Tannin Boiler Water Treatment
As a general rule of thumb, purified tannins are a “green technology” substitute for conventional boiler chemistry, but what’s the catch?
The water treatment industry has been utilizing tannin chemistry for decades, although drastically increasing in popularity only recently. And no, this isn’t referring to the polyphenolic dry taste that tannins can produce in certain wines. Natural blends of hydrolysable and condensable tannins have gained attention for several advantages over traditional sulfite/phosphate treatment programs. While the corrosion inhibition properties of tannins have not extensively been studied, the current publications are extremely promising. At this time, there are several thousand power boilers and hydronic closed-loop systems being treated with purified tannins. Personal experience has been obtained with this treatment method, and the boiler inspections have been a glimmering reflection of their effectiveness.
Tannins are polyphenolic molecules found naturally occurring as a byproduct of cellular metabolism typical of plants. There have been thousands of biodegradable tannin molecules identified and some have been shown to form complexes with certain metals, hence the inhibition capabilities. Tannins can either be condensable or hydrolysable. Condensable tannins are the most abundant, comprising up to half of the dry weight of leaves. Hydrolysable tannins refer to a secondary group of plant metabolites that includes gallic acid and those containing acidic properties. A combination of the two is typically blended, extracted from tree bark, and together, create an effective boiler treatment product.
Figure 1) Hydrolysable Tannins: 3D molecular structure of tannic acid (C76H52O46)
Figure 2) Condensable Tannins: 3D molecular structure of gallic acid (C7H6O5)
The blatant advantage of using tannin products is the reduction in toxicity compared to traditional boiler water treatment programs. Tannins are found naturally in water systems as plant biomass decomposes and leaches. And being extracted from plant material, tannin products offer a renewable source that can be harvested sustainably compared to mining for phosphorus, sodium dioxide, or sodium carbonate. Unlike traditional programs, most of the boiler tannin products are produced here in North America.
Another advantage to purified tannin products is the overall conservation of resources. Successful tannin programs have been able to cycle up boilers much higher than the ASME guidelines without sacrificing steam purity. Rather than maintaining a conductivity setpoint of 4,000 uS/cm, tannin-based programs can efficiently run well over 8,000 uS/cm depending upon alkalinity. They do not disrupt the internal surface tension of boiler water compared to traditional treatment programs, so carry-over is not a limiting factor. The addition of tannin products do not contribute to TDS, further reducing water and energy consumption. Tannins serve multiple purposes in a boiler system and are effective as an oxygen scavenger and scale/corrosion inhibitor, although they are typically blended with additional polymer and steamline products. They function as a filming agent, creating an absorption tannate layer to mild steel or aluminum. Tannins have also been shown to be effective at crystal growth modification to remove scale.
Personal experience would suggest that tannin-based programs offer better protection than traditional programs to boilers in wet layup. It can be difficult to maintain a satisfactory residual of sulfite when boilers are in layup. A sufficient tannin residual is not only easier to maintain but continues to serve as a corrosion inhibitor and oxygen scavenger, possibly more efficient at variable temperatures.
Every opportunity comes with some drawbacks. For larger-scale applications, separating the oxygen scavenger, scale inhibitor and steamline treatment products will probably be the most economical choice. Sulfite products are more effective at oxygen scavenging, and so more tannin product would have to be introduced. The natural color of tannins makes it difficult to accurately test other parameters such as iron, copper. alkalinity and hardness. This inhibits a water treatment professional from visually being able to identify any water-related issues. Also, when tannins are not fed continuously, they tend to “gum” up chemical feed pumps and dosing equipment, making it difficult to maintain prime. For most small applications, purified tannins are offered in a concentrated formula, so manual onsite dilutions may be required which leads to more hands-on work.
Overall, purified tannins are another tool in a water treaters toolbox. There are certainly applications where tannins are a better option than traditional chemistry such as small boilers. As most tannin products are offered as an all-in-one, they simplify treatment programs for these scenarios, especially when boilers are not in continuous operation. Although I do favor tannins in certain applications, there are many scenarios where I would not hinder the performance, efficiency, and cost of traditional treatment programs, but having additional tools only allows us to better help serve our customer’s needs.
List of Resources:
 Amarowicz, Ryszard, and Michał Janiak. “Hydrolysable Tannin - an Overview | ScienceDirect Topics.” Science Direct, Encyclopedia of Food Chemistry, 2019, www.sciencedirect.com/topics/agricultural-and-biological-sciences/hydrolysable-tannin.
 Parkinson, Ben. “Sulphite vs Tannin Oxygen Scavenger Treatments - Feedwater Website.” Feedwater Website - Feedwater - The Water Treatment Specialists, 26 Feb. 2016, feedwater.co.uk/sulphite-vs-tannin-oxygen-scavenger-treatments.
 “Purified Tannins for Boiler Water Treatment.” FCT Water Treatment, 21 Apr. 2021, fctwater.com/purified-tannins-for-boiler-water-treatment.
 Dargahi, M., et al. “Green Technology: Tannin-Based Corrosion Inhibitor for Protection of Mild Steel.” CORROSION, vol. 71, no. 11, 2015, pp. 1321–29. Crossref, https://doi.org/10.5006/1777.
 Gaudreault, Roger & Dargahi, Mehdi & Weckman, Nicole & Olsson, Adam & Omanovic, Sasha & Schwartz, Gary & Tufenkji, Nathalie. (2013). Green Chemistry – with a Special Emphasis on Tannin Molecules for the Protection of Aluminum Boilers.