Miller Magazine Issue: 112 April 2019

85 COVER STORY APRIL 2019 rage, but it was stopped by the high cost of the system itself and the high cost of electricity. It was only in a few places, like Spain, that it became widely adopted, but even in places with warmer temperatures (like Brazil), it is hardly known. Recently, we are seeing many researchers studying (or just cleaning old studies) on the use of nitrogen (N2) and carbon dioxide (CO2) to create a “controlled atmosphe- re” (also known as “modified atmosphere system”) insi- de the grain bins. The trick is the same: filling the grain bins with a cheap gas that would deprive the insects and fungi of any oxygen. Both nitrogen and carbon dioxide are natural occurring gases that don’t harm the environ- ment. They are also non explosive. The issue with carbon dioxide is that it may cause corrosion in steel by increa- sing its alkalinity. And some people may complain about greenhouse effects, but the quantity is too small to care. The other caveat is that MAS doesn’t prevent spoilage af- ter the atmosphere comes back to normal, so you have to keep it sealed all the time you have grain inside the bin. As with refrigeration, this technology has been around for 40 years, but never took off. The reason is that it requires hermetic grain bins (smoothwall bins), and only a couple of bin manufacturers (worldwide) offer this kind. The easy way to have a hermetic grain bin is just using an old-fashioned tank, but they are very expensive. A liquid doesn’t lead to the dynamic stresses exerted by bulk solids in the walls and floor of a bin. This means a smooth wall grain bin would need thicker walls, meaning more steel, meaning more money. You can also cover the grain bin with a tarp, but on large bins it is not possible and it is cumbersome in every case. You also have to pay for the gas, which has to be supp- lied by a company like Linde or Air Liquide. However, you won’t have to pay for electricity, as the grain in a MAS doesn’t require aeration. Now bear with me as I try to make a comparison between the cost of using MAS and the cost of using standard aeration. Cost of nitrogen: EUR 0,70 per cubic meter. Grain bin size: 1.592 m3 Bin diameter: 12,23 m Wall height: 14,92 m Cost of electricity: EUR 30,5 per 100 kWh Standard aeration flow: 9,0 m3/m3/h Fan air flow: 9 * 1592 / 3600 = 3,98 m3/s Air speed: Flow * Bin volume / Bin area = 4,0 / 117,4 = 0,03 m/s Let’s round that up to 0,10 m/s just to be able to use the standard tables. So that gives 30 mm water per each meter height of stored wheat. Let’s call this “grain load”. Fan pressure: Bin wall height * Grain load = 15 * 30 mm water (ap- prox.) = 450 mm water = 44,1 bar The aeration system we need has to handle around 4 m3/s and 44 bar. Considering a global efficiency of around 50%, the fan power must be about 20 kW. Con- sidering 3 hours on everyday, it makes 1.800 kWh every month. That makes about 540 EUR every month of ae- ration. On the other hand, if the space occupied by gas insi- de the grain bin is 50% of it, then you need about 800 m3 of gas. 0,7 * 800 = 560 EUR. But the gas will keep your grain fresh as long as the bin is sealed. Then, every month after the first month is more money for you. All these are just napkin calculations, to show there is a need and a market for hermetic grain bins. I expect you to challenge them and adapt them to your bin sizes and local prices. These calculations may be performed much more pre- cisely, but my idea here is just showing that the order of magnitude is similar and purchasing hermetic silos might be attractive for long term storage. The results are more dramatic if you think that, in a MAS, grain may be stored wet in a warm weather. That’s why I believe the commercial availability of hermetic gra- in bins will lead to a paradigm shift in grain (and flour) storage. There is a lot to be discovered. Another innovation I expect to see in the near future is grain bins manufactured with composite materials. Plas- tics and steel would make a lighter structure reducing the cost of the concrete foundations, which makes a huge portion of the capital cost in a new facility. They would also provide better thermal insulation. A wire mesh would discharge static and provide structural strength,