Ozone Generators Feed Gas Economics

In general, larger ozone generation systems benefit from the use of liquid oxygen (LOX). In general engineering companies, especially in the drinking water field, automatically recommend LOX based systems. For larger drinking water systems this is almost always the proper approach. In smaller drinking water plants and for some other ozone water treatment applications this may not be the best option. This page discusses the economics of ozone generator feed gas selection.

Oxygen based ozone generators are smaller and use less energy than an equivalent air fed system. They are also simpler in the sense that gaseous oxygen is simply evaporated from LOX. For applications where ozone is mixed with water using a side stream injection system, the size and pump energy also favors. This would be especially true where the degassing takes place in the side stream versus the main contact vessel. As the size of the ozone generation system increases the benefit increases as well.

The benefits of oxygen use are off-set by the cost of purchasing oxygen versus the freely available air. In addition, once a system has been sized for oxygen there is no way to switch back to air feed except with a decrease in production capacity. On the other hand and air feed system can almost double its capacity by switching to oxygen. This means that the economics of a LOX fed ozone generators are dependent vagaries of the oxygen market. This is especially true if oxygen supplier controls the LOX equipment.

Less tangible factors include the reliability of supply for LOX due to plant outages or problems with delivery (floods, hurricanes, snow storms, etc.). Air, of course, is always available. For smaller users, even if there are small benefits to the use of LOX they may be off set by potential supply issues and price uncertainty.

The following analysis evaluates the economics of air fed vs. LOX fed ozone generators for drinking water treatment using bubble diffusers.

Assumptions:

Time Frame 20 Years
No Increase in the price of LOX was considered in this analysis.
Power Cost 0.07 $/kWh
Interest Rate 5%
Ozone Production Rate 450 lbs/day (with 100% redundancy)
Air Fed System
Ozone Concentration 2%
Power Consumption 10.1 kWh/lb ozone
Capital Cost $549,000
Oxygen Fed System
Ozone Concentration 10%
Power Consumption 4.5 kWh/lb ozone
Capital Cost $435,000
LOX System Rental $1,000/month (tank, evaporators, valves and controls)
In order to evaluate the two different systems, the lifetime costs in 2006 dollars was evaluated for each system at a different LOX prices. The results are shown in the table below. The cost of the air fed system is constant since it is not effected by the price of LOX. As LOX prices increase the lifetime cost of the LOX based ozone generator increases. The break even point between the systems takes place around $90/ton of LOX. it is important to note that LOX price increases were not considered, but it is likely that these prices will increase over time.
Net present Value of System
LOX Pricing ($/ton) Air LOX Difference
50 $2.15 MM $1.74 MM $410 M
100 $2.15 MM $2.25 MM ($100 M)
150 $2.15 MM $2.27 MM ($620 M)
If instead of bubble diffusers a side stream injection system were used for ozone water mixing, the booster pump cost and energy use would be higher for the air fed system since more gas would have to be injected into the water. The air fed system would probably be changed to produce 3-4% ozone at a higher capital cost. This would shift the break even point between the system to higher LOX pricing. Nonetheless, LOX prices can exceed $200/ton, so the air fed system may still be competitive.

The point of the analysis is to demonstrate that the overall economics of the gas feed system should compared before selecting the type gas supply. Other issues should also be considered such as reliability of supply, future pricing of LOX, lifetime of the system, need for expansion, etc.