Co-firing coal with biomass has gained much interest in recent times by power generators keen on exploiting the environmental and economic benefits. Various trials have been undertaken on small substitution levels of typically below 10% of the total thermal input. Higher substitution levels would expose potential problems in terms of slagging and fouling on heat transfer surfaces. The research study investigated the use of a novel small scale combustor to simulate the conditions of real industrial furnaces. The design and manufacture of the novel combustor is explained with detailed discussion on the developments to suit the combustor for co-firing trials. Successful simulations of a 500kW semi-industrial and a 235 MWe full scale furnaces were achieved. Co-firing trials were performed with three types of waste biomass dried sewage sludge, sawdust and refuse derived fuel. Numerous valuable deposition data was generated during the research study. The data included deposition observations, fouling deposition rates, fuel and fly ash analyses, slag deposition analyses and online flue gas analyses. These would form part of an advanced slagging and fouling predictor. References to traditional empirical indices for slagging and fouling are also included.