Low-Temperature Corrosion in Biomass Boilers Fired with Chemically Untreated Wood Chips and Bark

Abstract

Low-temperature corrosion often causes failures of cold-end parts (economizers, air pre-heaters, and fire tubes of hot water boilers) in biomass boilers firing chemically untreated wood chips and bark. The most relevant mechanisms causing low-temperature corrosion are condensation of acids and hygroscopic salts in the deposits on heat-exchanger surfaces. This article offers a short review on acid condensation and presents a detailed study on the formation of hygroscopic salts, which may absorb moisture from the flue gas to such an extent that they are dissolved and form highly concentrated salt solutions. Typical fuel compositions of wood chips and bark in combination with typical operating conditions of biomass grate furnaces have been used to estimate the compositions of ash deposits on heat-exchangers. Based on these compositions, thermodynamic equilibrium calculations have been conducted in the range of 70−400°C, which have revealed that the hygroscopic salts K 2 ZnCl 4 , KCaCl 3 , and CaCl 2 can be formed at these conditions. Finally, recommendations are given on how to minimize the risk for lowtemperature corrosion.

Corrosion Mechanisms and Materials Selection for the Construction of Flue Gas Component in Advanced Heat and Power Systems

“…First, the condensation of acids can cause severe electrochemical corrosion on component surfaces. Moreover, the ash deposits may contain large amounts of hygroscopic salts, such as ZnCl 2 and CaCl 2 , which can absorb water vapor from the flue gas at temperatures higher than the dew point of water . Greater quantities of liquid water subsequently trap more acid gases, making the condensed phase more corrosive to flue gas components.…”

“…Moreover, the ash deposits may contain large amounts of hygroscopic salts, such as ZnCl 2 and CaCl 2 , which can absorb water vapor from the flue gas at temperatures higher than the dew point of water. 111 Greater quantities of liquid water subsequently trap more acid gases, making the condensed phase more corrosive to flue gas components. Kish et al 112 tried to select materials for the construction of condensing economizers in a biomass combustion system in which aqueous H 2 SO 4 condensates were the major corrodants in condensed phase corrosion as the flue gas was chloride-free.…”

Investigation on the Hygroscopicity of Deposits at the Cold-End of Biomass and Coal-Fired Plants

“…The affected equipment mainly includes the economizer, air-preheater, flue gas cooler, precipitator, desulfurization unit, DeNO x unit, and flue gas reheater, where the flue gas temperature ranges from around 300 to 50 °C. In the past, especially for the boilers burning fossil fuels, low-temperature corrosion at the cold-end was typically attributed to the sulfuric acid condensation (sulfuric acid dew point corrosion) on the flue walls and heat transfer surfaces. − According to the results of some corrosion tests and failure analysis, the coal ash deposits containing alkaline metal oxides accumulated on the metal surface might act as a barrier against the condensed sulfuric acid. − However, some current findings suggest that the deposits at the cold-end might also lead to extensive corrosion of the underlying steel or alloy. − …”

“…Some previous studies on low-temperature corrosion in the boilers concluded chlorides, including CaCl 2 , KCl, ZnCl 2 , and FeCl 3 , as the major cause of corrosion of the metal parts at the cold-end. ,,,− Biomass-fired plants, incineration plants, and co-combustion boilers were the focus because of the large quantities of chlorides and other salts formed in biomass and waste combustion. The highly hygroscopic components in the fly ash and deposits interacted with water vapor and resulted in severe corrosion problems on the metal surface.…”

Hygroscopic Properties of Calcium Chloride and Its Role on Cold-End Corrosion in Biomass Combustion

“…In fossil fuel combustion, the limiting factor is mainly the sulfuric acid dew point, below of which sulfuric acid-induced corrosion occurs. However, in biomass combustion, the main limiting factor has recently been shown by several authors to be the formation of hygroscopic deposits. − There are several reasons why H 2 SO 4 (g) or its precursor SO 3 are not found in the flue gases of biomass boilers. Biomass fuels have low sulfur content, and the conversion of SO 2 to SO 3 is low due to the relatively low combustion temperatures and the air staging applied in many systems, particularly in fluidized bed combustion.…”