TWI Industrial Member Report Summary 977/2010
By Shiladitya Paul
The aim of this work was to design and build a high temperature corrosion test facility with a view to identifying coatings that are resilient to high temperature corrosion in biomass, waste incineration and co-fired combustion power plant environments. This report incorporates the results of, and supersedes, previous TWI report 18470.01/2008/1352.3.
Global renewable energy growth is linked to government targets for the reduction of greenhouse emissions and increase in domestic renewable energy production. In 2007, the UK government agreed with other Member States to an EU-wide target of 20% renewable energy by 2020. The UK share of this target, as proposed by the European Commission, would be the generation of 15% of the UK's energy from renewable sources by 2020, equivalent to almost a seven-fold increase in renewable energy consumption from 2008 levels. While such a target is ambitious, and will be challenging, the UK is fully committed to meeting its share of the target. One way of meeting this target is the combustion of biomass and municipal wastes for power generation. Utilisation of municipal wastes and biomass as fuel has two main advantages ie it reduces the dependence on landfill for waste disposal and produces carbon-neutral power in the case of biomass. Biomass is generally considered carbon-neutral because the carbon released during its combustion is offset by the carbon trapped during the growth of plants.
Several technological problems hinder the growth of power plants using biomass and municipal waste as fuel. The main materials performance issues for boiler applications that must be addressed are:
- High temperature corrosion via permeation of corrosive gases through the scale, and corrosion of the underlying substrate or via corrosive salts.
- Solid particle erosion of the tube due to ash generation.
- Slagging and fouling of the tube due to corrosive ash sticking to the boiler pipes.
Corrosion and slagging of superheater tubes are known to cause unplanned shutdowns of power plant. It is evident that corrosion resistant materials are key to the improved performance of power plant; be they biomass, waste-to-energy or conventional fossil fuel-powered. The increased use of such biomass and waste-to-energy technology is dependent to a great extent on the development of high temperature corrosion resistant materials and processing technologies. The power plant component size and complexity as well as the rather limited ratio of improved corrosion resistance versus increased costs in most cases exclude solutions based on bulk corrosion resistant materials. Under such circumstances, solutions based on low-cost substrate materials (eg steels with sufficient strength at the temperatures envisaged), tailored coating and weld overlay compositions are becoming technically and economically attractive. In addition to the development of new materials and coating systems, existing materials need testing by the industry to demonstrate that they are suitably corrosion resistant. Carrying out such testing in simulated waste or biomass incineration conditions will eliminate materials unsuited to such environments at low cost compared to full-scale industrial trials.
The initial aim of the project was to build a test facility to allow a comprehensive range of corrosion tests to be performed in representative or simulated environments comprising a vapour mixture of water and an active oxidising agent while the test samples are kept in a salt bath at elevated temperatures of ~525°C, ~625°C and ~725°C.
- Review the factors affecting high temperature corrosion of power plant boiler tubes and the coatings available to mitigate such corrosion.
- Design and build a test facility to allow a comprehensive range of corrosion tests representative of elevated temperature biomass combustion.