Oxygen reduction (or depletion) technology has long been utilized in the fruit and vegetable preservation sector, preserving perishable products naturally while maintaining their organoleptic characteristics. Now, it's finding new applications in fire prevention by dispensing inert gas mixtures to prevent combustion and by continuously monitoring the environment.

 

We explore this topic further with Mozzanica's Technical Director, Mario Mignacca.

 

Mr. Mignacca, what are the origins of oxygen reduction technology, and how has it evolved for fire prevention purposes?

Oxygen reduction systems originate from controlled atmosphere technology primarily used for food preservation. Fruit Control, Mozzanica's technological partner with 30 years of experience in this sector, traces its origins back to the research of Engineer Bonomi, a pioneer in the field. In 1954 he began constructing the first systems that modify the atmosphere for fruit and vegetable preservation.

When we talk about controlled atmosphere, we mean controlling the concentration of different gases (carbon dioxide, oxygen, nitrogen, ethylene); a comprehensive system that allows you to manage the metabolism of all sorts of perishable goods, thereby extending their shelf life.

Oxygen reduction for fire prevention has developed almost as means of preventing oxidation, working on ignition thresholds based on the protected material.

 

What does oxygen reduction entail for fire prevention?

The new oxygen reduction technologies involve systems that technically aren't much different from those used for oxygen reduction in controlled atmospheres for preservation purposes.

First of all, we need a sealed container because if we want to alter the atmosphere, there must be a confined space in which to adjust gas concentrations relative to the normal external atmosphere. This container is the environment to be protected.

Secondly, a machine capable of reducing the concentration of oxygen is required; typically, nitrogen generators are used. By starting with a closed environment containing air at a normal concentration - 78% nitrogen and 20.8% oxygen - if nitrogen is introduced and air is extracted, a mixture will form in which the concentration of inert gas progressively increases.

The third fundamental element is an analysis and control system to monitor concentrations and introduce nitrogen where and when needed.

 

Oxygen reduction systems work by saturating protected rooms with nitrogen produced on-site. What are the principles behind the technologies used to produce it?

Technologies enabling nitrogen production are based on the atmospheric breakdown of the air we breathe, composed primarily of 78% nitrogen and 20.8% oxygen.

With systems that allow the molecular breakdown of air into nitrogen and oxygen, these two elements can be concentrated. The technologies are divided into membrane separation and PSA (Pressure Swing Adsorption) separation.

Membrane technology consists of a real molecular sieve: compressed air is fed into the capillary sieve, and only oxygen molecules, which are smaller than nitrogen molecules, can pass through. Nitrogen, as it is larger, is forced elsewhere, stored in a tank, and then used to inertize the environment; oxygen, in this case, is considered waste and is released into the atmosphere. PSA technology also relies on atmospheric separation by molecular size. However, oxygen is forced into the pores of activated carbon, which are roughly the same size as oxygen molecules. In this case, nitrogen cannot be extracted by the activated carbon and passes through the system.

Membrane technology is a continuous production technology while PSA uses two phases: the production phase and the regeneration phase, since the number of pores is limited. Therefore, when all the activated carbon - which is in the form of small pellets - is “saturated” with oxygen, the regeneration phase begins. This is why a PSA system consists of two banks, one in operation while the other regenerates; with exchangers, a continuous flow of nitrogen can be ensured.

 

In fire prevention applications of oxygen reduction, which nitrogen production technologies are used?

In applications of oxygen reduction for separating the gases that make up the atmosphere, both membrane and PSA technologies are utilized. For smaller-scale applications, where there is a need for lower flow rates, membrane systems tend to be preferable because they are simpler in construction, more economical, and have lower maintenance requirements. The PSA system, being more refined, allows for a better balance and economic advantage because it uses, for the same flow rate, a lower amount of electrical energy, working at lower pressures where it is already highly efficient. It is used for larger plants with consistent flow rates.

 

In the design or modification of the building envelope for the installation of an oxygen reduction system, ensuring gas tightness is crucial. What factors influence it?

Addressing all aspects of gas tightness is not easy, especially because we often work in environments that were not originally designed to maintain gas concentrations different from atmospheric ones. You have to pay close attention to all the openings and all the details: floors, panel joints, masonry, openings, windows and all the architectural components.

Usually, a Blower Test is conducted by pressurizing the environment with a fan that introduces air, aiming to maintain an overpressure within the area. It's one of those tests performed to evaluate the energy efficiency of buildings, albeit more stringent in this case because it's not about heat dispersion but about gases, which are easier to obtain and more critical as they could undermine the project's purpose.

It is therefore crucial to ensure the sealing of all elements of the structure, especially in the building envelope where sandwich panels are present. It's important to check the joints between panels and panel-floor junctions; any hole made in the envelope, no matter how small, must be sealed. All the entrances and exits must not be underestimated either. For example, in the case of an intensive warehouse, the loading bays for material entry and exit need to be managed; strategies need to be implemented to ensure that not too much nitrogen is lost from these exits, which are normally opened and closed multiple times per hour.

An envelope composed of sandwich panels like those of a large intensive warehouse, in some ways, may be easier to make airtight; a traditional brick structure, with the opening of doors and windows, can be more complex.

To be continued in the next issue...