Effective exhaust ventilation with minimal energy consumption

Heat recovery is key

Incineration systems oxidize organic compounds in the exhaust air into carbon dioxide and water. Depending on the processes, this requires different temperatures. Conventional thermal incineration (TNV) operates at temperatures exceeding 760 degrees Celsius. In catalytic incineration (CPC), the temperature can be significantly lower, as oxidation using a catalyst requires temperatures of only 280 to 350 degrees Celsius. The RTO operates between 830 and 850 degrees Celsius and is designed for high energy efficiency. How does this work? Ceramic heat storage media absorb the heat from the purified exhaust air and transfer it to the incoming raw air. “In this way, heat recovery rates of more than 95 percent can be achieved. If the exhaust air contains sufficient combustible components, their oxidation provides the necessary energy. Thus, after preheating and provided the exhaust air has a sufficient concentration, the system can operate with virtually no additional fuel,” explains Dirk Jackisch.

Every process has its merits

However, the RTO cannot fully leverage these advantages in every application. Dust, condensing, or sticky components can place a strain on the ceramic heat storage elements. Furthermore, in cases of small exhaust air volumes, short operating times, or frequent starts, the investment and heating costs may make the processes less suitable. “The first step is always to analyze the exhaust air stream(s). Only once the volume, constituents, and operating conditions are known can the technically and economically suitable processes be selected,” emphasizes Jackisch. For example, while a TVN would require more fuel if lightly contaminated exhaust air needs to be heated to high temperatures, it is robust against varying constituents, fluctuating concentrations, and particles. The KNV, which operates at significantly lower temperatures, can be energy-efficient when the exhaust air has a consistent composition. The exhaust air treatment expert points out that certain constituents can damage the catalyst or impair its effectiveness.

Demand from Biomethane Processing

A growing area of application for RTO systems is the conversion of biogas into biomethane. As a result, the demand for technically reliable solutions for treating contaminated exhaust air and low-concentration gas streams is also increasing. While the total number of biogas plants in Germany has largely stagnated for years, the number of plants for upgrading biogas to biomethane is growing significantly, particularly at the European level. According to data from Gas Infrastructure Europe (GIE) and the European Biogas Association (EBA), the number of European biomethane plants has increased from 729 to 1,678 since 2020, representing growth of approximately 130 percent.

Raw biogas consists primarily of methane and carbon dioxide and contains other associated substances. To be fed into the gas grid, it must be processed to meet the required gas quality standards. Depending on the treatment process, the separation of carbon dioxide produces a residual or low-quality gas stream that may still contain methane. This so-called methane slip should be kept as low as possible, since methane is a particularly potent greenhouse gas. For the treatment of such low-concentration gas streams, the guide “Biogas Processing and Injection” lists, among other methods, catalytic incineration, RTO, and special low-concentration gas burners.

“We have already equipped numerous biogas plants with our systems. Currently, however, we are seeing increased demand from the biomethane industry, which is certainly related to the fact that renewable gas is expected to increasingly replace fossil natural gas in the future,” explains Jackisch. The European REPowerEU program, for example, aims to massively increase biomethane production in order to reduce dependence on fossil natural gas imports. National incentive programs and feed-in regulations support this expansion.

“Since methane is chemically very stable, the RTO must be specifically designed for this application. This requires a sufficiently high temperature, a defined residence time, and reliable mixing of the gas stream,” says Dirk Jackisch.

Safety Even Outside Normal Operation

Whether an RTO operates continuously or is primarily on standby for special operating conditions depends on the specific biomethane processing plant. If residual gas containing methane is continuously produced during normal operation, it must be treated or recirculated on an ongoing basis. Other plants primarily need the RTO during startup, shutdown, when gas quality is insufficient, or in the event of malfunctions.

“An RTO with few operating hours is not automatically underutilized. Its role may be to ensure compliance with emission standards precisely when the regular treatment process is deviated from,” explains Jackisch.

There is no general regulation mandating that every biomethane plant must have an RTO. However, the operator must demonstrate how emission requirements are met even under non-standard operating conditions. The technical solution required for this is determined by the plant design and the respective permit.

Practice Is the Deciding Factor

The RTO impresses with its high heat recovery, but it is not a one-size-fits-all solution. The TNV excels in robustness, while the KNV offers low reaction temperatures. Which technology operates most efficiently is therefore determined not solely by the processes themselves, but by how they interact with the actual exhaust air stream(s).

“The best system is the one that reliably cleans the exhaust air stream(s) over the long term while requiring as little additional energy as possible throughout its entire operation,” summarizes the Venjakob expert.