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Content archived on 2024-05-21

Fireside corrosion in coal-fired utility boilers (CORROSION)


Objectives and problems to be solved: N2O is a powerful greenhouse gas and a precursor of stratospheric ozone depletion. NOx are also responsible for acid rain and photochemical smog. Combustion technology is an important source of both pollutants. The main objective is the development of an advanced, compact, efficient and cost-effective technology, based on the simultaneous selective catalytic reduction of N2O and NOx from combustion power plants by hydrocarbons. This will enable companies to comply with legal regulations at a reasonable cost while an environmental friendly image is created. The project deals with the development/ improvement of active and selective catalysts stable over a meaningful time of continuous operation and with the acquisition of a fundamental understanding of process kinetics and mechanisms. Description of work: The proposed work comprises the following tasks:
· Catalyst Design and synthesis, including preparation of different type of supported and unsupported active catalysts, such as zeolite, clay, carbon, and metal/metal oxide based catalysts, in the form of powders, pellets, extrudates or monoliths.
· Lab-scale testing, which consists of different screening tests for assessment of the catalyst activity and selectivity. Experiments are also foreseen in order to understand the basic scientific phenomena such as reaction kinetics, mechanisms of reaction and the mechanisms of inhibition and deactivation over the catalysts.
· Catalysts Characterisation, using a series of physicochemical techniques, in order to:
a) ensure the effectiveness of preparation and the homogeneity and reproducibility of the fresh samples,
b) seek insights into deactivation of the catalysts and
c) gauge the effect of potential poisoning on used samples.
· Bench-scale testing aimed at the assessment of the lifetime of the catalysts. Two different tests, using simulated combustion gases, are designed for this purpose:
a) accelerated deactivation tests, to simulate any possible deactivation of catalysts during their exposure to real combustion exhaust gases and
b) long-term stability tests (up to 2000 hours) simulating the clean up of flue gases in conditions as close as possible to those found in industrial applications. Eventually, the performance of the catalysts will be evaluated with real combustion off-gases. Expected Results and Exploitation Plans:
· The development of selective, active, stable and durable catalyst prototypes, potentially transferable to various industrial user groups. This will put European industry in a position to offer an efficient, economic and advanced technique for the simultaneous removal of N2O and NOx in off-gas from combustion processes.
· The acquisition of deep knowledge of the nature of selective catalytic reduction of N2O/NOx by hydrocarbons. This should directly influence scientific and technological level throughout Europe and will bring about significant progress in the field of heterogeneous catalytic denoxing of exhaust gases from stationary combustion sources, and thus accelerate the discovery of technically relevant solutions.
The laboratory and combustion rig, corrosion test campaigns produced extensive information on the behaviour of a wide range of materials tested under a variety of conditions. The results provided an approximation of how these materials would perform in environments arising from a range of coal-firing conditions, in situations where biomass and waste materials were fired and co-firing of biomass and waste materials with coal. The data from the 40 MWth test boiler was unique in that it provided corrosion information for a suite of superheater materials exposed in real conditions, which were controlled within a desired temperature range. This work produced detailed information on the corrosion effects arising from the combustion of coal, and the co-combustion of coal with olive husks and coal with meat and bone meal, on the test materials. The test programme completed on the 30 MWth CFB boiler supplied information on corrosion mechanisms that affect superheater materials in biomass fired, circulating, fluidised bed boiler plant, and provided details of some possible remedial measures.

The effects of corrosion on a variety of candidate materials and coated materials were investigated during long-term probe tests, and following a detailed examination of the test specimens, an assessment of the suitability of individual materials and coatings was provided. The effects of fireside additives at reducing corrosive atmospheres were considered during short-term trials involving the addition of sulphur, kaolin and a test mixture to the supplied fuel. The test mixture additive was the most effective with no adverse effects. The addition of Sulphur was aloe effective, however there was also an increase in SO2 present in the flue gas, which could contribute to an increase in low temperature corrosion. The information from the lab and combustion rig experiments, and the industrial scale tests, were collated and compared in order to identify relationships between small-scale investigations and those conducted in industrial plant. These results were used to prepare a report, which provides a consensus view of the current state of knowledge of corrosion mechanisms which apply in superheater and reheater conditions operating at advanced steam conditions, furnace sidewall tubes exposed to conditions associated with advanced primary NOx control technologies, and superheater and reheater elements in boilers fired with biomass/waste materials and the co-firing of these materials. The report includes valuable corrosion rate data of relevant boiler tube materials under gas side conditions associated with these operating conditions.

A total of 5 diagnostic techniques were developed during the project, viz. 1. Two types of bolt-like probes, which are used for directly measuring corrosion rates at desired locations in the furnace and superheater regions of a boiler, were developed. These devices are inexpensive, simple to operate and provide an accurate corrosion rate determination, based on metal wastage measurement for periods of around 8,00 hours. 2. Two types of probes, which provide corrosion rate data, based on electrochemical measurements. These devices were not completely proven during this project, however there was sufficient evidence to suggest that both have potential for commercial application. 3. A probe device, which makes metal thickness measurements based on a pyrometric technique. Notable progress was achieved, but the system still requires further work before it can be tested on a commercial boiler. A computer model based on CCSEM fly ash data and Computational Fluid Dynamics, to predict fireside deposition and corrosion in coal-fired utility boilers was developed. The model consists of a number of components, each of which address a specific activity including, volatile matter species release from host particles, gas phase chemical reactions, mineral matter transformation, particle dispersion, particle deposition on furnace walls and heat transfer tubes, and flow within the tube bank zones and rate of corrosion build up.

All of the experience and knowledge gained from the project was used to produce a guideline document, which provides an overview of the current state of knowledge of high temperature corrosion occurring in advanced coal-fired plant, relevant to the following scenarios: 1. The operation of superheater and reheater elements at advanced steam conditions. 2. The deployment of advanced primary NOx reduction technologies in utility boilers. 3. The combustion of biomass and waste materials and the co-firing of these materials with coal. This document provides authoritative and quantitative information and guidance on the interpretation and use of the information by boiler designers and operators.

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Porterfield Road
United Kingdom

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Participants (5)