Effects of Fuel Staging on the NO Emission ... - ACS Publications

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Effects of Fuel Staging on the NO Emission Reduction during Biomass−Biomass Co-combustion in a Fluidized-Bed Combustor Kasama Sirisomboon*,† and Vladimir I. Kuprianov‡ †

Department of Mechanical Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhon Pathom 73000, Thailand ‡ School of Manufacturing Systems and Mechanical Engineering, Sirindhorn International Institute of Technology, Thammasat University, Pathum Thani 12121, Thailand ABSTRACT: Fuel-staged co-combustion of sunflower shells (as a base fuel) and coconut coir dust/moisturized rice husk (as a secondary fuel) was studied on a 205 kWth fluidized-bed combustor with bottom air injection. During the experiments, the energy fraction of both secondary fuels in the total heat input to the reactor was varied from 0 to 0.22, while the amount of excess air ranged from ∼20% to ∼80% for each co-firing option. Temperature and gas concentrations (O2, CO, CxHy, and NO) were measured along the reactor centerline, as well as at stack, to evaluate the emissions and combustion efficiency of the combustor for the specified operating parameters. The experimental results revealed significant effects of excess air on the combustion and emission characteristics of the reactor, whereas the influence of fuel staging on these characteristics was moderate. An optimization method minimizing “external” costs of the co-firing was used to quantify the optimal energy fractions of the co-fired fuels and optimal amount of excess air. When operated optimally, the combustor can exhibit high (∼99%) combustion efficiency at minimal “external” costs and reduced NO emission (by 25%, as compared to burning sunflower shells on its own), because of the fuel staging. in power generation units.12,13 The extent of the system improvement (in terms of environmental characteristics and thermal/ combustion efficiency) is dependent on the properties of co-fired fuels and their mass/energy shares in total fuel supply, co-combustion method, and operating conditions.13,14 However, there is a lack of information on the co-firing of two or more biomasses in fluidized-bed combustion systems. As reported by some pioneering studies, fluidized-bed combustion systems can be effectively used for co-firing problematic biomass fuels (e.g., with unacceptable emission characteristics and/or very low calorific value), where individual burning is accompanied by operational problems and/or leads to substantial environmental impacts.10,15−17 Staged combustion methods have been proposed for reducing NO emission from various combustion systems fired with fossil/ biomass fuels and wastes. These methods are generally classified into (1) air staging (using overfire air injected downstream of the main combustion zone), (2) fuel staging (e.g., fuel biasing and burners out of service), and (3) their combination, which is commonly termed “reburning”. All of these methods have found wide application in boiler technology.18 Some pilot studies on air-staged combustion of biomass in grate-fired and fluidized-bed systems have reported a substantial reduction in NOx emissions (up to 70%), which has been achieved due to the air staging.19−21 However, because of elevated temperatures in the main combustion zone, the air staging technique has some limitations, mainly associated with biomass ash fusibility, and can therefore be recommended for (co-)firing biomasses with high-melting-point ash.19

1. INTRODUCTION Sunflower seeds is one of the important agricultural crops cultivated for the production of sunflower oil generally used for human consumption. In 2014, the world production of sunflower seeds was ∼40 million tons, with Ukraine, Russia, China, and Argentina accounting for 57% of the total amount grown.1 Sunflower shells (or hulls), the byproduct of sunflower seed processing, are reported to exhibit excellent combustion properties and potential as a biomass fuel in direct combustion systems.2−4 Taking into account the fuel availability (∼20% of the seed weight) and calorific value (assessed as 16 MJ/kg), the world annual energy potential of sunflower shells can be roughly estimated as 130 × 1015 J. However, as observed with many other biomasses containing elevated/high fuel-N, the combustion of sunflower shells is accompanied by substantial NOx emissions, whose level is expected to be dependent on the combustion method and operating parameters (mainly, excess air).5−7 Co-firing (or co-combustion), which is defined as the burning of two or more different fuels in a single combustion system (furnace/combustor), has been proven to be an effective technique of improving combustion and emission characteristics of systems. The co-firing technique is rather adaptable for various fuel types (fossil and biomass fuels, refuse-derived fuels, etc.) and has shown its suitability in stoker-fired, pulverized-fuel-fired, cyclone-fired, and fluidized-bed combustion systems.8,9 The method of fuel feeding/ injection (either as a premixed feedstock or via separate lines) is reported to affect the combustion and emission performance of a system.8,10,11 Co-firing of coal with biomass has been extensively studied previously.8 Apart from the environmental benefits (a substantial reduction of NOx, SO2, and CO2 emissions), coal−biomass co-firing offers some economic advantages, such as lowered fuel costs and improved efficiency of biomass conversion to electricity © XXXX American Chemical Society

Received: October 10, 2016 Revised: November 20, 2016

A

DOI: 10.1021/acs.energyfuels.6b02622 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of the experimental facilities for fuel-staged co-combustion tests.

biomasses with high K and Si (at low contents of other ashrelated elements) and those rich in K, Ca/Mg, and P (at low contents of Si and others).29,30 Co-firing biomass (as a base fuel) and a secondary (biomass) fuel with a low tendency for bed agglomeration seems to be an effective alternative measure to mitigate the bed agglomeration process.31,32 This objective can be achieved when an interaction of ashes of the co-fired fuels leads to the formation of highmelting-point compounds, preventing ash-related problems (such as bed agglomeration, slagging, and fouling) in a combustion system. Of special interest is a relevant example of the co-firing of high-alkali wheat straw (having a severe tendency to bed agglomeration) with municipal sewage sludge in a fluidized bed of quartz sand, which resulted in a substantial reduction of the propensity to bed agglomeration, compared to burning wheat straw on its own.33,34 This work was performed to explore the potential of fuel staging for the reduction of NO emission when co-firing sunflower shells and coconut coir dust/moisturized rice husk in a fluidized-bed combustor with bottom air injection. The effects of fuel staging and excess air on the gaseous emissions (CO, CxHy, and NO) and the combustion efficiency of the reactor were investigated for the specified ranges of operating parameters. A method aimed at minimizing total emission (“external”) costs of the co-firing of the selected biomass fuels was proposed to quantify the optimal energy fraction of the co-fired fuels and optimal excess air.

When using reburning, a base (or primary) fuel is burned in the main combustion zone under quasi-stoichiometric conditions, while a reburn (or secondary) fuel is introduced into the upper (reburn) zone to generate fuel-rich conditions facilitating NOx reduction. To complete combustion within the furnace/ combustor volume, secondary air is injected into combustion products at the exit of the reburn zone. The method has been tested in distinct biomass-fueled combustion systems and revealed a rather high, up to 80%, NO emission reduction.19,22−24 A more complicated control of fuel and air supply to a furnace/combustor (as compared to conventional combustion systems) and elevated temperatures in the reburn zone are major drawbacks of the reburning technique.19 However, as follows from literature reviews, limited attention has been paid to the fuel staging technique (e.g., axial biasing) in biomass-fueled combustion systems with conventional (bottom) air supply. These systems are expected to be free of the abovementioned disadvantages.25,26 Note that most biomasses exhibit a propensity for bed agglomeration, particularly when fired in a fluidized bed of silica/quartz sand (a typical bed material).27 “Coating-induced” agglomeration and “melt-induced” agglomeration are two extreme types (scenarios) of this undesired phenomenon often observed during combustion of high-alkali biomasses.28 With conventional bed material, the bed agglomeration type is dependent on the chemical composition of fuel ash. For instance, “coatinginduced” bed agglomeration basically occurs in combustion of biomasses with high contents of alkali metals in biomass ash (at rather low contents of Si and P) and most wood residues (typically, rich in Ca and K at minor Si and P).28,29 However, “melt-induced” agglomeration is often observed when burning

2. MATERIALS AND METHODS 2.1. Experimental Facilities. In the experimental part of this work, co-combustion tests were performed on a fluidized-bed combustor with B


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Energy & Fuels a cone-shaped bed (referred to as a “conical FBC”), using quartz sand as bed material. Compared to a prismatic/cylindrical fluidized-bed combustor with similar bed material and static bed height, the conical FBC can be operated (i) at lower pressure drop across the bed, (ii) lesser amount of bed material, (iii) shorter start-up time, all leading to lower operating costs of this reactor.35,36 Because of elevated superficial gas velocities in the conical bed, the combustor is generally operated in a turbulent fluidized-bed regime, resulting in significant expansion of the fluidized bed, thus ensuring “in-bed” biomass fuel injection.37 Figure 1 depicts the schematic diagram of the combustor with auxiliary equipment assuring its proper operation. The combustor consisted of a conical (bottom) section and a cylindrical (upper) section, both insulated internally to reduce heat losses to the environment. To avoid spouting of the bed material in the conical section, the (sieve) particle size of the sand was in the range of 0.3−0.5 mm, with the average particle size of 0.42 mm. In all experiments, the static bed height was 0.3 m. During biomass−biomass co-firing tests, biomass fuels were supplied into the combustor via separate routes using primary and secondary screw-type feeders. Air was delivered at a required flow rate to the reactor by a 25-hp blower. A bubble-cap air distributor, with a 0.25-mdiameter plate located at the combustor bottom, was used to induce bed fluidization in the bottom section. It can be seen in Figure 1 that the primary fuel was injected into the conical section at 0.65 m above the air distributor, while secondary fuel was introduced into the lower module of the cylindrical section at a higher level (1.15 m), thus ensuring fuel biasing, which is a type of fuel staging. Prior to co-firing testing, the fluidized sand bed was preheated with a start-up burner to a bed temperature of ∼700 °C. The start-up burner then was turned off, and the combustor was operated using either the primary fuel (in combustion experiments) or two fuels (in co-combustion tests). Fly ash generated during the tests was collected by a cyclone, which was fixed downstream from the combustor. The fly ash was analyzed for the unburned carbon content required for the assessment of an associated heat loss (as discussed below). During the combustor start-up mode and co-combustion testing, the temperature was monitored along the reactor height, using stationary chromel−alumel Type K thermocouples. In each trial, under fixed operating conditions, O2, CO, CxHy (as CH4), and NO were recorded along the reactor centerline using a “Testo-350” gas analyzer to investigate the effects of fuel staging and excess air on the formation and oxidation/reduction of major gaseous pollutants inside the reactor. To quantify the major emissions and amount of excess air (as a percentage of stoichiometric air) of the combustor, the above-listed gas concentrations were also measured in different test runs at the gas exit of the ash-collecting cyclone (i.e., at the stack). The measurement accuracies of the gas concentrations were: ±0.2 vol % for O2, ±5% for [CO] = 200−2000 ppm, ±10% for [CO] > 2000 ppm, ±10% for CxHy (as CH4), and ±5% for NO. 2.2. The Fuels. The following principles are generally important when selecting primary and secondary biomass fuels for fuel-staged fluidized-bed co-combustion with the bottom injection of combustion air: • The calorific value of primary fuels should be substantially higher than that of secondary fuels, to ensure stable and high-efficiency combustion in the fluidized bed (primary combustion zone) and prevent high peaks of temperature inside a reactor; • It is desirable (but not compulsory) to use secondary fuels with the fuel-N content lower than that in primary fuels, to avoid intensive NO formation in the overbed region (secondary combustion zone) of a reactor; • When using secondary fuels with elevated/high ash content, the fuels should not cause or enhance undesirable ash-related processes in a fluidized-bed combustor/boiler, such as bed agglomeration, slagging, and fouling. In this work, sunflower shells (SFS) were selected as a base (or primary) fuel to be co-fired with coconut coir dust (CCD) and moisturized rice husk (MRH), both used as a secondary fuel in the selected

co-combustion options. From the biomass structural analyses, all these fuels were typical lignocellulosic biomasses. With 14.0 wt % hemicellulose, 40.6 wt % cellulose, and 21.7 wt % lignin (all on dry and ashfree basis), SFS basically exhibits high thermal and combustion reactivity.5 The two secondary fuels, CCD (containing 4.9 wt % hemicellulose, 31.9 wt % cellulose, and 35.9 wt % lignin) and MRH (with 9.7 wt % hemicellulose, 52.0 wt % cellulose, and 19.6 wt % lignin), were also expected to be highly reactive biomasses. Table 1 presents the composition (as proximate and ultimate analyses), as well as the lower heating value of the fuels used in the co-combustion

Table 1. Properties of Biomass Fuels Used in the Co-combustion Tests on the Conical FBC property

sunflower shells, SFS

coconut coir dust, CCD

moisturized rice husk, MRH

Proximate Analysis (wt %)a fuel moisture 8.4 53.5 27.0 ash 2.7 1.5 11.7 volatile matter 72.5 40.0 50.7 fixed carbon 16.4 5.0 10.6 Ultimate Analysis (wt %, on dry and ash-free basis) carbon 48.8 52.6 48.9 hydrogen 6.69 4.16 5.11 oxygen 43.8 42.7 45.0 nitrogen 0.62 0.49 0.83 sulfur 0.11 0.07 0.11 lower heating valuea

16 420 kJ/kg

6580 kJ/kg

9810 kJ/kg

a

On an as-received basis for SFS and CCD, and on an as-fired basis for MRH.

experiments. Moisturizing of “as-received” rice husk (with an original fuel-moisture content of 12 wt % and rather high calorific value) was performed to meet the first of the above-listed requirements of reducing the calorific value of this secondary fuel to a sufficiently low level (9810 kJ/kg). However, because of the spongy texture and high water holding capacity of coconut coir (fibers), “as-received” CCD had a significant moisture content (53.5 wt %) and, consequently, a rather low calorific value (6580 kJ/kg). Because of lower fuel-N at a lesser percentage of fuel-O affecting the NO formation during biomass devolatilization,27 both CCD and MRH showed a substantially lower potential of NO formation, compared to SFS in individual burning of these biomasses. Because of the very low fuel-S content in all of the fuels, the emission of SO2 and its impacts were neglected. Note that the particles of SFS and MRH were rather consistent in shape and size. On average, SFS particles had a width of 6 mm, a thickness of 0.5 mm, and a length of 10 mm, whereas these dimensions of the rice husk particles were 2.5 mm, 0.3 mm, and 8 mm, respectively. However, the particle size of CCD was variable, from 0.007 mm to 1.5 mm, and ∼90 vol % of the particles had sizes in the range of 0.1−0.9 mm with a dominant volumetric diameter of ∼0.4 mm, as quantified with a “Mastersizer 2000” particle size analyzer. 2.3. Characterization of Fuel Ashes and the Potential for Bed Agglomeration. Table 2 shows the ash composition of SFS, CCD, and rice husk (as representative oxides, wt %) determined using a wavelength dispersive X-ray fluorescence (XRF) spectrometer. With a rather high potassium content in the fuel ash (K2O = 32 wt %), SFS were expected to display a propensity for bed agglomeration when using quartz/silica sand as a bed material. However, previous studies on the burning of this biomass in the conical FBC with a quartz sand bed have reported no occurrence of bed agglomeration/defluidization in relatively long-running combustion experiments.5 This fact can be basically attributed to a rather low content of Cl in the SFS ash and, therefore, an insignificant yield of low-melting-point KCl and eutectics, such as KCl−K2SO4 and KCl−CaCO3, from burning fuel-chars and ash particles,32,38 resulting in a low probability of bed agglomeration. C


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Energy & Fuels Table 2. Composition of Fuel Ash of Sunflower Shells, Coconut Coir Dust, and Rice Husk Composition (wt %) biomass fuel

K2O

CaO

MgO

P2O5

SO3

Cl

Al2O3

SiO2

Fe2O3

sunflower shells, SFS coconut coir dust, CCD rice husk, RH

32.0 45.8 6.0

26.9 6.8 1.3

15.2 3.6 2.1

3.7 1.3 2.8

10.4 1.6 0.9

3.3 25.3 0.5

1.7 1.8 0.1

4.1 5.8 85.6

1.9 1.1 0.2

The co-combustion tests were performed for five values of EFf2: 0 (i.e., for firing pure SFS), 0.06, 0.11, 0.17, and 0.22. Table 3 shows the

Another factor affecting the bed agglomeration tendency was the formation of ternary K2O−CaO−SiO2 systems (often with low melting points) in fuel char/ash particles and bed grain coatings. At low Cl in the SFS ash, the proportions of K2O, CaO, and SiO2 in a ternary system were likely in a quasi-linear correlation with their percentages in the fuel ash.39 As a substantial content of CaO in the SFS ash resulted in the high melting point of a K2O−CaO−SiO2 system, combustion of SFS at typical temperatures was not expected to exhibit sintering problems.40 Furthermore, during ash transformation, all the potassium compounds likely reacted with Mg, P, S, and Fe, contained in the SFS ash in noticeable proportions (see Table 2). These reactions led to the formation of various salts with high melting points (>1000 °C) (e.g., Ca9KMg(PO4)7, K2SO4, K2Fe2O4), which can mitigate “melt-induced” bed agglomeration.33,34 Yet, substantial/elevated Ca, P, and Mg in the SFS ash decreased the “coating-induced” agglomeration tendency. Even though a (primary) K-rich sticky coating layer was formed on the quartz grain surface, this layer can be promptly covered by an outer nonsticky layer enriched with Ca, P, and Mg, all facilitating the formation of highmelting-point compounds preventing bed agglomeration.28,41 Thus, at typical bed temperatures in a biomass-fueled reactor, SFS can be treated as not problematic, in terms of bed agglomeration in a combustor/furnace. With high contents of K and Cl in the fuel ash of CCD, KCl seems to be a major ash-derived compound that can be vaporized from the CCD ash in the vicinity of secondary fuel injection. However, because of noticeable Al and Si in the ash, some potassium can be retained in the fly ash in the form of aluminosilicates (such as KAlSi2O6 and KAlSi3O8) with high melting points.33 Furthermore, when co-firing SFS and CCD, some Ca3(PO4)2 can be formed in the primary zone, due to noticeable Ca and P in the SFS ash.34 This compound may react with KCl in the secondary combustion zone forming high-melting-point compounds, such as Ca5(PO4)3Cl and Ca10K(PO4)7, joining the fly ash.33 Thus, one can expect no substantial impacts of vaporized KCl on the bed behavior, mainly because of insignificant ash content in CCD and overbed injection of the secondary fuel into the reactor. With high SiO2 (85.6 wt %) at rather low K2O (6.0 wt %) and CaO (1.3 wt %), resulting in high infusibility of the fuel ash (over 1400 °C), rice husk can be treated as a nonproblematic fuel, with regard to bed agglomeration.32,40,42 Therefore, the co-firing of SFS and MRH in the proposed combustor was expected to be safe (with no bed agglomeration) when using quartz sand as the bed material. 2.4. Experimental Methods and Planning. Three groups of experimental tests (or test series) were performed in this work: (1) combustion of SFS, (2) co-combustion of SFS and CCD, and (3) co-combustion of SFS and MRH. As the biomass fuels had different calorific values, all experiments for different co-firing options were performed at an identical total heat input to the combustor (205 kWth). This allowed using the energy fraction of secondary fuel (EFf2) as an independent operating parameter in different test series. Apparently, EFf2 was affected by the mass fraction of secondary fuel (MFf2) in the total fuel supply, which was calculated using the feed rate of primary fuel (ṁ f1) and secondary fuel (ṁ f2) as MFf2 =

ṁ f2 ṁ f1 + ṁ f2

Table 3. Fuel Feed Rate of Primary and Secondary Fuels in the Co-combustion Test Series at Different Energy Fractions of the Secondary Fuel Primary Fuel Feed Rate (kg/h)

Feed Rate (kg/h)

Mass Fraction

sunflower shells, SFS





energy fraction

45.0 42.8 40.5 38.3 36.0

0 6.4 12.7 19.1 25.5

0 4.3 8.5 12.8 17.1

0 0.13 0.24 0.33 0.41

0 0.09 0.17 0.25 0.32

0 0.06 0.11 0.17 0.22

mass flow rate of the primary and secondary fuels, as well as the mass and energy fractions of the secondary fuel, for all co-combustion options. For given (fixed) EFf2, the test runs were conducted at four specified amounts of excess air (EA): 20%, 40%, 60%, and 80%. However, for each trial at fixed operating conditions (EFf2 and EA), the actual excess air ratio (α) and corresponding percentage of actual EA were quantified using O2, CO, and CxHy (as CH4), measured at stack, as introduced in Appendix A. Note that the excess air ratio was used to determine the volume of dry flue gas (Vdg) originated from the combustion of 1 kg of individual fuel, as provided in Appendix B. The CO, CxHy, and NO emissions from the combustor were measured at variable EFf2 and EA. A relationship between the NO emission reduction and the total CO and CxHy emissions was also derived to assess the percentage of NO emission reduction for the ranges of EFf2 and EA. 2.5. Determining the Combustion-Related Heat Losses and Combustion Efficiency. In this study on the co-combustion of two biomasses injected into the combustor at different levels above the air distributor, a concept of “equivalent fuel” was applied when predicting the combustion-related heat losses. Based on this concept, all relevant properties for the co-firing experiments were determined using the individual properties and mass fraction (as a weighting factor) of the co-fired fuels, as presented in Appendix C. For each co-combustion test run, the heat loss due to unburned carbon (quc,cf, %) was predicted according to Basu et al.,43 by using the content of unburned carbon in fly ash (Cfa, wt %), as well as the ash content (Acf, wt %) and the lower heating value (LHVcf, kJ/kg) of the “equivalent fuel” (both introduced in Appendix C), as

quc,cf =

32866Cfa ⎛ Acf ⎞ ⎜ ⎟ 100 − Cfa ⎝ LHVcf ⎠

(3)

The heat loss due to incomplete combustion (quc,cf, %) was quantified for a trial according to Basu et al.43 based on the actual CO and CxHy (as CH4) emissions (both in ppm) and using quc,cf, Vdg,cf (see Appendix C), and LHVcf, as given below:

(1)

The EFf2 was then determined by taking into account the lower heating value of the primary fuel (LHVf1) and that of the secondary fuel (LHVf2) as

MFf2LHVf2 EFf2 = (1 − MFf2)LHVf1 + MFf2LHVf2

Secondary Fuel

qic,cf = (126.4CO + 358.2Cx Hy) × 10−4Vdg,cf (2)

(100 − quc,cf ) LHVcf (4)

D


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Figure 2. Axial temperature profiles in the conical FBC when co-firing sunflower shells with (a) coconut coir dust and (b) moisturized rice husk for variable energy fractions of secondary fuel (EFf2) in excess air of ∼40% (upper graphs) and ∼80% (lower graphs).

Figure 3. Axial profiles of O2 in the conical FBC when co-firing sunflower shells with (a) coconut coir dust and (b) moisturized rice husk for variable excess air (EA) at EFf2 = 0.17 (upper graphs) and EFf2 = 0.22 (lower graphs). of a combustion system.44 In the present work, the effects of CO2, SO2, and particulates on the “external” costs were ignored, mainly due to (i) a rather weak dependence of the respective emissions on the operating conditions, (ii) a relatively low specific “external’’ cost of CO2, and (iii) quite low emissions of SO2 and particulate matter from the co-combustion of the selected biomasses. Therefore, the objective function for the optimization can be represented as

The combustion efficiency (as the percentage of LHVcf) was then determined by subtracting the combustion-related heat losses from 100%, as

ηc = 100 − (quc,cf + qic,cf )

(5)

2.6. Model for Optimizing Operating Parameters. The major operating parameters, EFf2 and EA, were optimized using a cost-based optimization model aimed at minimizing the emission (“external”) costs E


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Figure 4. Axial profiles of CO in the conical FBC when co-firing sunflower shells with (a) coconut coir dust and (b) moisturized rice husk for variable excess air (EA) at EFf2 = 0.17 (upper graphs) and EFf2 = 0.22 (lower graphs).

Figure 5. Axial profiles of CxHy (as CH4) in the conical FBC when co-firing sunflower shells with (a) coconut coir dust and (b) moisturized rice husk for variable excess air (EA) at EFf2 = 0.17 (upper graphs) and EFf2 = 0.22 (lower graphs).

Jec = Min(PNOxṁ NOx + PCOṁ CO + PCxH yṁ CxH y )

(6)

The mass fluxes of the CO, CxHy (as CH4), and NOx (as NO2) emissions were determined for a co-combustion test run, by using ṁ f1 and ṁ f2, as well as actual CO, CxHy (as CH4), and NOx (as NO) measured at the stack (all given in ppm), as shown below:

ṁ CO = 1.25 × 10−6(ṁ f1Vdg,f1 + ṁ f2Vdg,f2)CO

ṁ CxH y = 0.71 × 10−6(ṁ f1Vdg,f1 + ṁ f2Vdg,f2)Cx Hy

(8)

ṁ NOx = 2.05 × 10−6(ṁ f1Vdg,f1 + ṁ f2Vdg,f2)NOx

(9)

where Vdg,f1 and Vdg,f2 are the volumes of dry combustion products (Nm3/ kg) at the cyclone exit formed during individual burning of primary and

(7) F


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Figure 6. Axial profiles of NO in the conical FBC when co-firing sunflower shells with (a) coconut coir dust and (b) moisturized rice husk for variable EA at EFf2 = 0.17 (upper graphs) and EFf2 = 0.22 (lower graphs). secondary fuels, both determined from eq B.1 by taking into account the actual amount of EA quantified from eq A.2. The specific “external’’ costs of NOx and CxHy (as CH4) were assumed as PNOx = 2400 US$/t and PCxHy = 330 US$/t, respectively.45 However, no reliable data on the CO externalities were found in the literature. In the meantime, some studies revealed that PNOx and PCO show an apparent correlation, with the PNOx/PCO ratio being in the range of 5−8.46 Therefore, it was decided to assume PNOx/PCO = 6 in this optimization analysis, resulting in PCO = 600 US$/t.

a gradual decrease along the combustor height, mainly due to the heat loss across the reactor walls. In the test series with distinct secondary fuels, switching EA from 40% to 80% (at fixed EFf2) resulted in a noticeable temperature drop at different points in the reactor, obviously caused by air dilution. Meanwhile, during co-firing of SFS and CCD, an increase in EA led to a substantial carryover of fine (light) particles of coconut coir dust to the upper part of the combustor, shifting the location of the temperature maximum to the level Z ≈ 2 m, as seen in Figure 2a (lower graph). However, when co-firing SFS and MRH at different EA values, the axial temperature profiles in the reactor showed similar behaviors at fixed EFf2. 3.1.2. Axial Profiles of Gas Concentrations. As a matter of fact, O2, CO, CxHy, and NO inside the reactor showed moderate effects from fuel staging, but exhibited a rather strong influence of excess air. In the discussion below, the axial profiles of these gaseous compounds were therefore represented for the co-firing tests at the two highest energy fractions (EFf2 = 0.17 and EFf2 = 0.22) of both secondary fuels, with EA ranging from ∼20% to ∼80% for each co-combustion option. Figure 3 shows the axial O2 profiles inside the conical FBC when co-firing SFS and CCD/MRH at the selected operating parameters. Despite the secondary fuel injection, O2 decreased gradually along the reactor height. A substantial (negative) axial gradient of O2 was observed in the lower part of the reactor (Z < 1.6 m), comprising both primary and secondary combustion zones, where O2 was consumed intensively during oxidation of the co-fired fuels. However, in the upper part of the combustor, O2 decreased at an insignificant rate to a level essentially dependent on EA. Figure 4 compares the axial CO profiles in the reactor between the two co-firing options for the same operating conditions as those described in Figure 3. In all of the test runs with CCD/MRH,

3. RESULTS AND DISCUSSION 3.1. Effects of Fuel Staging and Excess Air on the Distribution of Temperature and Gas Concentrations within the Reactor. 3.1.1. Axial Temperature Profiles. Figure 2 depicts the axial temperature profiles in the conical FBC when co-firing SFS and CCD/MRH in different mass/energy fractions in excess air (∼40% and ∼80%). As seen in Figure 2, the behavior of temperature inside the reactor was noticeably affected by both EFf2 and EA. With increasing EFf2 within the range (at fixed EA), the temperature at different points in the conical section (occupied by an expanded turbulent fluidized bed37) decreased by some 40 °C in all test runs, mainly due to the reduced feeding of SFS into the bed and, therefore, reduced the heat release in this section. An injection of the secondary fuel (CCD/MRH) in the test runs at EA ≈ 40% resulted in a similar maximum temperature inside the reactor, 930−950 °C (regardless of EFf2), observed at Z ≈ 1.5 m above the air distributor (see Figure 2, upper graphs). This fact can be explained by a similar heat input (205 kWth) to the combustor in all trials, when a reduction in the energy contribution of SFS was compensated by CCD/MRH injected into the secondary combustion zone. At higher levels (Z > 1.5 m), the temperature at any point on the reactor centerline was nearly the same at EA ≈ 40% for the two secondary fuels, showing, however, G


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Figure 7. Effects of the energy fraction of the secondary fuel (EFf2) and excess air (EA) on the CO emission (top row), CxHy emissions (as CH4; middle row), and NO emission (bottom row) of the conical FBC when co-firing sunflower shells with (a) coconut coir dust and (b) moisturized rice husk, using fuel staging.

to CO2 (mainly, via reactions with O2 and OH)47 was much higher than that of CO generation, which resulted in a substantial decrease of CO along the reactor height. The formation of the second CO peak (at Z ≈ 1.6 m) in the vicinity of secondary fuel injection was caused by processes/reactions similar to those in the primary combustion zone. At the reactor top (Z > 2.5 m), CO was oxidized at a quite low rate, mainly due to reduced O2 in this region.

the profiles showed similar shapes (with two peaks of CO) and noticeable effects of the operating parameters. At Z < 0.7 m, CO originated from the SFS volatile matter, as well as from oxidation of the volatile CxHy and fuel chars,27 showed a drastic increase to its first peak (at Z ≈ 0.7 m), primarily due to the prompt devolatilization of SFS at a substantially lower rate of CO oxidation. In the region of 0.7 m < Z < 1.15 m, where the devolatilization process slowed down, the rate of CO oxidation H


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and CxHy in this zone, which enhanced the above-mentioned NO reduction reactions, and thus “cut off” the second NO peak, regardless of the fuel-N content in the two secondary fuels. As a result, all axial NO profiles were rather similar, in terms of both the shape and the effects from operating parameters. For the two co-firing options, the NO peak (at Z ≈ 0.7 m) decreased as EFf2 was increased at fixed EA, mainly because of the lowered bed temperature and higher CO and CxHy (responsible for NO reduction), despite the increased EA in the primary combustion zone. The second peaks CO and CxHy sustained a further reduction of NO along the combustor height, which occurred at almost the same rate in all the test runs, as seen in Figure 6. However, with increasing EA at fixed EFf2, NO at any point inside the reactor was higher, which complied with the fuelNO formation mechanism.27,47 3.2. Characterization of Major Gaseous Emissions from the Conical FBC. Figure 7 presents the CO, CxHy (as CH4) and NO emissions from the combustor (all at O2 = 6%, on a dry gas basis) when co-firing SFS and CCD/MRH for the ranges of operating parameters. These emissions represent, in effect, a net result of formation and oxidation/reduction of the pollutants inside the reactor. For the two co-firing options, the emissions exhibited quite similar trends, in response to the two independent variables. Compared to firing pure SFS (EFf2 = 0), the co-firing of this biomass with CCD/MRH at similar EA led to an increase of the CO and CxHy emissions from the combustor as EFf2 was increased. This result was due to the effects of the CO and CxHy second peaks in the vicinity of CCD/MRH injection (see Figures 4 and 5). From Figure 7, the extent of the emission increase (in response to EFf2) was different: it was substantial at EA ≈ 20%, but moderate at EA = 40%−60%, and insignificant at EA ≈ 80%. In the test at the highest EFf2 and the lowest EA, the CO and CxHy emissions were substantially higher than those for firing pure SFS. However, during the co-firing at EA ≈ 80%, the effects of EFf2 on these emissions were insignificant for both secondary fuels. With increasing EA at any arbitrary EFf2, the CO emission showed a significant reduction, mainly due to the acceleration of homogeneous CO oxidation by O2 in all regions inside the reactor. However, the most drastic reduction in the CO emission was observed at EA = 20%−60%, whereas a further increase in EA (from 60% to 80%) resulted in an insignificant (200− 250 ppm) reduction of this emission. At first glance, the response of the CxHy emissions to a variation in EA (at fixed EFf2) was similar to that of CO. At EA = 20%−60%, one can observe a strong reduction of the CxHy emissions as EA was increased. However, unlike with CO, as EA increased from 60% to 80%, the CxHy emissions in both co-combustion studies (see Figure 7) slightly increased, likely due to a greater carryover of fine (light) fuel chars to the reactor top. In the co-firing tests with both secondary fuels, the least NO emissions were achieved at the highest EFf2 and the lowest EA, i.e., when the CO and CxHy peaks in the secondary zone (and, therefore, the CO and CxHy emissions) attained their maximums, which indicated the important roles of the abovementioned reduction reactions in mitigating the NO emission via fuel staging in this combustor with the bottom air supply system. Apart from this, with lowering EA, the content of unburned carbon in chars of the co-fired fuels was higher, thus enhancing the catalytic activity of the chars and, eventually, increasing the rate of NO reduction in the two combustion zones. However, an increase in EA led to the higher emission of NO at any EFf2

Figure 8. Dependence of the NO emission reduction on total CO and CxHy emissions of the conical FBC when co-firing sunflower shells with coconut coir dust (CCD) and moisturized rice husk (MRH), using fuel staging at variable values of energy fraction of secondary fuel (EFf2) and excess air (EA).

For both secondary fuels, the first CO peak showed a noticeable increase as the energy share of CCD/MRH was increased from EFf2 = 0.17 to EFf2 = 0.22 (at fixed EA). This fact can be generally attributed to the lowered bed temperature (see Figure 2), which, despite the reduction in the SFS feed rate (causing an increase in local EA), resulted in a decreased CO oxidation rate within the primary combustion zone. In the trials at similar EA, an increased feed rate of the secondary fuel (injected into the reactor at Z = 1.15 m) led to a higher CO concentration in regions above the primary combustion zone. However, in the test runs at fixed EFf2, EA showed rather strong effects on the axial CO profiles. With increasing EA, CO was lower at all points along the combustor height, generally because of enhanced rates of the CO oxidation reactions. The axial CxHy profiles for all of the test runs are depicted in Figure 5. These profiles revealed the behavior (with two concentration peaks) and the effects of operating parameters similar to those of CO. However, unlike with CO, CxHy originated from the only source, the volatile matter in the fuel, and decomposed in breakdown/oxidation reactions, forming CO as an intermediate product.47 Because of a continuous “supply” of CO from the CxHy oxidation, the concentration of CO at all points inside the reactor was noticeably higher than that of CxHy for the ranges of operating conditions, as can be compared between Figures 4 and 5. Figure 6 presents the axial NO profiles in the conical FBC for the co-firing options and operating conditions, as in Figures 3−5. Like CO and CxHy, NO was rapidly formed along the centerline in the combustor bottom region, resulting in a significant NO peak in the vicinity of SFS injection into the reactor (at Z ≈ 0.7 m) in all the trials. At temperatures observed in this region (see Figure 2), NO mainly originated from volatile NH3 and HCN in the primary fuel, via the fuel-NO formation mechanism, with the proportional effects from fuel-N, EA, and temperature.27 A substantial reduction of NO in this and subsequent (upper) regions was primarily due to the catalytic reaction of NO with CO (on the fuel-char and ash surfaces), as well as homogeneous reactions of NO with CH/CH2 and NH2/NH radicals, all converting NO to N2.27,47 From Figure 6, at Z > 0.7 m, the rate of the reduction reactions prevailed over that of NO formation, which resulted in a gradual decrease of NO along the combustor height. However, no peak of NO was observed in the secondary combustion zone during the co-firing tests. Such a result was due to the second peaks CO I


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Table 4. Performance Characteristics of the Conical FBC in the (Co-)Combustion Tests at Variable Energy Fraction of Secondary Fuel for the Range of Actual Excess Air Values Concentration at the Stack energy fraction of secondary fuel

excess air (%)

unburned carbon in fly ash (wt %)

CO (ppm)

CxHy (ppm)

Heat Loss (%) due to unburned carbon

due to incomplete combustion

Combustion of Sunflower Shells 4.5 2320 630 0.25 3.3 1170 370 0.18 2.5 570 160 0.14 2.5 330 150 0.14 Co-combustion of Sunflower Shells and Coconut Coir Dust As Secondary Fuel 4.5 2710 940 0.26 4.1 1420 530 0.23 2.6 620 210 0.15 1.5 370 210 0.09

0

20 38 59 79

0.06

21 39 60 81

0.11

19 39 59 79

3.2 3.4 2.6 1.4

3040 1580 650 390

1190 670 270 240

0.17

18 41 58 79

4.0 3.3 2.2 1.5

3460 1760 730 400

1420 730 340 310

0.22

19 38 59 81

0.06

19 41 58 79

0.11

18 39 58 78

3.8 3.6 2.5 3.3

2900 1350 630 370

1020 510 250 230

0.17

18 38 58 78

4.0 3.3 2.6 2.6

3090 1560 680 400

0.22

18 41 60 79

4.1 3.5 2.9 2.8

3240 1660 720 440

1.53 0.97 0.51 0.44

98.2 98.8 99.4 99.4

2.06 1.29 0.62 0.55

97.7 98.5 99.2 99.4

0.18 0.20 0.15 0.08

2.45 1.55 0.72 0.62

97.4 98.3 99.1 99.3

0.23 0.20 0.13 0.09

2.87 1.75 0.87 0.75

96.9 98.0 99.0 99.2

3.05 1.96 0.97 0.87

96.7 97.8 98.9 99.0

1.93 1.05 0.58 0.50

97.9 98.7 99.2 99.2

0.21 0.34 0.24 0.31

2.17 1.23 0.68 0.58

97.6 98.4 99.1 99.1

1130 560 300 290

0.22 0.38 0.29 0.28

2.35 1.39 0.77 0.68

97.4 98.2 98.9 99.0

1170 580 320 340

0.23 0.47 0.39 0.36

2.47 1.48 0.83 0.79

97.3 98.0 98.8 98.8

4.0 3640 1460 0.22 3.3 2030 810 0.20 2.2 770 380 0.13 1.5 450 360 0.09 Co-combustion of Sunflower Shells and Moisturized Rice Husk As Secondary Fuel 3.3 2610 900 0.18 3.5 1190 420 0.26 2.7 610 200 0.20 3.4 340 190 0.26

(in accordance with the fuel-NO formation mechanism and due to the weakening effects of NO reduction reactions), which made the influence of fuel staging on the NO emission reduction somewhat lower. The opposite trend in the behaviors of the NO emission and the CO/CxHy emissions in their response to EFf2 and EA indicated an opportunity to optimize the mass/energy fraction of the co-fired fuels and EA, with the objective to minimize environmental impacts by the combustor. 3.3. NO Emission Reduction. Figure 8 shows the dependence of the NO emission reduction percentage on the total CO

combustion efficiency (%)

and CxHy emissions for the two (co-)firing options at different amounts of excess air. It can be seen in Figure 8 that, at the lowest EA, the NO emission reduction was substantially affected by the fuel staging and exhibited a rather wide range, from 20% to 60%, as EFf2 was increased from 0.06 to 0.22 for both secondary fuels. In contrast, at the highest EA, the effects of fuel staging was rather weak, resulting in the 5%−25% NO emission reduction. Thus, the fuel-staged biomass−biomass co-combustion with the conventional (nonstaged) air supply shows a potential in reducing the NO emission from the combustor, despite the presence of excess air in all regions inside the combustor. J


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Figure 9. Effects of the energy fraction of the secondary fuel (EFf2) and excess air (EA) on the emission (“external”) costs of the conical FBC when co-firing sunflower shells with (a) coconut coir dust and (b) moisturized rice husk using fuel staging.

pure SFS) NO emission, while controlling the CO and CxHy emissions at reasonable levels (see Table 4).

As revealed by this study, a significant NO emission reduction can be achieved at substantial EFf2 and relatively low EA. However, under such conditions, the total CO and CxHy emissions may be unreasonably high. Therefore, optimization of the operating parameters is required. 3.4. Heat Losses and Combustion Efficiency. Table 4 shows the predicted heat losses and combustion efficiency of the reactor, together with the content of unburned carbon in the fly ash and actual CO and CxHy (as CH4) in the (dry) flue gas (i.e., at stack) used in the calculation of heat losses, from the three test series at actual EA values. From Table 4, the effects of heat loss due to unburned carbon on the combustion efficiency were substantially lower, compared to that associated with incomplete combustion. For both secondary fuels, the two heat losses somewhat decreased as EA was increased (at fixed EFf2), following the variation of carbon content in the fly ash and that of CO and CxHy emission concentrations. As a result, in all test series for fixed EFf2, the combustion efficiency attained a maximum at EA = 60%−80%, which, however, slightly decreased, from 99.4% (for firing pure SFS) to 98.8%−99.0% (for co-firing SFS with CCD/MRH), as EFf2 was increased. Therefore, it can be concluded that the combustion efficiency of the conical FBC was weakly affected by the fuel staging. 3.5. Optimization of the Operating Parameters. Figure 9 shows the emission costs (US$/h) of co-firing SFS and CCD/ MRH in the conical FBC at variable EFf2 and EA, which were predicted using actual CO, CxHy, and NO in the flue gas and other relevant variables used in the optimization model. It can be seen in Figure 9 that at elevated EFf2 and relatively low EA, the emission costs for the two co-combustion options were characterized by significant values and showed the rather strong effects of the operating variables. This fact can be primarily attributed to the high CO and CxHy emissions from the combustor under such conditions (as seen in Figure 7). However, at EA > 50%, the influence of both EFf2 and EA was rather weak. Nevertheless, the three-dimensional (3-D) surfaces indicated the occurrence of an “external” costs minimum (roughly 0.40 US$/h in both graphs), for which the optimal operating parameters were quantified to be 0.16−0.18 for EFf2 and ∼60% for EA for the two secondary fuels. Under these conditions, the conical FBC can exhibit high (∼99%) combustion efficiency at the minimum emission costs and reduced (by 25% as compared to burning

4. CONCLUSIONS The effects of fuel staging at bottom air injection on the co-combustion of sunflower shells (as a base fuel) and coconut coir dust/moisturized rice husk (a secondary fuel) have been studied in a fluidized-bed combustor with a cone-shaped bed using quartz sand as the bed material. The mass/energy share of the secondary fuel and excess air have noticeable/substantial effects on formation and oxidation/reduction of CO, CxHy, and NO in the primary and secondary combustion zones, and, eventually, on the emissions and combustion efficiency of the combustor. With fuel staging, the second peaks of CO and CxHy are formed in the secondary combustion zone, facilitating NO reduction in this region and resulting consequently in lower NO emission from the reactor. The CO and CxHy emissions from the combustor increase as the mass/energy fraction of the secondary fuel increases at the lowered excess air level, while the NO emission shows a reducing trend. A method aimed at minimizing “external” costs of the combustor was used to optimize the key operating parameters of a practical relevance, the energy fraction of the secondary fuel and the amount of excess air. Under optimal operating conditions (0.16−0.18 energy fraction of the secondary fuel and excess air of ∼60%), high (∼99%) combustion efficiency at the minimal “external” costs and 25% NO emission reduction can be achieved through the fuel-staged co-firing of sunflower shells with coconut coir dust/moisturized rice husk, as compared to individual burning sunflower shells in the proposed co-combustion technique.

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APPENDIX A. ACTUAL EXCESS AIR FOR AN INDIVIDUAL TEST RUN When the volume percentages of O2, CO, and CxHy (as CH4) in the (dry) flue gas are known, the excess air ratio (α) can be determined according to Basu et al.,43 by neglecting H2 and assuming 79% N2 in the gas, as 21 α= 21 − (O2 − 0.5CO − 2CH4) (A.1) The actual amount of EA at the stack (vol %) is then quantified as EA = 100(α − 1) K

(A.2) DOI: 10.1021/acs.energyfuels.6b02622 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

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(4) Netherlands Enterprise Agency. Bioenergy & Biobased Opportunities in Ukraine, June 2013 (download). Available via the Internet at: http:// english.rvo.nl/topics/sustainability/sustainable-biomass/publicationsresults/countries, accessed on Oct. 28, 2016. (5) Arromdee, P.; Kuprianov, V. I. A comparative study on combustion of sunflower shells in bubbling and swirling fluidized-bed combustors with a cone-shaped bed. Chem. Eng. Process. 2012, 62, 26−38. (6) Sirisomboon, K.; Kuprianov, V. I.; Arromdee, P. Effects of design features on combustion efficiency and emission performance of a biomass-fuelled fluidized-bed combustor. Chem. Eng. Process. 2010, 49, 270−277. (7) Qian, F. P.; Chyang, C. S.; Huang, K. S.; Tso, J. Combustion and NO emission of high nitrogen content biomass in a pilot-scale vortexing fluidized bed combustor. Bioresour. Technol. 2011, 102, 1892−1898. (8) Sami, M.; Annamalai, K.; Wooldridge, M. Co-firing of coal and biomass fuel blends. Prog. Energy Combust. Sci. 2001, 27, 171−214. (9) Munir, S.; Nimmo, W.; Gibbs, B. M. The effect of air staged, co-combustion of pulverised coal and biomass blends on NOx emissions and combustion efficiency. Fuel 2011, 90, 126−135. (10) Chakritthakul, S.; Kuprianov, V. I. Co-firing of eucalyptus bark and rubberwood sawdust in a swirling fluidized-bed combustor using an axial flow swirler. Bioresour. Technol. 2011, 102, 8268−8278. (11) Madhiyanon, T.; Sathitruangsak, P.; Soponronnarit, S. Co-firing characteristics of rice husk and coal in a cyclonic fluidized-bed combustor (ψ-FBC) under controlled bed temperatures. Fuel 2011, 90, 2103−2112. (12) Harding, N. S.; Adams, B. R. Biomass as a reburning fuel: a specialized co-firing application. Biomass Bioenergy 2000, 19, 429−445. (13) Yang, H.; Wu, Y.; Zhang, H.; Qiu, X.; Yang, S.; Liu, Q.; Lu, J. NOx emission from a circulating fluidized bed boiler cofiring coal and corn stalk pellets. Energy Fuels 2012, 26, 5446−5451. (14) Ghani, W. A. W. A. K.; Alias, A. B.; Savory, R. M.; Cliffe, K. R. Co-combustion of agricultural residues with coal in a fluidised bed combustor. Waste Manage. 2009, 29, 767−773. (15) Kuprianov, V. I.; Janvijitsakul, K.; Permchart, W. Co-firing of sugar cane bagasse with rice husk in a conical fluidized-bed combustor. Fuel 2006, 85, 434−442. (16) Vainio, E.; Brink, A.; Hupa, M.; Vesala, H.; Kajolinna, T. Fate of fuel nitrogen in the furnace of an industrial bubbling fluidized bed boiler during combustion of biomass fuel mixtures. Energy Fuels 2012, 26, 94− 101. (17) Vamvuka, D.; Sfakiotakis, S.; Kotronakis, M. Fluidized bed combustion of residues from oranges’ plantations and processing. Renewable Energy 2012, 44, 231−237. (18) IEA Clean Coal Center. Clean coal technologies. Available via the Internet at: http://www.iea-coal.org.uk/site/2010/database-section/ clean-coal-technologies, accessed on Oct. 8, 2016. (19) Salzmann, R.; Nussbaumer, T. Fuel staging for NOx reduction in biomass combustion: experiments and modeling. Energy Fuels 2001, 15, 575−582. (20) Chyang, C.-S.; Qian, F.-P.; Lin, Y.-C.; Yang, S.-H. NO and N2O emission characteristics from a pilot scale vortexing fluidized bed combustor firing different fuels. Energy Fuels 2008, 22, 1004−1011. (21) Duan, F.; Chyang, C.-S.; Wang, Y.-J.; Tso, J. Effect of secondary gas injection on the peanut shell combustion and its pollutant emissions in a vortexing fluidized bed combustor. Bioresour. Technol. 2014, 154, 201−208. (22) Nussbaumer, T. Combustion and co-combustion of biomass: fundamentals, technologies, and primary measures for emission reduction. Energy Fuels 2003, 17, 1510−1521. (23) Casaca, C.; Costa, M. NOx control through reburning using biomass in a laboratory furnace: Effect of particle size. Proc. Combust. Inst. 2009, 32, 2641−2648. (24) Han, K.; Niu, S.; Lu, C. Experimental study on biomass advanced reburning for nitrogen oxides reduction. Process Saf. Environ. Prot. 2010, 88, 425−430. (25) Baukal, C. E., Jr. The John Zink Combustion Handbook; CRC Press: New York, 2001.

APPENDIX B. VOLUME OF GASEOUS PRODUCTS FROM COMBUSTION OF INDIVIDUAL FUELS For firing a single fuel, the volume of flue gas (Vdg, in units of Nm3/kg, on a dry basis) can be predicted according to Basu et al.43 by using C, S, and N from the fuel ultimate analysis, the theoretical (stoichiometric) volume of air (V0, in units of Nm3/kg), and the EA ratio, as Vdg = 0.01866(C + 0.375S) + 0.79V 0 + 0.008N + (α − 1)V 0

(B.1)

0

In eq B.1, V is determined using C, H, S, and O from the fuel ultimate analysis as

■

V 0 = 0.0889(C + 0.375S) + 0.265H − 0.0333O

(B.2)

APPENDIX C. PROPERTIES OF “EQUIVALENT FUEL” FOR CO-COMBUSTION TESTS For the co-firing of primary and secondary fuels in a combustor treated as a control volume, the relevant properties of “equivalent fuel” required for assessing the combustion-related heat losses were predicted as follows: • ash content (wt %): Acf = (1 − MFf2)A f1 + MFf2A f2

(C.1)

• lower heating value (kJ/kg): LHVcf = (1 − MFf2)LHVf1 + MFf2LHVf2

(C.2)

• volume of flue gas (Nm /kg, on a dry basis) from firing 1 kg of “equivalent fuel”’: 3

■

Vdg,cf = (1 − MFf2)Vdg,f1 + MFf2Vdg,f2

(C.3)

AUTHOR INFORMATION

Corresponding Author

*Tel.: + 66 3 425 9025. Fax: + 66 3 421 9367. E-mail: [email protected]. ORCID

Kasama Sirisomboon: 0000-0002-1048-2520 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from the Thailand Research Fund, Commission on Higher Education, Ministry of Education, Thailand, and Silpakorn University, Thailand (Contract No. MRG 5380019). Special thanks to Mr. Piyanat Jarernporn for his valuable contribution to the experimental part of this work.

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REFERENCES

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