Modelling of co-firing wood and non-woody biomass in an industrial grate furnace

Co-firing non-woody biomass residues in grate-fired combined heat and power plants offers a promising route to improve resource efficiency. However, it can introduce significant challenges in combustion stability and furnace operation ability due to biomass diversity in moisture content and composition. This study investigates the potential of co-firing non-woody biomass in a 15 MW industrial grate furnace originally designed for woody biomass combustion. A numerical framework is developed that couples a stand-alone 0D bed model, based on empirical pyrolysis relations, with a CFD model of the freeboard region of the furnace. Quantitative temperature measurements at six locations in the furnace, as well as qualitative measurement trends, are accurately reproduced, validating the model at full load. The validated model is applied to assess the influence of co-firing 30% non-woody biomass, including grape, pepper, and cucumber residues. Results show that fuel moisture content is the dominant parameter governing combustion behaviour, rather than the composition of the non-woody biomass. Pepper residues, with a moisture content of 43.2 w-% similar to wood (45.0 w-%), can be co-fired without significant changes in furnace behaviour. In contrast, co-firing dry grape residues (9.0 w-% moisture) requires reducing oxygen by 6.1% and increasing flue gas recirculation by 83.3%, whereas co-firing wet cucumber residues (78.0 w-% moisture) requires the opposite adjustments of an increase in oxygen and reduction of recirculation to maintain stable, high-conversion combustion. Overall, this work demonstrates that co-firing non-woody biomass in industrial grate furnaces can achieve high conversion rates of above 0.9964 at consistent operating temperatures ( ± 1 . 5 % ), either through direct substitution or by adjusting operational conditions.