Downward and Upward Combustion of Biomass on a Grate: A case study

XXIV International Symposium on Combustion Processes, September 23-25, 2019, Wroclaw, Poland

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Downward and Upward Combustion of Biomass on a Grate: A case study

Miladin Markovic,, _{Tamoor Mughal}*_{, Artur Pozarlik, Gerrit Brem}

1 _{University of Twente, Faculty of Engineering Technology, Thermal Engineering, P.O.Box 217, 7500 AE, Enschede, The} Netherlands

* corresponding author: m.t.mughal@student.utwente.nl

Keywords: Biomass, Reverse Combustion, Grate Furnace

Grate firing is widely used technique for biomass combustion. This is mainly due to its high flexibility and minor requirements regarding fuel pre-treatment. In industrial grate fired furnaces, the main combustion configuration is downward combustion on the grate, the solid combustion is in contra flow with the combustion air. However, conventional grate combustion faces some problems with complete combustion and process control. The processes of drying, devolatilization and burnout occur irregularly over the grate, which may result in non-uniform and unsteady release of volatiles. This can lead to formation of fuel rich zones in the freeboard height and thus some volatiles may leave the boiler unburnt [1]. The upward combustion is a new concept and is very promising because it is less sensitive to fuel moisture content, has low ash entrainment with rapid conversion of solid fuel and decouples drying, devolatilization and burnout phases [2].

In this research, gaseous combustion of volatiles in freeboard, applying Computational Fluid Dynamics (CFD), is investigated. Commercially available CFD code Ansys Fluent v19.2 is used. A base case model is developed for the conventional downward combustion and validated with warranty measurements from the commercial boiler. The results show overall good agreement between experimental and numerical data with average error of 7.2% for thermocouple measurements and 1% for Acoustic Gas Measurements (AGAM). This validated model is then used to study upward combustion by application of gas profiles obtained from the laboratory experiments [2]. The comparison of the temperature contours for downward and upward combustion obtained from the CFD computations is shown in Fig.1.

Fig. 5. Temperature contours for: a) conventional combustion and b) reverse combustion

The numerical results show that high local temperatures are achieved for upward combustion which results in high thermal NOx. This behaviour is consequence of utilization of upward combustion technique into the

boiler not optimized for such application. In oxy-fuel combustion, different ways are reported for regulating NOx by controlling combustion parameters such as primary air flow rate [3], fuel type [4], air-fuel ratio [5] and

XXIV International Symposium on Combustion Processes, September 23-25, 2019, Wroclaw, Poland

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will not affect thermal conversion processes in the bed. This is also supported by experience of industry. Thus, in total, nine simulations are performed for different mass flow rates of the secondary air to optimize the boiler performance. The investigated cases are presented in Table 3.

Table 3.Effect of secondary air flow rate on outlet temperature and NOx concentration Simulation # Secondary Air [kg/s] Outlet Temperature [K]

Outlet thermal NOx Concentration [mg/Nm3_] Outlet O2 Concentration [wt.%] 1 1 1356.02 335.4675 2.1964 2 3 1284.13 295.9967 3.6172 3 5 1224.50 268.7964 5.3097 4 7 1211.90 247.6863 6.5476 5 9 1166.59 219.5124 7.5495 6 11 1114.50 193.6311 8.4596 7 13 1040.70 172.5171 9.2582 8 15 1011.18 144.8774 9.9702 9 17 1003.31 132.0484 10.5006

The simulation number 6 shows optimized results because both the temperature and NOx emissions reach

an acceptable value. The temperature at the outlet of furnace is 1115 K and thermal NOx emissions are 193

mg/Nm3 _{respectively, confirming that upward combustion is a promising and feasible technique for industrial}

furnaces.

References

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