Corrosion in Biomass Combustion Systems

There is growing concern over the effects of global warning. In response the power generation sector is having to consider a wider range of systems and fuels for use in generating heat and power. One of the classes of solid fuels that is being increasingly developed is biomass, which is regarded a both sustainable and carbon neutral. In fact, the term biomass covers a wide range of fuels from waste products, such as straw, forestry wastes and sawdust, through to purpose grown energy crops, such as coppiced willow and miscanthus. To maximise combustion plant efficiency it is necessary to use high temperature/pressure steam turbines. However, to generate such steam conditions, the high heat exchanger surface temperatures can interaction with the various potential products of biomass combustion to cause excessive deposition and corrosion of these surfaces. This paper considers the range of heat exchanger operating environments that can be produced by the combustion of different potential biomass fuels, especially the effects of the higher K and Cl contents of the faster growing biomass fuels. This paper reports the results of a series of laboratory corrosion tests that have been carried out to assess the effects of various types of biomass on the corrosion of high temperature heat exchanger materials in combustion plants. The corrosion tests have been carried out using the deposit recoat method in controlled atmosphere furnaces. Six 1000 hour tests have been carried out at typical superheater / reheater and evaporator conditions (450-600°C) using simulated deposit and gas compositions, which have been selected on the basis of potential biomass fuel compositions. The five metals exposed in this study are widely used in power plant heat exchangers: 1% Cr steel, 2.25% Cr steel (T23), 9% Cr steel (T91), X20CrMoV121, TP347HFG and alloy 625. During the course of the tests, the material degradation was monitored using traditional mass change measurements. In order to produce statistically valid data on the actual metal loss from the materials, the performance of the materials in these tests was determined from dimensional metrology before and after exposure: pre-exposure measurements were made using a micrometer; post-exposure measurements were made using an image analyser system. SEM/EDX and XRD analyses have been used to confirm corrosion mechanisms and their association with corrosion damage levels. For each material, the dimensional metrology data have been used to determine the sensitivity of the corrosion damage to changes in the exposure conditions (e.g. deposit composition, gas composition) to generate models of the corrosion performance of the materials. The corrosion data and model outputs have been compared with data available from power plants operating on coal, straw or wood fuels.