In the last posts of this series we covered phosphate-based internal treatment programs, presenting the main characteristics of the Coordinated, Congruent, Equilibrium and Continuum Phosphate programs. These four programs have in common that they add suspended solids to the boiler water, which then have to be removed by blowdown. From this post on, we turn to AVT, All Volatile Treatment, a treatment that adds no solids at any stage.
Following the technological development of boilers, we reach a pressure range in which the supercritical condition is attained, 22.06 MPa / 221 bar and 374 °C, where there is no distinction between liquid and vapor.
The main advantage is higher thermal efficiency. Working at higher pressures and temperatures, the boiler extracts more work from the fuel and reduces specific consumption and CO₂ emissions per MWh. On the other hand, water quality must be very strict, because there is no blowdown/concentration mechanism like the one typical of a subcritical drum boiler.
Under these conditions, phosphate-based programs reach their limit. Without blowdown, suspended solids would accumulate and travel entirely to the highest-temperature sections of the boiler and to the turbine, forming deposits on critical heat-transfer surfaces and damaging the blades. In this scenario, phosphate is not just an unsuitable choice. It is incompatible with the architecture of the cycle.
AVT, because it uses exclusively 100% volatile substances, is not subject to this limit. pH is controlled with ammonia or neutralizing amines, which travel with the steam without leaving solid residues in the remaining liquid phase.
Since no solids are introduced into the system, concentration control by continuous blowdown is no longer a requirement. This characteristic makes AVT compatible with both drumless cycles and conventional ones, and explains why it is also adopted in some subcritical drum boilers. However, its use in subcritical boilers raises some concerns, which will be addressed in the next posts.
AVT became a reference practice through EPRI and ASME along with the expansion of supercritical units in thermal power generation from the second half of the 20th century. In those plants, the absence of a drum was not a design option, it was an inherent characteristic of the steam generator technology. This turned the development of a volatile-substance-based treatment into an engineering necessity, not an operating preference.
AVT is divided into two variants: AVT(R), reducing, and AVT(O), oxidizing. The fundamental difference between AVT(R) and AVT(O) is the redox potential of the circuit, that is, whether the water chemistry is kept in a reducing or an oxidizing condition.
In AVT(R), the aim is to keep the circuit with practically no dissolved oxygen, using a reducing agent. It is used when there are copper alloys in the feedwater system. The reason is that oxidizing conditions can favor the oxidation/corrosion of certain copper components.
In AVT(O), no reducing agent is added to eliminate oxygen. On the contrary, a small controlled concentration of dissolved oxygen is maintained. It is used in systems that are entirely iron-based. The goal is not to "corrode" the steel, but precisely to allow the formation of a very thin, adherent oxide layer, mainly on carbon steel surfaces. This layer works as a protective barrier. One of the great benefits is the reduction of FAC (Flow-Accelerated Corrosion).
AVT is not an evolution of phosphate, nor a universal substitute for it. It is a response to a design constraint that solids-based treatment simply cannot meet. Where there is a drum and continuous blowdown, phosphate remains a solid option, with decades of documented history, as discussed in the previous posts of this series. Where those conditions do not exist, AVT is the technically viable path.
In the next posts we will detail AVT(R) and AVT(O), the objective criteria for choosing between them, pH control with ammonia and neutralizing amines, and the reducing agents available on the market.
Does your plant run a once-through or supercritical cycle, and are you unsure which AVT variant applies to your case? Talk to us for a technical assessment.