A novel Friday 13th (Fr 13) risk assessment is used for the first time to investigate the fuel-to-steam efficiency of a coal-fired-boiler (CFB). The aim was to quantitatively determine the risk to reduced CFB efficiency in the face of naturally occurring random (stochastic) changes in key parameters. Data from a new commercial plant in East Java, Indonesia are used. The approach was to define an underlying unit operations model based on the indirect heat loss method, together with an efficiency risk factor (p). Process behaviour is simulated using a refined Monte Carlo (Latin Plypercube) sampling of 20 key input parameters that include coal feed and quality. The CFB is expected to operate at an efficiency of eta=82.82%. Below 77.82% the process is characterized as 'fail' due to the greater coal needed to produce sLearn quantify and qualify, and the potential for damage to Lhe boiler. Results reveal an underlying risk of 73 failures in CFB efficiency per 10,000 operations. This equates on average to three failures each year over a prolonged period. This new information cannot be obtained from alternative hazard and risk approaches. A number of changes to the CFB to cut efficiency failures are illustrated using Fr 13 methodology. It is shown to be a practical design tool for improving process reliability and costs. A major benefit is that it can be used in design synthesis and analysis. Findings will be of benefit to operators and manufacturers of boiler equipment. Crown Copyright (C) 2015 Published by Elsevier Ltd. All rights reserved,