Electricity and Control November 2020

PLANT MAINTENANCE, TEST + MEASUREMENT

samples are sent to a lab to perform ultimate and proximate analyses on these samples. The fuel and ash testing, a combination of ultimate and proximate analyses, can be done at Bureau Veritas coal laboratories. The analyses define the following: - Fuel constituents (carbon, sulphur, nitrogen, moisture, ash etc.) based on mass fractions - Ash constituents (sulphide minerals, carbonate minerals, oxide minerals etc.) based on mass fractions - Inherent and surface moisture in the fuel - Total volatile matter - Net and gross calorific values of the fuel - Carbon mass fraction (lost energy) in the ash samples. Once the online flue gas analyses and the fuel and ash analyses results are available, the boiler efficiencies can be calculated. All results are processed using the techniques and cor- relations given in BS 845 parts 1 and 2, which is the stand- ard for assessing thermal performance of boilers for steam, hot water, and high temperature heat transfer fluids. The same techniques can be used on various boiler designs and combustion types, such as: pulverised fuel, CFBC (circulating fluidised bed combustion), chain grate, natural gas, bagasse, peat, black liquor, and others. A tried and tested assessment process To start with, documented information related to the boil- er/s to be tested needs to be provided and time given for a process of familiarisation with all aspects of the particular boiler design, past reports and boiler modifications, for as- sessment purposes. This is done offsite. A plant visit, meet- ing the client and a plant walk down with the client follows to check out the operations on site. Preferably, the boiler should be offline and open for an internal inspection to assess the conditions of the heat transfer components. This will be conducted on a separate date from the flue gas analysis because the boiler must be online for the latter tests to be performed. At a time when the boiler is online, an online flue gas analysis is performed, which entails measuring and record- ing flue gas properties at various boiler locations at differ- ent operating loads. Coal and ash samples are taken at the same time. The ultimate and proximate coal and ash analysis is per- formed offsite at the Bureau Veritas laboratory. Once all results are available, indirect and direct boiler efficiency calculations based on measured plant information are performed offsite. Finally, a comprehensive report detail- ing the boiler and combustion efficiencies is compiled and recommendations are made to optimise boiler performance. Regular assessments There are many benefits in conducting regular performance tests on boilers and combustion equipment. These provide peace of mind for clients and the employees on site.

ƒ Alternatively, the indirect method is used which entails measuring all the common losses on a boiler and then subtracting all the losses from 100%, to determine the boiler efficiency. The common boiler efficiency losses include heat losses due to: - dry flue gas - evaporation of water formed due to H 2 in fuel Industrial steam boilers used in process plants also require regular checks and testing to maintain optimum operating efficiency.

- evaporation of moisture present in fuel - unburnt combustible gases in the flue gas - unburnt fuel in fly ash - unburnt fuel in bottom ash - radiation and other unaccounted loss.

To quantify most of the efficiency losses, a flue gas analy- sis is performed. This entails taking flue gas measurements during online boiler testing. In the flue gas analysis, the fol- lowing are measured: - Boiler flue gas temperature - Oxygen (O 2 ) In the case of coal or other solid fuel firing, in order to quan- tify the heat loss caused by unburnt fuel in the bottom ash and to determine the moisture content in the fuel, a fuel and ash analysis must be performed. Samples of the fuel (be- fore combustion) and ash (during online boiler testing) are sampled while boiler testing is performed. After testing, the Jan-Hendrik Fourie has worked in the power and utilities industry for more than a decade. He holds a Master’s degree in Nuclear Engineering, a Bachelor’s degree in Mechanical Engineering and qualifications in a host of high-tech courses. Fourie is an accredited Professional Engineer with the Engineering Council of South Africa (ECSA) and is registered at the Association for Energy Engineers (AEE) as a Certified Energy Manager. At Bureau Veritas South Africa he is a member of the Technical Centre Africa team. He has a range of experience unique to pressure equipment such as boilers, piping, heat exchangers and pressure vessels, including condition monitoring of utility boilers. - Carbon dioxide (CO 2 ) - Carbon monoxide (CO)

24 Electricity + Control NOVEMBER 2020