Desenvolvimento de revestimento para tubulação de caldeiras de centrais termelétricas que operam a carvão mineral

Abstract

Generation is the segment of the electricity industry responsible for producing electricity and injecting it into the transmission (distribution and distribution) systems to reach consumers. Of the thermoelectric generation, approximately 9% of the generation of energy comes from thermal power plants that use coal as fuel. Brazilian coal has a high content of impurities and a low calorific value. The "water wall" pipes of the boilers, in which the water circulates, will become the steam that will drive the turbine are subject to high temperature and aggressive environment of the erosion caused by the gas and particulate fluxes resulting from the incomplete burning of the coal. The metal parts of the interior of the boiler, and especially the pipes of the water wall, suffer from marked wear due to the ash composition of the coal, particles rich in silica, alumina and iron oxide. The process of attacking the metal surfaces, potentially, is able to reduce the thickness of the pipes and, consequently, increase the frequency of occurrence of perforations causing unit unavailability. The objective of the present study was to elaborate a composite coating with ceramic resin and carbon fiber fabric in order to protect and delay the wear process of the metallic surfaces of coal fired boilers. For the characterization and validation of the coating, tests of abrasion wear, abrasive blasting wear, adhesion tests, tensile tests, thermal efficiency evaluation, cost analysis and thermal simulation on the critical temperature of the boilers were performed. The adhesion tests met ASTM D4541 and coatings were considered to be successful in the adhesion to substrate aspect. In the results obtained in the continuous friction and abrasive blast abrasion tests, the coating schemes did not demonstrate significant difference between them in terms of prolongation of the substrate protection time. In this manner, the use of the first coating scheme is indicated. The use of the coating implies a reduction of 8% tube wall exchange and an increase in fuel-related expenses of approximately 3.3%. In the thermal simulation test it was found that the coating degraded after a few hours exposure to high temperature (550°C). Therefore, despite the reduction of thermal exchange efficiency and cost were considered relatively high, this deleterious result on the materials obtained in the thermal simulation at the boilers operating temperature apparently disqualified the proposed coating for use in the specified situation.