Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Comparative Study of the MILD Combustion Characteristics of Biomass and Brown Coal Manabendra Saha,* Giovanni Gitto, Alfonso Chinnici, and Bassam B. Dally Centre for Energy Technology, School of Mechanical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia ABSTRACT: The present paper reports a comparative study on the burning characteristics of pulverized biomass and coal under moderate or intense low-oxygen diluted (MILD) combustion conditions. Two types of biomass fuelsnamely, grape marc and almond huskand a high volatile Victorian brown coal were used as pulverized fuels to burn in a vitiated coflow inside a vertical MILD combustion furnace. The furnace walls, as well as coflow temperature, and local oxygen concentrations were controlled by a secondary swirling burner. Fuels were introduced into the furnace employing CO2 as a carrier gas with a constant velocity (i.e., bulk jet Reynolds number, Rejet = 20 000) and a fixed range of particle sizes (250−355 μm). Detailed measurements of in-furnace and exhaust temperatures and chemical species (i.e., O2, CO, CO2, and NO) are demonstrated and discussed, together with optical images at the top, middle, and bottom sections of the furnace. It was found that MILD combustion was successfully established for all of the fuels investigated without any visible flame inside the furnace. Under similar experimental conditions, biomass volatiles are released earlier leading to a difference of the maximum temperature within the furnace of ∼150 K along the centerline. The largest NO emission was measured to be ∼185 ppmv (db at 3% excess O2) for grape marc case, because of the higher value of in-fuel N of grape marc and the lowest was ∼125 ppmv (db at 3% excess O2) for coal case. From the comparison of CO emission, biomass shows more eminent burning characteristics than brown coal under MILD combustion conditions. can offset its emissions during combustion.6 Hence, the use of biomass for electricity generation is a significant option to reduce the production of GHGs. Biomass resources are mainly composed of wood and wood wastes, followed by agricultural crops and domestic solid wastes.7 Present research is focusing on implementing and optimizing environmentally friendly and sustainable technologies for biomass exploitation as an energy resource.1 The present work focuses on the exploitation of the grape marc and almond husk as biomass fuels and Victorian brown coal as a conventional fossil fuel for energy generation. Grape marcwhich is also known as grape pomaceis a solid waste product from grape pressing during the wine-making process and is made of skins, seeds, pulp, and stalks. Almond husk, which is also called almond hull, is the outer leathery flesh of the almond kernel obtained through the mechanical removal process of almond. On the other hand, the brown coal is collected from Loy-Yang, Victoria, Australia. The Victorian brown coal deserves particular attention, because of its high reactive nature and combustion characteristics, despite its high water content in nature. The grape marc and almond husk have a substantially higher volatile matter content and less fixed carbon, in comparison with Victorian brown coal.8,9 However, all the fuels have a high moisture content (∼65%), which reduces the thermal efficiency of the burning process. The combustion of solid
1. INTRODUCTION The world’s energy demand is predominantly supplied by the combustion of conventional and renewable fuels in the combination of solid, liquid, and gaseous form. Among all the available fuel sources, solid fuels play an essential role in the world’s energy supply, because they are (i) the leastexpensive fossil fuel source and (ii) abundant. As a conventional fossil fuel, coal is an abundant fuel resource in the world. Although conventional combustion of coal contributes to a substantial number of adverse effects on the environment and public health, global warming is the most serious impact of coal combustion, in terms of its universal and potentially irreversible consequences. In contrast, biomass is considered to have the highest potential to replace traditional fossil fuels and significantly contribute to meeting the global energy requirement1,2 among the available renewable energy sources. A report published by the International Energy Agency (Task40)3 estimates that 10% of the world energy is generated using biomass, with industry utilizing a third of the resource and the rest is for domestic use. The interest in biomass as a viable alternative has been increasing in the recent years, because of the need for a more-sustainable energy production.4 Moreover, its relevance is expected to further grow as international energy policies focus more on a greater exploitation of renewable energy sources.4 Several advantages differentiate biomass from other renewable fuel sources.5 Biomass energy extraction does not require expensive conversion technologies, and energy can be delivered through different energy vectors such as liquid and gaseous fuels, heat, and electricity. Moreover, biomass is a carbon-neutral energy source and can even act as a greenhouse gases (GHGs) negative emitter.6 The amount of carbon dioxide absorbed by biomass during its growth, through the photosynthesis process, © XXXX American Chemical Society
Special Issue: 6th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: October 15, 2017 Revised: December 8, 2017
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DOI: 10.1021/acs.energyfuels.7b03158 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
biomass minimizes it. Furthermore, Ponzio et al.36 studied the combustion reaction mechanism with different operating conditions for the MILD combustion of wood pellets. Three different ignition mechanisms have been identified: spark ignition for high oxygen concentration and temperature, flaming ignition for low oxygen concentration and high temperature, and glowing surface ignition for low oxygen concentration and temperature. Dally et al.37,38 achieved MILD combustion conditions burning saw dust as a fuel in a self-recuperative vertical furnace with a parallel jet burner without air preheating. It has been found that the parallel jet burner allows a fast dilution of the pulverized fuel stream with the hot combustion products at high jet momenta, reducing the residence time. Ramona et al.39 analyzed the combustion behavior of wood pellets in a HiTAC furnace. The NOx formation, CO and CO2 concentration in the exhausts, ignition delay, and flame behavior were investigated under the variation of the combustion conditions. Choi and Katsuki40 studied the effect of high-temperature air preheating of NOx emission. The obtained results show that low NOx level can be achieved, even at high temperature. This is an advantage of the MILD conditions, with respect to conventional combustion technologies, that are strongly affected by thermal NOx formation. It has been stated that the reason behind this observation is the high dilution of the reactants. Furthermore, Wang et al.41 found that an increase in the exhaust recirculation ratio reduces the NOx production. Kakietek42 investigated the effect of the combustion temperature and oxygen concentration on the ignition delay. The results obtained shows that an increased oxygen concentration delays the combustion ignition, and this effect is stronger for lower temperatures. Blasiak et al.43 studied the combustion characteristics of wood pellets under MILD conditions. It has been proved that short ignition delay values and fast reaction rates can be gained when the temperature is increased above 1000 °C. The ignition delay seems to be independent of oxygen concentration at high temperatures, while a strong correlation has been highlighted for low-temperature values. MILD combustion utilizing gaseous and liquid fuels has been widely implemented in various industrial sectors (e.g., beam furnaces at Degerfors in Sweden, annealing furnaces for the steel industry in Italy, MILD combustion reformer at the Munich airport in Germany, rotary hearth furnaces in the United States, etc.). A review paper by Cresci et al.,45 representing a major burner manufacturer operating under a gaseous flameless oxidation regime, has highlighted the latest development and potential for application of this technology to new thermal systems. However, MILD combustion technology using solid fuels is still in an infancy stage, because of the challenging combustion characteristics of char particles. Therefore, more fundamental research is needed to successfully implement MILD combustion of solid fuels on an industrial scale. The application of the MILD combustion technology to solid fuels in a particular biomass has been explored only marginally, and there is a need for further analysis. Specifically, a substantial gap has been identified on the interaction of the turbulence field inside the combustion chamber on the flame and combustion process. Therefore, it is quite interesting to perform a detailed experimental investigation of the aforementioned issues in order to implement the technology in the field of renewable as well as conventional solid fuels.
biomass is conventionally performed with technologies developed for traditional fossil fuel such as coal.10 Only small changes and adjustments are applied to these systems, and this affects the amount of pollutant emitted.10 Commonly employed technologies include fixed bed combustion, traveling gate combustion, pulverized fuel combustion, fluidized bed combustion, and cofiring with fossil fuels. The prominent issues associated with these systems are NOx and particulate matter emissions,10 as well as low combustion temperature and ash gathering.11 When burning biomass fuels in conventional jet flame systems, a high burnout rate, similar to typical values for the coal, can be achieved.12 However, the burning characteristics are known to have lower temperature levels, as well as greater NOx emissions and ash deposition.12 Therefore, there is a pressing need for new technologies to be implemented to achieve high-efficiency and low-emission combustion of biomass fuels. A simple, but effective, way to increase the thermal efficiency of a combustion process consists of significantly increasing the combustion temperature. This can easily be done by recovering the waste heat exiting with the flue gases in order to preheat the reactants.13 However, a higher temperature in the combustion chamber means enhanced thermal-NOx formation, resulting in higher pollutant emissions. A viable and successful solution to ensure high combustion efficiency while keeping the NOx production at acceptable levels is the direct recirculation of the exhausts inside the combustion chamber.14,15 This hot combustion products flow heats up the reactants coming into the combustion zone and increases the average combustion temperature. At the same time, the flue gas stream mixes with reactants before the reactions start, generating a highly diluted low-oxygen environment.16 The high temperature and the low oxygen concentration due to the dilution of the reactants with inert gases are essential for the establishment of the moderate or intense low-oxygen diluted (MILD) combustion conditions. These specific conditions lead to a distributed reaction regime often referred to as “volumetric combustion”. Hence, a semiuniform thermal field without high peak temperature zones is obtained and this ensures improved thermal efficiency and very low NOx production.17 The above-mentioned features make the MILD combustion an attractive and high-potential combustion technology to be further studied and optimized. MILD combustion is also known in the literature as “volumetric combustion”, “flameless combustion”, or “flameless oxidation”,18,19 because of the already mentioned distributed reaction at moderate temperatures, which generates a flame that is barely visible or audible. Various experimental and computational studies have been conducted on the MILD combustion of pulverized solid fuels such as coal.20−30 The potential for NOx emission abatement has been reported widely and two main effects have been identified, namely, the lower and semiuniform temperature distribution leading to a reduced thermal NOx contribution and the reburn mechanism due to relatively long residence time at elevated temperatures.31−34 Only a few specific studies have been carried out for the application of the MILD technology to the combustion of biomass. Luo and Ma35 performed a detailed analysis of the effects of the temperature, oxygen concentration, and moisture content on the NOx formation in the MILD combustion of rice husk and saw dust. They found that higher air temperature values make the NOx formation increase, while a high water content in the B
DOI: 10.1021/acs.energyfuels.7b03158 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
The main MILD combustion chamber resides on top of the duct. This has a cross-sectional area of 260 mm × 260 mm, with a height of 1.2 m. The last 100 mm of the central jet is located inside it, surrounded by the ceramic plate at the bottom. The chamber has 9 ports along its length on the middle line, spaced 100 mm apart, starting from 70 mm above the jet exit. These slots allow the introduction in the furnace of the thermocouples and the gas sampling probe. On the other three walls, there are 8 equally spaced apertures with dimensions of 80 mm × 120 mm to allow optical access inside the furnace. These can be provided with UV-grade quartz windows if needed and can be insulated with metal plates covered in aluminum foil. A square section converging duct is then located on top of the furnace. This has a length of 500 mm and allows the exhaust gas flow leaving the combustion chamber to exit and reach the hood. All the furnace walls are thermally insulated with a 100-mm-thick hightemperature ceramic fiber board refractory layer to minimize heat losses to the surrounding. Air and natural gas at ambient temperature are metered and introduced into the swirl burner, according to the desired conditions of temperature and excess oxygen. An electric igniter is plugged in through the ignition port next to the burner in order to provide the spark to ignite the flame. Once the flame is stabilized, the flame igniter is removed. The ceramic plate ensures the uniformity of the flow. The coflow brings the amount of oxygen and diluents required to reach MILD conditions to the primary combustion chamber. It also controls the wall temperature. Once steady conditions and thermal equilibrium are obtained and the in-furnace temperature above the central jet exit is stabilized above a threshold value of ∼1300 K, the injection of the pulverized fuel starts. The fuel mass flow rate is regulated by varying the rotational speed of the screws in the particles feeder in order to obtain the desired value. The particles are then injected in the carrying gas flow rate at the exit of the feeder. This mixture enters the central jet at the bottom of the furnace and passes through it, up to the combustion chamber. The high temperature and high dilution environment generated by the hot coflow coming up from the secondary burner allows MILD conditions to be obtained in the combustion chamber. The exhaust gases derived from the process exit the furnace top through the converging exhaust duct and are sucked into the exhaust hood. A summary of the operating conditions and measure gas concentrations in the coflow is shown in Table 1.
The main aim of this research is to compare the MILD combustion characteristics of pulverized biomass fuels and brown coal through a comprehensive experimental investigation. Two types of biomass fuelsnamely, grape marc and almond huskand a high volatile Victorian brown coal were used as particulates fuels to burn within a vitiated coflow vertical MILD combustion furnace to examine the flame structure, stability, and pollutant emissions. Meanwhile, the experimental conditions, such as the turbulent bulk jet Reynolds number (i.e., Rejet = 20 000), local oxygen concentration (i.e., 6% excess coflow O2), particle size (i.e., 250−355 μm), and residence time of the particles (i.e., 136 ms), was kept constant for all cases. For each case, in-furnace temperature distribution, in-furnace gas concentration, and exhausts’ emissions of O2, CO2, CO, and NOx are measured, analyzed, and discussed.
2. EXPERIMENTAL SETUP AND METHODOLOGY The experimental investigation has been carried out on a laboratoryscale coflow vertical MILD combustion furnace. A detailed scheme of the furnace is reported in Figure 1. The system is composed of a main
Table 1. Operating parameters Value/Comment
Figure 1. Sketch of the coflow MILD combustion furnace. vertical combustion chamber for the MILD combustion, coupled with a secondary burner located underneath. The secondary burner, which supplies the hot and diluted gases to the coflow, stabilizes a turbulent nonpremixed swirl flame of natural gas. The hot combustion product is carried by an insulated duct from the secondary burner to the MILD combustion chamber. A silicon carbide ceramic foam plate 50 mm thick in the junction between the duct and the combustion chamber ensures the uniformity of the flow of hot products. An isolated, water-cooled, vertical jet with an internal diameter of 19 mm passes through the duct from the bottom of the furnace up to the ceramic plate. It is positioned along the combustion chamber centerline and allows the input of the fuel’s particles flow rate mixed with the carrying gas into the reaction zone. Two thermocouples monitor the cooling water temperature at the inlet and outlet. A volumetric, twin-screw particles feeder is connected to the central jet. This system regulates the fuel flow rate and introduced the particles to the carrying gaseous flow.
parameter
Case 1
Case 2
Case 39
fuel type particle size (μm) particle residence time (ms) fuel jet exit velocity (m/s) bulk jet Reynolds number, Rejet heat input by solid fuel (kW) global equivalence ratio, ϕ wall temperature of furnace (K) co-flow velocity (m/s) co-flow temperature (K) co-flow O2 (% (v/v), db) co-flow NO (ppmv)
grape marc 250−355 μm 136 8.84 20 000
almond husk 250−355 μm 136 8.84 20 000
brown coal 250−355 μm 136 8.84 20 000
10
8.1
10
0.84 ± 0.02 1100−1240
0.82 ± 0.01 1110−1225
0.96 ± 0.02 1112−1236
1.5 1295 6 106
1.5 1290 6 104
1.5 1284 6 98
Type-R ceramic-sheathed thermocouples are employed to measure in-furnace temperatures at heights of 70, 170, 270, 370, 470, 570, 670, 770, and 870 mm above the fuel jet exit. The thermocouples were traversed in the horizontal directions at three positions (Z = 0 mm, Z = 50 mm, and Z = 100 mm) across 9 different planes of various perpendicular locations of the furnace to provide a total of 27 points of measurement. The in-furnace gas concentrations and global C
DOI: 10.1021/acs.energyfuels.7b03158 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels emissions are measured using a portable gas analyzer, while the absolute errors are ±0.8% (by volume) for O2, ±10 ppmv for CO, and ±5 ppmv for NO of these measurements, according to the manufacturer. 2.2. Characteristics of Solid Fuels. Two types of biomass fuelsnamely, grape marc and almond huskand a high volatile Victorian brown coal were used as particulate fuels for this work. The grape marc used in this work was collected in April 2015 from the Riverland region of South Australia. The grape marc was dried in an oven at 105 °C for 6 h to remove all of the moisture. The gross dry calorific value of grape marc is found to be 22.2 MJ/kg. Almond husk is treated as the combination of almond hull and almond shell, which are mechanically removed from the almond kernel. The almond husk for this experiment was collected from the Laragon Almond Huller and Sheller Pty Ltd. (Lindsay Point, Victoria, Australia). Laragon collects almond husk from the Riverland region of South Australia between March to July. All the moisture of almond husk was removed before the experiment employing an electrical oven where almond husk was dried at a temperature of 105 °C for 7 h. The gross dry calorific value of almond husk is found to be 14.5 MJ/kg. The Victorian brown coal was collected from Latrobe Valley, Victoria, Australia for this experimental study. The coal was predried in an electric oven at 105 °C for 4−5 h to remove all moisture. The gross dry calorific value of Victorian brown coal is found to be 24.5 MJ/kg. All three solid fuels are milled and sieved into the 250 μm < d ≤ 355 μm size range. Proximate and ultimate analyses of the solid fuels are demonstrated in Table 2.
observations at the three optical accesses of the furnace. Nonetheless, a more luminous combustion, in the form of ghostly flames, is captured for biomass cases in comparison with the brown coal case. This difference points toward a higher reaction rate that is caused by the high volatile contents of biomass and, consequently, directly impacted on the rate of devolatilization and volatile combustion. An occasional individual streak of bright yellow light appeared in some of the images. Those are believed to be extraneous large (or agglomerated) particles that are burning bright. Their appearance was sporadic. There were no apparent ghostly flames, which have been reported in the past for gaseous fuels under similar MILD conditions. These flames were found to be directly related to ineffective mixing and longer local residence time, and such phenomenon can cause an increase in NOx emission, because of the elevated temperature of the reaction at these locations.8,44 In the current case, such ghostly flames did not appear for any of the cases. 3.2. Thermal Field. Figure 3 illustrates a comparison of experimentally measured mean temperature profiles between the grape marc, almond husk, and brown coal combustion cases. The temperatures are measured at several perpendicular positions (Y-direction) along the furnace centerline, as well as 50 and 100 mm away from the furnace centerline. The maximum/peak temperature is found to be ∼1285 K at 270 mm above the jet exit plane along the centerline of the furnace for the grape marc case in comparison to other cases. The peak temperatures point to the intensive volatile combustion of the grape marc at this location. It is not surprising, because the volatile contents of the grape marc are ∼11% higher than that of almond husk and ∼27% higher than that of brown coal. A comparison between biomass and coal MILD combustion reveals a significantly higher furnace temperature is obtained for the biomass cases at upstream locations (Y < 370 mm), even though the coflow temperature was constant for all cases. The reason for low in-furnace temperature for brown coal combustion case can be explained by the slower rate of reactions along the furnace centerline, in addition to its lower volatile contents. When the combustion reactions are slower, the heat is distributed over a larger volume and consequently leads to a reduction in peak temperatures. Moreover, the higher CO production (as seen in Figure 5, presented later in this work) by the endothermic heterogeneous reactions of solid carbon, for the coal combustion case, contributes to the lower in-furnace temperature. At downstream locations, the difference in the temperature distribution becomes less pronounced (away from the centerline), although a different trend can still be observed at the centerline. This is to be expected as more coflow products are mixed with products of the main jet flames. Figure 4 shows the measured temperature contours for the three fuel cases. A semiuniform temperature is found in the furnace, and the temperature discrepancy at various positions along the furnace is quite low, with a maximum difference of
