- Requires a gas-tight sealed silo – meeting Australian standard AS2628 being a five minute half-life pressure-test.
- Nitrogen generators are required to produce 99% concentration N gas from the ambient air which is: 78% N, 21% O₂, 0.9% Ar, 0.04% C0₂ and water vapour.
- Nitrogen generation units, two designs:
- “Pressure swing adsorption” (PSA) is the traditional style used by CBH at the export port. Requires an annual ‘pressure vessel’ inspection by authorities
- “Membrane” – advantage is it is a ‘flow through’ system with no pressure vessel required.
- These units for smaller silos are typically sold in the 30 to 60 m³/ hour nitrogen output range. Large capacity units can be purchased. They are powered by either diesel or electricity.
- Capital costs of the 30 to 60 m³/ hour Nitrogen generators range from $50,000 to $75,000 from China, or approx. $95,000 Australian built.
- Diesel / electricity operating costs for the generators ranges from $0.70 to $1.00 / tonne.
- An oxygen concentration monitor is required (e.g. Drager XAM 5000 – cost approx. $2,000)
- To achieve effective storage grain insect pest control the aim is to use the 99% nitrogen gas to purge oxygen levels in the storage from 21% to below 1% oxygen concentration.
- Nitrogen gas (99%) is delivered into the base of the silo with the aim of completing at least two air changes to reduce oxygen levels measured in the head-space at the top of the silo to less than 1% O₂.
- Example: Using 60 m³/ hour nitrogen generator on a 100 tonne wheat capacity silo (130 m³ silo). Two flushes of 130 m³ = 260 m³ takes the generator just over 4 hours. Silo is monitored, aiming for less than 1% O₂ at the top of the silo with the top lid just open. When 1% O₂ concentration target is met, the silo top lid is sealed shut.
- Silo remains sealed for a period of 14 – 15 days with grain temperatures of 25°C and above. If grain temperatures are lower e.g. 20°C and below, insect pest control of the full life cycle may not be effective. Extension of time, 3 weeks plus, should help with pest control results.
- While the silo is sealed, O₂ concentration is monitored regularly to check that less than 2% O₂ is maintained. If very small silo leaks allow O₂ concentration to rise to 3 – 4% O₂, the generator needs to be reconnected to the silo and ‘nitrogen flushed’ to achieve the less than 1% O₂ target. Less than 3% oxygen over 28 days is adequate to control all insect life stages. However less than 2% is even better and will shorten the required exposure period.
- Silos sealed to a high ‘gas-tight’ standard will reduce the time, cost & labour to achieve a good result.
Advantages to using Nitrogen:
- Grain quality – high nitrogen atmosphere help to maintain good ‘colour’ in stored pulses and oilseeds – Lupins, Chickpeas, Canola.
- Oil content quality – with a low oxygen environment in storage there is less oxidation of oils in oilseeds – canola, sunflower, linseed.
- Pest control effective when low oxygen levels are maintained for the required time.
- No WHP or venting of grain required. Reduced work place health & safety risks when compared to using phosphine fumigation for pest control.
- A suitable alternative to using carbon dioxide (C0₂) for the organic grains industry.