION 
The subject to be dealt with is divided into three par t s . In the first 
chapter several aspects of CO£ apparatus will be discussed. In the 
second chapter we will look at the distribution of CO2. In the third 
part attention will be paid to the CO2 concentration, which is a func-
tion of distribution. 
APPARATUS 
Demands upon the apparatus. In view of human health, in many 
countries demands are made upon the concentration of gases in the 
working environment. Some of these gases occur in the combustion 
from burners, which are used for CO2 enrichment. This implies that 
burners should not produce too much of harmful components. The 
demands are expressed in the so-called MAC values (maximum allow-
able concentration), at which labourers can work without harm during 
8 hours every day. 
Plants are also sensitive to some components of flue gases, where 
certain concentrations are exceeded. But the harmful concentrations 
for man and plantare different. This is shown in the following example. 
The MAC value for CO is 100 ppm (see Table 1). This means that a 
burner with the combustion gases not piped to the open, should give 
no r ise of the CO-content beyond 100 ppm. With regard to CO, plants 
are less sensitive than men. Beyond values of 500 ppm we may expect 
phytotoxical effects. In many cases plants are far more sensitive than 
human beings. For example MAC-values for SO2, NO2, NH3, O3 and 
formaldehyd are 5, 5, 50, 0.1 and 5 ppm, respectively. But for plants 
the maximum tolerable concentrations for the same substances are 
0.2, 20, 10, 0.2 and 0.7 (see Table 1). Plants are said to be very 
sensitive to ethylene (Crocker et al . 1932). At concentrations of about 
0.05-0.10 ppm, injury on tomato plants should occur. However, recent 
fumigation experiments showed that tomato plants are less sensitive. 
333 
Proceedings of the XVII International Horticultural Congress Volume m. 1967. 
334 XVIIth INTERNATIONAL HORTICULTURAL CONGRESS 
Table 1. Some MAC values and some critical values for plants (ppm) 
MAC va lues (ppm)* 
C r i t i c a l va lues for 
p lants (ppm)** 
SC-2 
5 
0.2 
N 0 2 
5 
20 
N H 3 
50 
10 
o3 
0.1 
0.2 
C H 2 0 
5 
0.7 
CO 
100 
500 
C 2 H 4 
0.05 
C 3 H 6 
50 
*Taken from Archives of environmental health, 1964. 
**Taken from Thomas, 1961, but for CO and C3H6 from Crocker et al., 1932. 
But not only for human health, also for possible danger from ex-
plosion certain demands are made. Distinct rules are given especially 
on the equipment for gases, such as propane. 
Incomplete combustion. Incomplete combustion can be brought 
about in two different ways: (a) The burners do not work well, be-
cause of bad construction, pollution or wrong relation between air 
and gas supply, (b) Gas from leakages s treams along the outside of the 
flames of the burners . Some aspects of incomplete combustion are: 
1. Products of incomplete combustion. The main products of com-
bustion are carbon dioxide and water. But combustion gases always 
contain minor quantities of other products such as carbon monoxide, 
aldehydes, ethylene, nitrogen oxides and propane or methane. The 
better the combustion, the lower the contents of these minor 
components. 
2. Injury to plants. Several cases of plant injury are known. They 
were always caused by burners which did not work properly, or were 
caused by leakages in pipes or pipe-connections. The most striking 
case was with tomatoes. One set of burners out of a range of 22, all 
supplied with natural gas, stopped working in the night, while the 
others kept working normally. The following morning all plants were 
badly injured. The oldest leaves had been burnt, younger leaves were 
partially damaged. Damage might be aggravated by the presence of 
temperature radiators above the plants. 
All other known cases of damage were less severe and arose more 
gradually. On cucumbers we saw leaves gradually yellowing—at first 
the oldest, later on the younger leaves. The same was observed on 
tomato plants, where abortion of t russes also occurred. In both cases 
damage was done by leaking of the gas. Yellowing of leaves and 
abortion of t russes were also observed when burners worked badly. 
After incomplete combustion of natural gas, ethylene appeared to be 
present in relatively higher concentrations than other harmful 
components. 
3. How to prevent injury, (a) The total equipment should be in-
stalled by reliable firms. The best burners should be mounted and 
VAN BERKEL: ASPECTS OF CO2 ENRICHMENT 335 
the best materials employed, (b) Good service must be given. At least 
once a year the total installation should be tested to be sure every-
thing is in a good working condition, (c) Stop CO2 enrichment when an 
acid smell is perceived, which indicates that something must be wrong. 
Call the installer to repair the burner or the installation. Even with 
very little incomplete combustion, one can detect the characteristic 
smell, (d) Buy only burners which are certified as reliable. This 
certificate must be issued preferably by an official bureau. In the 
Netherlands proposals have been made for the foundation of a bureau, 
where all burners for CO2 enrichment will be tested on the presence 
and quantity of harmful components in the combustion gases. Only 
certified burners should be used in horticulture. 
Appliances for paraffin. There are two different burning principles. 
The first is known as burning by evaporating. A well-known repre-
sentative is the "hylo" (see Fig. 1). In the beginning of the C02-boom 
in 1961 this was the only paraffin burner. It was not suitable for 
automatic control and the maintenance costs were high. Besides it 
did not work very reliably. This type is gradually pushed aside by a 
type of burner based on another principle: burning by automizing the 
paraffin, with a fan to provide a desired air stream. It is automatically 
controlled and works very well (see Fig. 2). 
Fig. 1. Paraffin evaporating burner. 
336 XVIIth INTERNATIONAL HORTICULTURAL CONGRESS 
\*"?ÏM- <ts „ V Î T ^ Ç ^ i f s 
Fig. 2. Paraffin atomizing burner. 
Appliances for propane. Many propane appliances are available. 
Mentioning all of these is not necessary. The combustion of the dif-
ferent propane burners can vary greatly however. On this field we 
feel a strong need for a bureau which will test all burners . The con-
struction is based on two principles. On one hand there are high p r e s -
sure burners with pressures for propane of 1 or 2 kgf/sq cm. There 
is a single gas outlet, and there is but little chance for pollution. 
Safeguarding is easy and reliable. When it is in use, it makes an 
annoying noise; this appeared to be an objection to this type of burner. 
On the other hand there are low pressure burners . The gas p r e s -
sure is lower than 30 gf/sq cm. Always a number of gas outlets have 
been joined to one unit. The construction may easily lead to plugging, 
but there is no annoying noise. Many types are known. 
Appliances for pure CO2. In most cases pureC02 is supplied from 
a battery of steel bottles or some other container. The CO2 is led to 
the cultural space, where it flows from very small orifices (inter-
spaced at 30-60 cm) in plastic pipes. 
VAN BERKEL: ASPECTS OF C0 2 ENRICHMENT 337 
DISTRIBUTION OF C 0 2 
The CO2 which is brought into glasshouses must be distributed as 
uniformly as possible. To achieve this one can: (1) Use a fan; (2) leave 
the distribution to convection, which will arise by temperature dif-
ferences between combustion gases and glasshouse air ; or (3) leave 
the distribution to diffusion and accidental air movement caused by 
insolation, heating pipes and wind speed. 
With paraffin as a CO2 source, a fan is always used because only a 
large apparatus is employed. With propane, small units are common 
and convection is the only way of distribution. 
From C02~readings it appeared that one fan of a good capacity can 
give a good distribution when the glasshouse area does not exceed 
about 3000 sq m. On larger a reas more fans are necessary. When 
making use of convection a uniform distribution of CO2 will be ob-
tained, when an apparatus is installed at about every 500 sq m. The 
distribution from diffusion only is very poor. Only in a restr icted area 
near the C02-outlet a clear increase in CO2 concentration will occur. 
For this reason a large number of C02-outlets is necessary in a 
glasshouse. This is illustrated with Fig. 3. 
In a Dutch vinery house (8 x 70 sq m) two propane burners without 
fans were installed at about 18 and 54 meters from the front wall along 
the length of the house. In an identical house pure CO2 streamed out 
of two openings at corresponding places. 
C09(%) 
0.3Û L 
N propane 
pure CO2 
5 172o 35 5255 65 70m 
— , m 
Fig. 3. CO2 concentrations from pure CO2 and propane. (0 = gas outlet; 
B = burner). 
338 XVIIth INTERNATIONAL HORTICULTURAL CONGRESS 
From the C02-readings it appeared that with propane the highest 
CO2 concentrations were found in the middle of the house by accumu-
lation of readily spread gases, while in the case of pure CO2 the 
highest values were found near the outlets caused by "poor" diffusion. 
It is clear, that with pure CO2 one gets only local distribution, while 
one covers a much wider field with combustion gases. 
CONCENTRATION OF C 0 2 
The distribution of carbon dioxide will lead to a certain increase 
in concentration of this gas in the glasshouse. The concentration finally 
reached is influenced by several factors. We will distinguish factors 
influencing the height of the concentration and those which govern 
uniformity. 
In the following it is assumed that the ventilators are closed. 
Height of concentration. Fi rs t we mention two factors, involved in 
dc Fick's wellknown diffusion law: dS = Da — • dt dx 
a = diffusion area; 
D = diffusion constant; 
-r- = concentration gradient; 
dS = diffused quantity; 
dt = time. 
1. Concentration gradient (-7-). The losses to the outside air are 
proportional to differences in concentration between in- and outside 
air . The higher the concentration that is wanted, the more CO2 will 
escape into the open. 
2. Total diffusion area (a). The losses are also proportional to 
the total diffusion area in the glasshouse cover. In the Netherlands we 
observed striking differences between old Dutch light structures and 
modern houses at this point. 
3. Windspeed. The main factor governing the concentration is 
windspeed. It increases the diffusion rate via the openings in the 
glasshouse cover. In a modern glasshouse we observed a drop of the 
CO2 concentration from 0.18% on a still day to 0.06% on a stormy day 
with the same C02~dosage. Near the coast the effect of wind speed is 
of course more striking than on the inland. 
4. CO2 source. The choice of the CO2 source is of less influence 
on diffusion. When pure CO2 is employed the gas enters the glasshouse 
VAN BERKEL: ASPECTS OF CO2 ENRICHMENT 339 
, Km 
Zo.09 
/o.17 
ƒ 0,21 
ƒ 0,21 
/ XB 
/ 0,21 
/0.15 
• 
0,10 
0,17 
0.20 
0,21 
0,22 
0,18 
(%>) 
93m 
IT m 
Fig. 4. CO2 gradient caused by glasshouse shape (in %; 
B = burner). 
at room temperature or below, while CO2 from combustion gases has a 
much higher temperature than the surrounding glasshouse air. The 
pure CO2 has a tendency to stick near the soil because of "poor" 
diffusion, and the combustion gases tend to gather near the top of the 
house because of convection. In the latter case more gas will diffuse 
to the open. From CO2 readings it appeared that going from soil level 
to the top of the glasshouse, the CO2 concentration increased when 
combustion gases were used and no fan is applied. With the application 
of pure CO2, an inverse gradient was found when the CO2 was emitted 
from small plastic tubes near the soil. 
5. Fan. When using a fan the glasshouse air is set into motion and 
diffusion to the open will increase (compare 3, windspeed). In com-
340 XVIIth INTERNATIONAL HORTICULTURAL CONGRESS 
paring the dosing of pure CO2 via tubes with small openings near the 
soil with dosing CO2 from one central opening at a height of two 
meters, where distribution took place by means of a fan, it appeared 
that with the use of a fan about 10% more gas was needed to reach the 
same CO2 concentration in the house, as with dosing via pipes on the 
soil. 
6. Photosynthesis. The effect of photosynthesis on CO2 concentra-
tion varies strongly with the extent of leaf surface and light intensity 
(Wittwer and Robb). 
7. Watering. It is said that watering by overhead sprinkling will 
result in a clear drop of the CO2 concentration. 
8. Soil born CO2. It is known that in cucumber houses large 
quantities of CO2 can be released from the masses of fresh organic 
material used. The amount of CO2 can be so high that CO2 enrichment 
from other sources is superfluous for a long time. With most of the 
other crops the share of soilborn CO2 is of little importance upon 
CO2 concentration. 
Normally the ventilators are closed when extra CO2 is given. Ini-
tial readings showed that a distinct r ise of the CO2 concentration 
occurs, when CO2 enrichment takes place when the ventilators are 
kept open not beyond a width of about 5 cm, and also if the windspeed 
does not exceed a moderate value (8 m/sec) . 
Uniformity of concentration. Some factors are known governing 
uniformity: (1) Shape of the house. If length/width relations are un-
favourable concentration may be uneven, as can be seen from Fig. 4. 
(2) Number of apparatus. The number of apparatus with or without 
fans, together with their spacial arrangement are also of importance 
upon uniformity of the CO2 concentration. The same applies to gas 
outlets for pure CO2. (3) Wind-speed. Especially with high windspeeds 
local uneveness of the CO2 concentration may occur by inducing an 
air flow in the glasshouse opposite to the outside wind direction. (4) 
Height of the crop. The higher the plants, the stronger the influence 
upon the air movement. This implicates, that with increasing plant 
lengths the uniformity in CO2 concentration decreases. 
SUMMARY 
Three aspects of CO2 enrichment have been discussed. (1) Appara-
tus. Demands are made upon combustion gases by man and plant. 
Causes of incomplete combustion, products of incomplete combustion 
and damage done on plants. Several types of burners. (2) Distribution 
of CO2, by means of a fan or without. The influences of apparatus, 
glasshouse cover, windspeed, photosynthesis and weather on (3) CO2 
concentrations. 
VAN BERKEL: ASPECTS OF CO2 ENRICHMENT 341 
REFERENCES 
1. Archives of environmental Health Ü. 1964. Threshold limit values for 1964. 
545-554. 
2. Berkel, N. van. Apparatus for CO2 in greenhouses. To be published in 
Acta Horticulturae. 
3. Crocker, W., Zimmermann, P. W., and Hitchcock, A. E. 1932. Contrib. 
Boyce Thompson Inst. 4: 177-218. 
4. Thomas, M. D. 1961. Air pollution, ed. World Health Organization. Geneva 
233-278. 
5. Wittwer, S. H., and W. Robb. 1964. Carbon dioxide enrichment of green-
house atmospheres for good crop production. Economic Botany 18(1): 34-56. 


Apparatus for CO2 in greenhouses.

Archives of environmental Health Ü.

Carbon dioxide enrichment of greenhouse atmospheres for good crop production.

Some technical aspects of CO2 enrichment

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