NO pollution in greenhouses using boiler flue-gases

• Ducting flue-gases into greenhouse atmospheres from centralised, gas-fired boilers has become an increasingly attractive technique for practising summer CO₂ enrichment on long-season crops of tomatoes and sweet peppers
• The atmospheric concentrations of nitrogen oxides (NO_x) such as nitric oxide (NO) and nitrogen dioxide (NO₂) were monitored for nine weeks (from early August to mid October 1990) and for seven weeks (from early May to late June 1991) at Runcton (long-season tomatoes) and Angering (long-season sweet peppers) nurseries.
• In general, the level of NO_x pollution rose and fell concomitantly with the rise and fall in the CO₂
• The level of NO_x pollution was similar at both nurseries and varied directly from a mean value of 0.06 to 0.14vpm when the greenhouse CO₂ concentration was maintained within 350-500vpm, i.e. the normal range of CO₂ concentrations used for summer CO₂ enrichment of long-season crops of tomatoes and sweet peppers.
• Regression analysis was used to show that within the range 350-500vpm CO₂, the NO_x level increased linearly by 0.000554vpm and 0.000506vpm for each vpm increase in mean CO₂ at the Runction and Angmering nurseries respectively.
• The ratio of the concentrations of NO:NO₂ averaged 4.7:1 and 2.5:1 at the Runct6ion and Angemering nurseries respectively.
• The risks of injury to long-season crops of tomatoes and sweet peppers when flue-gases from gas-fired boilers are used for summer CO₂ enrichment are probably slight provides that the CO₂ concentration is kept within the recommended range of 350-500vpm.
• Ventilation of greenhouses influecnes the levels of CO₂ and NO_x present in the atmosphere when boiler flue-gases are used for summer CO₂ NO_x levels of 0.5vpm and above were recorded at both the Runction and Angmering nurseries on those occasions (usually dull weather) when low rates of ventilation contributed to a 4- to 5-fold increase in atmospheric CO₂ concentration. Levels of NO_x pollution above 0.5vpm may be injurious to the foliage if the exposure is too prolonged in dull light.

Plentiful supplies of natural gas have encouraged growers to use this premium fuel for greenhouse heating. Natural gas burns cleanly and CO₂ enrichment can be readily achieved by extracting a proportion of the combustion gases, diluting these with air and expelling the mixture into the greenhouse atmosphere through a ground-level network of inflated tubes. The use of flue-gases from centralised, gas-fired boilers from CO₂ enrichment is the preferred method in Holland.

This study was commissioned to establish the NO_x levels present in greenhouse atmosphere when the flue-gases from centralised, gas-fired boilers are used for summer CO₂ enrichment. For this purpose, the managements of John Hall Nurseries Ltd and Van Heyningen Bros Ltd generously made suitable glasshouse facilities available at their Runcton and Angmering nurseries respectively.