De-risking a tailored firing concept: Advanced CFD engineering for a wood-fired power plant with grate firing
Wood-fired power plants that pair a grate with additional dust burners are rarely built the same way twice. Both firing systems share one furnace, one air supply and one set of emission targets, and each one changes the conditions for the other. A Grate Firing CFD engineering makes these interactions visible while the design can still change, as a recent study for a planned plant illustrates.
The Challenge
A plant engineering contractor was planning a wood-fired power plant for an end customer, with a grate firing and additional wood dust burners in one furnace. At a comparable plant in operation, slagging and deposits had formed in the furnace. The new design was therefore to be examined before its geometry was fixed.
The questions spanned the arrangement of the injection firings, the position and inclination of the nozzles in the side wall, the distribution of air and recirculation gas, and the effect of a refractory lining compared with a bare membrane wall on furnace temperature. On the process side, the study considered temperature hot spots, the position of the fire front under hot impregnation air as primary air, the slagging potential in the front of the furnace and the influence of the interacting firings on CO and NOx.
Our Approach
We built a CFD model of the planned furnace in which a one-dimensional grate model is coupled bidirectionally with the three-dimensional gas phase. On the grate, drying, devolatilization and char burnout run through consecutive control volumes while the solids and their heat travel with the grate. The gas phase receives fuel surface conditions and combustion products from the grate and returns radiation and gas conditions, with fuel particles and CO and NOx formation models part of the same calculation.
The base design revealed several weak points. One of them sat at an injection lance: its large particles landed in a concentrated spot near the ash discharge, where little air was available, putting complete burnout before the ash discharge at risk. We varied the position, rotation and inclination of the lance toward the solids feed and compared the variants iteratively on their temperature and velocity fields.
Results for the Lance Example - Grate Firing CFD Engineering
Base design | Modified design | |
Landing zone of large particles | Concentrated, near the ash discharge | Moved by about 1.5 m against the grate's transport direction |
Fuel mass from the lance reaching the grate | Reference | Almost unchanged |
Air supply at the landing zone | Little | Higher |
Residence time on the grate | Reference | Longer |
Secondary air at the boiler front | Reference | More even, with a better oxygen distribution above the secondary air level |
All results come from the design phase and describe the planned plant in the model.
What It Means for the Design
The lance position and the other weak points could be addressed while the design was still on paper. Landing position, residence time and local air supply are coupled, and the model made that coupling visible before anything was built. The decision on the final design remains with the project team, and the effect in operation becomes measurable once the plant runs.
Your Next Step
Check the design in the model before anything is built. If your project combines one or several firing systems in one furnace, get in touch to discuss which interactions belong in the study.
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