Abstract
Experimental and numerical investigations were performed for pure pulverized biomass combustion in a 300 kW laboratory swirl burner with a pre-combustion chamber. This work investigated a bluff body at the burner tip and how that affected the combustion characteristics in comparison with a conventional annular orifice burner. The combustion performances were assessed by measuring the temperature distribution in a pre-combustion chamber and furnace, oxygen concentration, and emissions (CO and NOx). Simulations were carried out and validated, providing insight on flow aerodynamics, particle trajectories, species concentrations, and temperature in a pre-combustion chamber and furnace. It was concluded that the bluff body provided a superior performance in terms of flame attachment and combustion efficiency. However, the emissions were high due to the contribution of thermal NOx.
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Development of Pulverized Biomass Combustion for Industrial Boiler: A Study on Bluff Body Effect
Niwat Suksam and Jarruwat Charoensuk *
Experimental and numerical investigations were performed for pure pulverized biomass combustion in a 300 kW laboratory swirl burner with a pre-combustion chamber. This work investigated a bluff body at the burner tip and how that affected the combustion characteristics in comparison with a conventional annular orifice burner. The combustion performances were assessed by measuring the temperature distribution in a pre-combustion chamber and furnace, oxygen concentration, and emissions (CO and NOx). Simulations were carried out and validated, providing insight on flow aerodynamics, particle trajectories, species concentrations, and temperature in a pre-combustion chamber and furnace. It was concluded that the bluff body provided a superior performance in terms of flame attachment and combustion efficiency. However, the emissions were high due to the contribution of thermal NOx.
Keywords: Pulverized biomass; Swirl burner; Bluff body; Pre-combustion chamber; CFD
Contact information: Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520 Thailand;
* Corresponding author: jarruwat.ch@kmitl.ac.th
INTRODUCTION
Coal consumption has become a debatable issue over recent years because it is a non-renewable resource. The concern regarding the impact of emissions from coal combustion on climate change has increased the interest in biomass as an alternative because it is renewable, considering that it is a carbon-neutral energy resource (Sami et al. 2001). However, biomass has lower calorific value because the composition of biomass has a lower carbon content than coal. Additionally, when converting chemical energy to thermal energy via combustion, its thermal performance should be considered, i.e., its flame stability, emissions, etc.
Pulverized biomass has been adopted for co-firing with pulverized coal in the furnace of an industrial boiler for power generation and process steam. Yi et al. (2013) conducted a study with thermogravimetric analysis (TGA) on a blend of ramie residue and coal. The effects of coal blending ratio from 0 to 30 wt% on performance of cyclone furnace were investigated. Ndibe et al. (2015) had successfully run a 300 kW combustor firing 100% pulverized torrefied spruce, 100% pulverized coal, and 50% blending of these two types of fuel with pre-heated secondary air at 195 C. A study using a drop tube furnace was also carried out by Wang et al. (2015) on coal and coal-biomass blending combustion performance (coal, straw, and wood). Results suggested a positive effect of blending on combustion efficiency and NOx emission. The effect of air staging was also evaluated. Aziz et al. (2016) had adopted computational fluid dynamics (CFD) for simulation of combustion performance of pulverized fuel blends between palm kernel and coal in an existing power plant. In their study up to 15% of the biomass could be adopted for co-firing without adverse effects on temperature distribution. Darmawan et al. (2017) performed experimental and numerical investigation on combustion of hydrothermally treated empty fruit bunch (HT-EFB) blended with coal in a drop tube furnace. A feasibility study was also carried out to integrate fuel processing into power plant system. Pu et al. (2017) recently attempted to use coal blended with biomass as a feedstock for an oxy-fuel bubbling fluidized bed combustor.
The existing plants have not been designed for firing pure pulverized biomass. This is due to the low heating value of volatiles as compared with that from coal, leading to problems with its flame stability. Char burnout was relatively slower due to both a larger biomass particle size and slower reaction kinetics. Firing pure pulverized biomass has been tested in the laboratories of some research institutes.
Ballester et al. (2005) carried out experiments on pulverized combustion in a semi-industrial scale furnace with three types of fuel: bituminous coal, lignite, and oak sawdust. Due to the difference in the stoichiometric air ratio of sawdust from coal, the operating conditions for bituminous and lignite were similar; however, there was a significant difference observed for sawdust. The aim of that study was to investigate the effect of different fuel types on some important flame characteristics, such as visible flame shape, distributions of temperature, and some major species (i.e., O2, CO, NOx unburned hydrocarbon, and N2O). It was found that a high volatiles content of lignite led to more intense combustion near the burner when compared with that of the bituminous flame. Such effects were even more prominent in the biomass flame due to it having the highest value of volatiles content. In addition, the results of hydrocarbon concentration and CO concentration confirmed this finding. Therefore, an expanded combustion zone of biomass had been found. However, according to the percentage of unburned carbon 1 m from the burner outlet, the combustion of bituminous particle was the lowest, followed by lignite, and the biomass particles gave the highest percentage of unburned carbon. There were two distinct regions of combustion, with the fist zone exhibiting intense volatiles combustion, followed by the region with trace amounts of delayed volatiles release and the burning of solid char.
Computational fluid dynamics has been widely used for the research and development of a pulverized combustion system. Ma et al. (2007) simulated the combustion of biomass in an existing pulverized coal-fired furnace by using a developed CFD model. The prediction showed a reasonable agreement with experimental data. Yin et al. (2012) investigated the combustion characteristics when firing pure coal and firing pure wheat straw in a 150 kW swirl-stabilized burner flow reactor under nearly identical conditions. Their results showed dissimilar combustion characteristics between the coal flame and the straw flame. Li et al. (2013) investigated the combustion characteristics of pulverized torrefied-biomass firing with high-temperature air by experiment and by simulation using CFD. They reported on the effect of drag force on the devolatilization of biomass and on the flame characteristics. The effect of oxygen concentration in an oxidizer and of air velocity on the flame characteristics were additionally discussed.
Elfasakhany et al. (2013) studied the combustion of pulverized biomass by conducting experimental trials and performing a simulation with CFD. There were three groups of particle size distributions ranging from 0.02 mm to 1.2 mm with an aspect ratio greater than 5. The study found a high volatiles release with high concentrations of hydrocarbon and carbon monoxide. The flame resembled a gaseous premixed flame. Three distinctive zones are identified: the preheated zone, the devolatilization zone, and the char oxidation zone. The heat transfer mechanism of the preheated zone for biomass was different from that of the gaseous combustion because the biomass particle was heated up by the radiation mode of heat transfer from the flame and furnace wall prior to undergoing the process of devolatilization. Therefore, pollutant emissions were affected by the burner configuration, particle size, and the furnace chamber. In addition, there was a “rocket effect” during the devolatilization process of the biomass particles.
Weber et al. (2015) studied the variety of combustion behaviors in mixed wood, sawdust, fermentation-process residues and grain residues, and South African Middleburg coal. The pulverized coal was injected through an annular nozzle surrounded by the combustion air without swirl, and the biomass was injected through a central pipe nozzle. The furnace was cylindrical with a vertical orientation. The firing rate was 15 kW for all cases. Important flame characteristics such as ignition, temperature rise and distribution, devolatilization, NOxemission, combustible burnout, fly ash, and slag were investigated. The peak temperature in the volatiles combustion zone when firing coal was 100 C higher than that of biomass, with an earlier ignition observed for coal. However, all types of fuel fired under this experimental setting achieved a burnout greater than 94.2%. Approximately 50% to 60% of the nitrogen content in sawdust and mixed wood was converted to NOx, while 16% to 18% was converted for the coal, fermentation-process residues, and grain residues. The bottom ash deposition was found at 950 C to 1200 C, and the deposition for the biomass combustion was three times greater than that of the coal combustion.
Karim and Naser (2018) developed user-defined subroutines on a commercial CFD code (AVL Fire) for simulation of woody biomass combustion on moving grate boiler. They found an agreement between the simulation result and the experimental data. Gómez et al. (2019) developed a mathematical model for porous media. The model was used together with a commercial software, FLUENT, to simulate combustion, heat and mass transfer in a 35 kW wood pellet furnace. The effect of exhaust gas recirculation (EGR) and excess air was investigated. The model had reasonably represented the phenomena taking place in the reactor.
Elorf and Sarh (2019) found the effect of excess air on the flow aerodynamics and the combustion characteristics of pulverized biomass to be made of olive cake in a vertical furnace with a cylindrical cross section. Fuel was injected radially from the bottom of the furnace to increase the residence time. The study adopted CFD to perform a 3D reacting flow simulation. Biomass with an average particle size of 70 µm was injected radially and combusted under excess air ratios of 1.3, 1.7, 2.3, and 2.7. The simulation suggests that the increase in the excess air ratio would dilute the CO and CO2 concentrations, decrease the flue gas temperature, and shorten the flame length.
Combustion of biomass in pulverized form has the potential for replacement of conventional fossil fuel. In a conventional burner configuration, fuel is fed into the combustion chamber by pneumatic transport surrounded by combustion air. Introduction of swirl motion to the primary or secondary air stream would help spreading the solid particle around, leading to rapid heat transfer as well as the enhancement of devolatilization and its reaction rate (Zhou et al. 2003; Gu et al. 2005). However, when combusting biomass with different ignition and char burnout characteristics, additional modification is necessary to improve the combustion performance of the burner.
This paper reports on an achievement made on combustion of pure pulverized biomass with a new burner specifically designed and constructed for pure biomass combustion. The burner is equipped with a pre-combustion chamber, which is used to accommodate ignition of biomass volatile compounds. Apart from the result of the base case, development work is also carried out by installing a bluff body at the burner tip locating at the center of primary air inlet. Discussion is made on how this bluff body affects to the aerodynamics at the central region of a pre-combustion chamber, which leads to improvement of combustion stability and efficiency. The result is verified by temperature distribution and CO emission taken from experiment performed by the authors. CFD is used to investigate aerodynamics, particle trajectories, devolatilization, and combustion when the burner is operated with and without bluff body. Species concentration and NOx emission are also reported. This simulation enable an in-depth understanding on how the bluff body affects to aerodynamics in the near burner region and eventually to combustion stability and emissions.
EXPERIMENTAL
Experimental Setup
The pulverized biomass swirl burner assembled with a pre-combustion chamber had diameters of 0.385 m and 0.465 m. The furnace had 1 m in inner diameter and 1.215 m in length, as shown in Figs. 1 and 2.
Fig. 1. Pulverized biomass combustion system: (1) pulverized biomass milling and feeding system, (2) primary air and fuel inlet, (3) secondary air inlet, (4) tertiary air inlet, (5) swirl burner and pre-combustion chamber, (6) furnace, (7) flue gas treatment and wet scrubber, (8) air supply system, (9) ID fan, and (10) flue gas stack
Fig. 2. Pulverized biomass combustion burner and furnace
The burner was operated at 300 kW thermal throughput (net) at an equivalence ratio of 0.834 (20% excess air). The fuel and air entered the furnace at 313 K. Primary air and pulverized biomass entered through a central pipe with diameter of 0.0525 m. The primary air and fuel were fed at 0.0186 kg/s and 0.0134 kg/s, respectively. Secondary air travelled through swirl vanes, creating a swirl flow, with a swirl number equal to 0.83, through the annular pipe before entering the pre-combustion chamber at 0.0869 kg/s. Tertiary air entered the furnace from the connector between the pre-combustion chamber and furnace at 0.0334 kg/s, as presented in Table 1. The induced draft (ID) fan was used to draw flue gas from the furnace to the stack. The vacuum outlet of flue gas was controlled at 1500 Pa below atmospheric pressure. The flame temperature was measured using a type K thermocouple with a Yokogawa XL100 Portable Data Station (Yokogawa Electric Corporation, Tokyo, Japan). The combustion gas species were measured using a Testo 330-2 LL flue gas analyzer (Testo AG, Lenzkirch, Germany). The monitoring locations of the pulverized biomass furnace are illustrated in Fig. 3.
Table 1. Operating Conditions of the Furnace
