Detailed study of the impact of co-utilization of biomass in a natural gas combined cycle power plant through perturbation analysis

Co-utilization of fossil fuels and biomass is a successful way to make efficient use of biomass for power production. When replacing only a limited amount of fossil fuel by biomass, measurements of net output power and input fuel rates will however not suffice to accurately determine the marginal efficiency of the newly introduced alternative fuel. The present paper therefore proposes a technique to determine the marginal biomass efficiency with more accuracy. The co-utilization of natural gas and a small perturbing fraction of biomass in an existing Combined Cycle Plant (500MW_th Drogenbos, Belgium) is taken as case study. In this particular plant, biomass is introduced into the cycle as fuel for a primary steam reforming process of the input natural gas. This paper proposes a perturbation analysis that has been developed to allow for an accurate assessment of the marginal efficiency of biomass by using only accurately measurable variables. To achieve this, effects of co-utilization were studied in each component of the gas turbine down to its steam bottom cycle to identify the components most affected by the limited perturbing amount of biomass. The procedure is validated through process simulation, where accurate marginal efficiencies can be compared with the efficiency obtained from the perturbation analysis. A full off-design simulation is required to achieve this result, which was validated through experiments at part load and at variable air inlet temperatures. Through the use of process simulation, the accuracy of the mathematical model could be verified for each formula and each assumption. The model was found to accurately predict marginal efficiencies of the introduced biomass for biomass shares as low as 0.1%.