Regimes of Nonpremixed Combustion of Hot Low-Calorific-Value

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Regimes of Nonpremixed Combustion of Hot Low-Calorific-Value Gases Derived from Biomass Gasification Kamil Kwiatkowski*,†,‡ and Epaminondas Mastorakos*,§ †

Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warsaw, Poland Interdisciplinary Centre for Mathematical and Computational Modelling, University of Warsaw, Prosta 69, 00-838 Warsaw, Poland § Hopkinson Laboratory, Engineering Department, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK ‡

ABSTRACT: Combustion of the hot, diluted gases derived from biomass gasification was investigated in mixture fraction space. The composition of the fuels corresponds to those measured in industrial gasification plants operated on wood chips and organic residuals. The temperatures of both the fuel and the oxidizer are within the range accessible to gasifiers which naturally produce hot gases. Simulations of counterflow laminar nonpremixed flames show that only the maximum temperatures and temperature profiles of the biomass syngases are comparable to those obtained for methane diluted with nitrogen to the same calorific value. In contrast, the profiles of the heat release rate (HRR) are significantly different, being widely spread across the mixture fraction space when syngas is burning but with a narrow distribution for the methane equivalent. There are also differences in profiles of OH, important from an experimental point of view. From this perspective, methane equivalents cannot be treated as an analog of real biomass-derived gases. Wood-derived gases are generally burned conventionally or, when both reactants are highly preheated, in the high-temperature air combustion (HTAC) regime. Lower calorific-value gases derived from organic residuals are most commonly burned in pilot-assisted regimes, but the favorable regime of moderate and/or intense level of dilution (MILD) can be achieved when the reactants are sufficiently preheated. Moreover, even when the oxidizer is cold, highly diluted and preheated residual-derived gases can be burned within the MILD combustion regime. Wood-derived gases typically do not achieve MILD combustion, even when additional exhaust gas recirculation (EGR) is introduced or when the scalar dissipation rate (SDR) is increased. Although based on the simplified mixture-fraction approach, a survey of the combustion regimes qualitatively agrees with the properties of the combustion processes observed in industrial gasification plants. Gases from wood chips are burned conventionally, whereas less calorific-value gases derived from organic residuals achieve MILD combustion for three different geometrical configurations of the industrial combustion.

1. INTRODUCTION The replacement of fossil fuels with renewable materials for fueling thermochemical processes has resulted in strong interest in feedstocks such as low-quality biomass and organic residuals, which were previously treated as waste.1 Despite their low quality and low calorific value, such renewable materials are often attractive feedstocks for robust fixed-bed gasification processes.2−4 The virtually unlimited diversity of potential feedstocks, from refuse derived fuels to wood materials,5,6 organic residuals,2,4 and preprocessed biomass;5 however results in different gasification-derived gases. Industrial fixedbed biomass gasifiers always proceed with a presence of inert gases such as carbon dioxide or water vapor, which are constantly mixed with produced hydrogen, carbon monoxide, methane, and other combustible species. In the case of the airblown process the gasification-derived gases are also diluted by nitrogen, moreover part of the combustible gases react with oxygen which additionally increases the level of dilution. As a result the gases produced from industrial air-blown fixed-bed gasifiers fed by low-quality biomass are usually hot and naturally diluted by inert species such as nitrogen, carbon dioxide, or water vapor. In cases when the fuel, oxidizer, or both reactants are hot, the combustion of such gases potentially drives drastic temperature increases and high nitrogen emission. Combustion with high-temperature reactants has long been investigated, and promising methods ensuring low nitrogen oxide emissions have © XXXX American Chemical Society

been proposed. In fact, different approaches such as hightemperature air combustion (HTAC)7−9 and flameless oxidation (FLOX)10,11 have become industrial standards for combusting methane and other gases. Despite their technical differences, the two technologies historically have one feature in common: the oxidizer is supplied strongly preheated and, at the same time, diluted. In flameless oxidation, frequently referred to as moderate and/or intense level of dilution (MILD) combustion,12,13 the fuel and/or oxidizer are diluted and heavily preheated above a temperature that ensures autoignition of the mixture, such that temperature increase due to exothermic combustion reactions is relatively small.12 Using preheated but diluted air raises the maximum temperature of combustion but lowers the gradients of temperature and oxygen content, thus the combustion is more homogeneous than in the conventional mode. More homogeneous combustion and relatively uniform temperature profiles promotes significant reduction of the NOx emission. In fact, the same effect can be achieved via the strong internal recirculation of combustion products11 or when very diluted Special Issue: International Symposium on Combustion Processes Received: November 2, 2015 Revised: April 1, 2016

A

DOI: 10.1021/acs.energyfuels.5b02580 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Composition of Syngases Produced in Fixed-Bed Gasifiersa gasification plant in Szepietowo

Olsztyn

syngas produced from waste wood chips

poultry feathers

mass fraction

SG1

SG2

SG3

SG4

SG5

SG6

SG7

SG8

nitrogen carbon monoxide hydrogen carbon dioxide methane LHV [MJ/kg] ξstoic methane equivb

0.567 0.277 0.006 0.121 0.012 4.14 0.478 0.08

0.562 0.319 0.006 0.085 0.010 4.42 0.465 0.08

0.529 0.333 0.007 0.097 0.012 4.77 0.445 0.09

0.572 0.271 0.006 0.097 0.010 4.26 0.495 0.08

0.588 0.282 0.005 0.072 0.008 4.13 0.507 0.08

0.541 0.298 0.005 0.089 0.011 4.39 0.489 0.08

0.549 0.030 0.005 0.396 0.020 1.91 0.630 0.04

0.599 0.068 0.003 0.299 0.015 1.80 0.655 0.04

a

Samples SG1−SG6 were collected in Szepietowo, and samples SG7−SG8 were collected in Olsztyn.2 bMethane equivalent is defined as the mass fraction of methane diluted with nitrogen with the same calorific percent value as syngas.

syngases include a wider region of HRR than for the methane equivalent. A weak correlation between the location of the stoichiometric MF and the peak in the HRR profiles, characteristic of MILD combustion, is observed. The results of the proposed simplified mixture-fracture analysis are the basis of a survey of the combustion regimes of the low-calorificvalue biomass syngas for different values of fuel and oxidizer inlet temperatures. The survey summarized the differences between gases derived from wood gasification, typically burned in the classic regime, and gases from organic residuals burned in the pilot-regime of combustion with the possible achievement of MILD combustion. The survey is extended with an analysis of the influence of scalar dissipation rate (SDR) and exhaust gas recirculation (EGR) on the predicted combustion regime. To understand why the furnace has such a different behavior under the different fuels used provides additional motivation to explore the structure of the flames of these fuels through the fundamental canonical problem of the strained diffusion flame. Although the approach is simplified, it has recently been shown that the classification of the combustion regime is rather generic and depends little on the actual configuration of the combustion chamber.28 In fact, the combustion processes observed in industrial gasification plants qualitatively confirmed the presented survey. Thus, the survey result is of practical importance for industrial installations in which both temperatures are controlled and can be adjusted to optimize the process.

fuel is burned. One observable feature is the generation of a distributed colorless flames, which caused the process to be frequently referred to as flameless.7,10,11,14,15 These advantageous properties of MILD combustion have recently inspired even more numerical,16 experimental,17 and theoretical18 investigations. For practical reasons, the most commonly explored configuration leading to MILD combustion is when the oxidizer is preheated but the fuel remains cold, with one or both reactants being diluted. Recently, the usual studies have been extended to cases in which both the fuels and oxidizers are preheated simultaneously.19−22 Combustion with preheated but diluted fuels, so-called hot fuel diluted fuel (HFDF),21 has also been investigated numerically for methane mixed with nitrogen.19,20,23 It is worth noting that the majority of investigations on the MILD combustion of gaseous fuels are based on methane, usually diluted with nitrogen14,24 or carbon dioxide.15,16 Conversely, results for real low-caloric gases and diluted gas mixtures are much less commone.g., refs 25−27. In fact, recent investigations by Sabia et al.16,25 revealed that the impact of carbon dioxide or water vapor, which may possibly be included in realistic low-caloric fuels, on combustion properties were likely underestimated. This suggests that the usual mixture of methane diluted with nitrogen is in fact unrepresentative as an analog for a wide class of low-calorific-value gases derived from gasification processes. This present paper focuses on the combustion of low-calorific-value gases derived from two fixedbed biomass gasification plants fueled by wood chips and organic residuals. These gases, composed of methane, hydrogen, and carbon monoxide, are highly diluted with both nitrogen and carbon dioxide. Moreover, the gases are preheated in the process of their production.2 The counterflow laminar nonpremixed combustion of analyzed fuels was investigated numerically in mixture fraction (MF) space and contrasted with their methane equivalents defined as mixtures of methane and nitrogen diluted to the same calorific value. The profiles of temperature, heat release rate (HRR) and selected species along with their production rates for syngases have been compared with those obtained for a mixture of methane and nitrogen with a corresponding low calorific value. It has been shown that the simple methane-nitrogen equivalents cannot, from the perspective of the underlying chemical reactions, substitute for real gases. The reaction structures for gases derived from biomass and their methane equivalent allow for the investigation of the effect of the reactant temperature and composition on the key flame characteristics of classic and heavily preheated and diluted flames. The key features for

2. MATERIALS AND METHODS 2.1. Analyzed Fuel Gases. The main factors determining the low heating value (LHV) of these gases are the use of air as the gasification agent, leading to a high nitrogen content, and the initial high water content of the biomass. Thus, the gases produced in fixed-bed biomass gasification from waste biomass or organic residual are vastly diluted but usually hot. The gases analyzed in this paper consist of small amounts of flammable components, mainly hydrogen, methane, and carbon monoxide, which are highly diluted in carbon dioxide and nitrogen. Two types of syngas have been considered, both of which are produced in robust industrial gasification plants. The first plant, which is located in Szepietowo, Poland,29 and gasifies low-quality wood chips, produces gas corresponding to approximately 0.08 of methane equivalent defined as the mass fraction of methane diluted with nitrogen having the same calorific value. The second plant, located in Olsztyn, Poland,2,3,30 uses as a feedstock organic residuals, mainly feathers from an adjacent slaughterhouse. The derived gases are even more diluted, corresponding to approximately 0.04 of methane equivalent. The compositions of the samples taken are shown in Table 1. The bottom row shows the diluted methane equivalenti.e., B


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Energy & Fuels the mass fraction of methane in a methane−nitrogen mixture with the same energy content. As its value varies between 0.04 and 0.1, the syngases correspond to highly diluted methane. The presented values are mass fractions obtained from gas chromatography analysis of the gases, tar- and water-free, collected from the gasifier outlet through a set of scrubbers.5 Fluctuations in the syngas composition presented in Table 1, which are typical for this type of installation, are due to small changes in the gasification conditions and the biomass feedstock properties. The gasification plants contain a fixed-bed gasifier where flammable gases are continuously produced from biomass via thermochemical conversion; the gases then flow into an adjacent combustion chamber, where they are burned. Finally, the energy of the flue gases is recovered by a heat exchanger. The typical characteristics of the inlet and outlet streams are as follows: syngas flowing into the combustion chamber is hot Tf = 1000 ± 100 K; the oxidizing air is cold To = 290 ± 15 K; temperature of the recycled exhaust gas Tegr = 400 ± 20 K; temperature of the raw flue gases Tout = 1200 ± 50 K. A schematic configuration of the combustion chamber operating in a gasification plant is shown in Figure 1. At the gasification plant in Olsztyn, three

the auxiliary burner with continuous measurements of temperature and combustion completeness, allow for the estimation of the autoignition temperature of the syngases, Tign. This procedure is biased with high uncertainty, but long-term estimates of the autoignition temperatures were consistent: • Below 800 K (wood syngas) and 900 K (feathers syngas), the flames are always extinguished when the additional burner is off; • Above 900 K (wood syngas) and 1000 K (feathers syngas) the flames are sustained, even when the additional burner is off; • Between these temperatures, combustion is usually stable without the auxiliary burner when the combustion chamber is already heated up (when switching off) and unstable when the combustion chamber is cold (during startup). Thus, the following values of autoignition can be assumed: • samples of syngas from wood SG1−SG6: Tign = 850 ± 50 K; • syngas from feathers SG7−SG8: Tign = 950 ± 50 K. In the real industrial process, several parameters can be controlled, at least to some extent. The parameter that is easiest to adjust is fuel temperature. The properties of the gas produced in a fixed-bed gasifier are controlled by the amount of air supplied to the gasifier. When the oxygen concentration in the gasifier is sufficiently high, small amounts of hydrogen, carbon monoxide, and methane are burned, the temperature increases, and the syngas becomes more diluted. A qualitative analysis performed in an industrial gasifier2 revealed that the temperature of the produced gas might vary from 800 to 1400 K and is usually stably above 1000 K. The amount of external EGR is usually well-controlled in industrial plants. The EGR may increase the temperature and dilute the oxidizer, thus promoting homogeneous combustion, but its impact is smaller than the internal recirculation of the combustion products. The SDR can be increased by a suitable design of the combustion chamber flowe.g., by maximizing the intensity of the turbulence. In general, increasing the values of the SDR promotes MILD combustion.33 2.2. Numerical Model. In order to analyze and compare the flame structure of several syngases under different combustion conditions simplified but a robust approach has to be used. The approach is conceptually based on counterflowing jets of fuel and oxidized described in the one-dimensional MF space, where MF ξ = 0 means pure oxidized and ξ = 1 indicates pure fuel. This approach describes the transient laminar flamelet34 with a unity Lewis number. The set of governing equations for the mass fraction of the reacting species and for temperature (eqs 1 and 2) is as follows: ∂Yi ∂ 2Y ω = N (ξ) 2i + i = 0 ∂t ρ ∂ξ

(1)

⎛ 1 ∂cp ∂Ti ω ∂ 2T = N (ξ) 2 − T + N (ξ)⎜⎜ + ∂t ρcp ∂ξ ⎝ cp ∂ξ

Figure 1. Scheme of the combustion chamber: cross-jet configuration.

∑ cpi

∂Yi ⎞ ∂T ⎟ =0 ∂ξ ⎟⎠ ∂ξ

(2) The notations are as follows: Yi mass fraction of species indexed by i, ξ variable in MF space, ωi reaction rate for species i, ωT heat release rate, ρ density of gas mixture, cpi heat capacity of species i. The heat capacity of the gas mixture is defined as cp = ∑i Yicpi. The boundary conditions are such that Yi(ξ = 0) and T(ξ = 0) are equal to the composition and temperature of the oxidizer, and Yi(ξ = 1) and T(ξ = 1) are equal to those of the syngas. In total, seven different combinations of boundary conditions for the fuel and oxidizer temperatures have been considered, with special attention put on the combustion of fuel preheated to different levels from 800 K (case B), 1000 K (case C), 1200 K (case D), to 1400 K (case E). For these cases (B−E) air is cold (300 K). These set of syngas temperature is realistic for gasification plants. For completeness of the presented analysis the cases when both reactants are cold (case A), both reactants are preheated to 1000 K (case F), and the case with cold fuel and a oxidizer preheated to 1000 K (case G) has been considered. The scalar dissipation rate (SDR), denoted as N, is an important factor linking diffusion in MF space with

different geometrical configurations of inlets for the oxidizer and fuel were tested during the 6 y period of operation, including a coflow configuration31 with additional swirl air inlets, cross-jet configuration,32 and, most recently, a cyclonic configuration. The details of the geometrical configurations and their effects on the achieved combustion regime are discussed in section 2.3. The combustion chamber is equipped with windows that allow for direct observation of the flame characteristicsone at the top and one in the middle (or bottom, depending on the configuration) of the chamber. Because the flue gases are subjects to constant environmental monitoring, the level of nitrogen oxides can be continuously measured, as well as the indicators of combustion completeness, specifically, the content of carbon monoxide and total organic carbon (TOC). When the produced gases must be ignited, they become too diluted for stable burning or the combustion is incomplete, which occurs when starting up or switching off, and the auxiliary oil burner can be switched off. Direct observations of the flames during unstable operation periods, such as lighting up or extinguishing, along with manual control over C


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Figure 2. Profiles of temperature in MF space: (a) for syngas SG1 and (b) for syngas SG7. HRR (ωT) in MF space: (c) for syngas SG1 and (d) for syngas SG7. Note the difference in scale (right axis) for HRR profiles of methane equivalents. SDR is equal to 5 s−1. The cases with different values of fuel and oxidizer temperature are marked with A−G. previously used for multidimensional CMC modeling.35,36 The equations were solved numerically on a one-dimensional grid using the implicit ordinary differential equation (ODE) solver VODPK.37 This solver was used for two reasons: it can handle large systems of ODEs, and it was tested on various combustion systems.35,36 Mesh dependency tests showed that a homogeneous grid with 200 nodes was adequate to obtain accurate solutions. The relative and absolute tolerances were 1 × 10−5 and 1 × 10−10, respectively. The time step was fixed as 1 × 10−6 s, and the code was run for enough time to reach steady-state solutions of eqs 1 and 2. These reaction−diffusion equations in MF space are solved at atmospheric pressure for syngases SG1−SG8, whose compositions are given in Table 1. Because the syngases contain only trace amounts of higher hydrocarbons than methane, the GRI3 detailed mechanism of chemical reactions38 has been used. The mechanism is based on 53 species appropriate for syngas and includes more than 300 reactions. In ref 32, the results of syngas combustion based on GRI3 were compared with results from larger mechanisms with good agreement. Another comparison for the combustion of hot fuels with GRI3 and other mechanisms is presented in.20,32 Recently, the GRI3 mechanism was used in several simulations related to MILD combustion;15,24 but the new work of Sabia et al.25 has shown the mechanism is still not perfect for gases with a high content of CO2 or H2O.

mixing in real space. In physical space, the SDR is defined via gradients of Z, where Z is the ratio of fuel-originating compounds to all compounds,

⎛ ∂Z ∂Z ⎞ ⎟ N = D⎜ ⎝ ∂x ∂x ⎠

(3)

where D is the thermal diffusivity. When the counterflow configuration is assumed (one-dimensional case when fuel and oxidizer are flowing from opposite directions), eq 3 can be solved directly because in this particular configuration Z = ξ. The resulting function N(ξ) = −1

N0e−2(erfc (2ξ)) describing diffusion in the MF space is then used to solve eqs 1 and 2. Here, the maximum value of SDR (N0) is assumed to be relatively low, N0 = 5 s−1, which allows for comparing flame structures even for gases with very low calorific values. For higher values of N0, the combustion is easily extinguished, especially when the gases are strongly diluted. In practical systems very high values of the SDR are unlikely to be sustained because high N0 implies quick mixing which decreases gradients in the MF space. Numerical simulations of an industrial combustion chamber showed that the typical values of SDR are as low as 5 s−1 with local peaks on the order of 100 s−1.32 The effect of the SDR on the solutions of eqs 1 and 2 and consequently on the survey of the combustion regime is discussed in section 3.4. Equations 1 and 2 are solved using a reduced version of the code D


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Figure 3. Theoretically predicted combustion regime: (a) syngases derived from wood; (b) syngases derived from feathers. 2.3. Combustion Regimes. A simplified description of the combustion regimes was proposed by Cavaliere and de Joannon12 with two parameters: the reactants temperature Tin and the temperature increase during combustion, ΔT. The temperature Tin depends, obviously, on the temperatures of the two reactants and also on the proportion in which they are mixed. In MF space, To and Tf are the temperature boundary conditions for the fuel and oxidizer. To be consistent with the analysis in MF space, the stoichiometric value is always used to determine Tin.

Tin = (1 − ξstoich)To + ξstoichTf

noticeably narrower than profiles for the syngases, even when profiles of temperature in all cases remain broad. The HRR is significantly different in the two gases. In SG1, the main peak is at a stoichiometry, and there is a sharp secondary peak for higher ξ, which decreases with the increasing initial temperature. The second peak is not observed when the methane equivalent is burning. In SG7, the main peak is much narrower. It is nearly stoichiometric in cases with low initial temperature (B, C, and G), but it moves to the for higher value of MF when the fuel temperature is increased. For higher Tf, a broad second peak at ξ < ξstoich emerges that increases with increasing initial fuel temperature. The HRR profiles of the methane equivalent for SG1 reveals that prominent pyrolytic zone is present above the stoichiometry (negative value of HRR visible in Figure 2c). In the case of much more diluted SG7, the pyrolytic region almost disappears in the methane equivalent (Figure 2d). According to ref 39 the presence of pyrolytic region, observed when methane diluted in nitrogen is burned, together with flame thickening are the necessary conditions to achieve colorless and distributed combustion. This analysis showed that for gases more complex than methane diluted in nitrogen, like syngases derived from wood and feathers, this condition is not valid. The results for SG1 and SG7 showed the HRR peaks instead of a ubiquitous pyrolytic region. The HRR has negative values only in case E for SG1 but these values are negligible. No single reaction responsible for this peak has been identified, but it must obviously be related to carbon oxides, as presented in section 3.5. The absence of the pyrolytic region for complex gases was confirmed also in the analysis of Abtahizadeh et al.,19 who diluted fuel with flue gases instead of clear nitrogen. The results clearly illustrate the difference in the combustion of wood and feather syngases, despite the fact that characteristic flame thickening is visible in both cases. Flame thickening has previously been reported when cold gas (typically methane) was burned with preheated oxidizer40−43 or when the gas or oxidizer was diluted.44−46 The broadening is due to the dilution of the combustible components of the fuel in nitrogen and carbon dioxide, which slows down the reaction kinetics but makes even very low-calorific value fuels, such as SG7, flammable. The mixture of a 0.02 mass fraction of methane diluted in nitrogen is combustible when the fuel is preheated to 1400 K and the air is at 300 K21 or when the fuel is cold (300 K) but the air is hot (1300 K).47 Our work confirmed that a 0.04 mass fraction of methane (equivalent to syngas SG7) is combustible when the fuel temperature exceeds

(4)

The temperature increase during combustion is defined as ΔT = Tmax − Tin

(5)

where Tmax is the maximum temperature of combustion. When the temperature of the reactants Tin does not exceed the autoignition temperature Tign, the combustion is classical or pilot-assisted. Hightemperature air combustion (HTAC) and MILD combustion require that the reactants be preheated above the temperature at which autoignition occurs. Although this sharp demarcation does not consider the underlying physical and chemical complexity of the combustion processes of gaseous mixtures, it is useful for a quick quantitative description of the combustion behavior of diverse fuels such as gases derived from biomass gasification. In this paper, four combustion regimes are considered: • Classic (conventional) when Tin < Tign and ΔT > Tign. Flames are clearly visible in this mode. • Pilot-assisted when Tin < Tign and ΔT < Tign. Flames are clearly visible, but combustion is unstable and an additional source of heat is needed to sustain the process. • HTAC when Tin > Tign and ΔT > Tign. Flames are invisible. • MILD combustion when Tin > Tign but ΔT < Tign. Flames are invisible.

3. RESULTS 3.1. Laminar Counterflow Nonpremixed Flame Structure. In Figure 2 profiles of temperature (a and b) and HRR (ωT) (c and d) are plotted for all considered cases (A−G) introduced in section 2.2. Figure 2a and c shows results for syngases SG1 and its methane equivalent, while Figure 2b and d shows results for SG7 and its methane equivalent. The main feature of the flame structures of real low-calorific-value gases is that their temperature and the HRR profiles are broad. The analogous distributions for conventional fuelse.g., methanehave narrow peaks (see, e.g., ref 21). In fact, the HRR profiles for methane equivalents shown in Figure 2c and d are E


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Energy & Fuels Table 2. Summary of Combustion Regimes for Syngases SG1 and SG7 case

fuel temp Tf

oxidizer temp To

reactants temp Tin

combustion temp Tmax

increase in temp ΔT

combustion regime

Syngas SG1 A B C D E F G

300 800 1000 1200 1400 1000 300

K K K K K K K

300 300 300 300 300 1000 1000

K K K K K K K

300 539 635 730 826 1000 665

K K K K K K K

A B C D E F G

300 800 1000 1200 1400 1000 300

K K K K K K K

300 300 300 300 300 1000 1000

K K K K K K K

300 615 740 867 993 1000 560

K K K K K K K

1678 1858 1940 2025 2111 2174 1933

K K K K K K K

1378 1319 1306 1294 1286 1174 1268

K K K K K K K

classic classic classic classic classic HTAC classic

no reactions 1452 K 1562 K 1677 K 1790 K 1747 K 1427 K

837 821 810 797 747 868

K K K K K K

pilot assisted pilot assisted pilot assisted MILD MILD pilot-assisted

Syngas SG7

Figure 4. Combustion regime for syngas SG1: (a) no EGR is supplied; (b) oxidizer is composed of 30% EGR and 70% air; (c) equal amounts of EGR and air are supplied; (d) 70% EGR is supplied.

1200 K. Importantly, the results show that because of the hydrogen addition, the real gases are flammable even when their temperatures are lower than the temperature of their methane equivalents; e.g., SG7 is flammable at 800 K. The flame thickening, well visible in broad HRR profiles in MF space, implies more homogeneous combustion in physical

space. This was confirmed by numerical simulations performed in physical space presented in ref 32. 3.2. Survey of Combustion Regimes. The temperature profiles allow for the determination of combustion regimes according to the simplified approach described in section 2.3. The reactant temperature Tin, maximum temperature Tmax, and F


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Figure 5. Temperature increase ΔT of syngas SG1 as a function of the maximum SDR (N0): (a) cases A−E and G; (b) case F.

Figure 6. Profiles of OH in MF space for syngases (a) SG1 and (b) SG7. OH reaction rates for syngases (c) SG1 and (d) SG7. SDR is equal to 5 s−1.

the temperature increase ΔT were determined and are summarized in Figure 3 ((Tin − ΔT diagram) and Table 2. Wood syngases generally fall within the classic combustion regime though their heating value is small, less than 10% of that of methane. The value of ξstoich is smaller than that of feather syngas. Therefore, much air must be added, and the reactants are not sufficiently hot; i.e., Tin is insufficient unless the air is strongly preheated as in the HTAC regime in case F. For the feather syngas (SG7), the desirable MILD mode can be achieved more easily, even with a cold oxidizer. In this case, less air is required for stoichiometry, which is why it can burn in the MILD regime even with a cold oxidizer (case Eno preheating) provided that the syngas itself is sufficiently hot.

These conditions differ from the usual HTAC (SG1, case F) or MILD (SG7, case F) combustion setup in which the oxidizer is hot and the fuel is much colder, but they do in fact occur when the fuel comes from a gasifier and ambient air is drawn. The regime criteria described in section 2.3 are based on the fuel autoignition temperature (at atmospheric pressure as the installations are not pressurized), but precise measurements of Tign are unavailable. The estimated temperature, based on long-term measurements from gasification plants as discussed in section 2.1, are Tign = 850 ± 50 K for wood syngases SG1−6 and Tign = 950 ± 50 K for feather syngases SG7−8. The relatively high uncertainty related to the autoignition temperature does not affect the conclusion of the survey. As shown in G


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Figure 7. Profiles of CO in the MF space for syngases (a) SG1 and (b) SG7. CO reaction rates for syngases (c) SG1 and (d) SG7. SDR is equal to 5 s−1.

higher gradients in the physical space as defined in eq 3, accelerates diffusion in the MF space and consequently reduces Tmax and ΔT. Thus, high SDR can potentially affect the survey of the combustion regimes predicted based on the sharp criteria discussed in section 2.3. Moreover, an excessively large N0 may cause the abrupt extinction of the flame. To evaluate how strongly the predicted regime depends on the SDR, the dependence of ΔT on the maximum SDR N0 was determined and is presented in Figure 5. Figure 5a confirms that up to very high values of SDR, woodderived gases are burned conventionally. In contrast, Figure 5b shows that a nominal MILD regime would be achieved for wood syngas SG1 with strongly preheated air (case F Tin > Tign), but with a relatively high SDR. The SDR at which the flame is blown off depends strongly on the temperature of the reactants. In the cold syngas SG1, extinction occurs when N0 = 150 s−1, whereas for preheated fuel, combustion is sustained up to N0 = 500 s−1. When the gases are hot, there clearly is a large margin for increasing the SDR without the risk of extinction. Very high values of the SDR are, however, unlikely to be sustained in practice because high N0 implies quick mixing, which eventually homogenizes the MF gradients. The results of three-dimensional numerical simulations of burning in an industrial combustion chamber32 showed that the typical values of the SDR are as low as 5 s−1 with local peaks on the order of 100 s−1. Thus, the impact of the SDR on the survey of the combustion regimes for low-calorific-value gases is relatively small.

Figure 3, the MILD regime would still be achievable for feather syngases and HTAC for wood syngases. The borderline case E (wood syngases) suggests that the HTAC regime may be possible with slight air preheating, which would be technologically feasible. 3.3. Exhaust Gas Recirculation. The external EGR increases the flexibility of the combustion process and is often used in combustors.48 In gasification plants, it introduces an extra parameter, relatively easy to control, enabling adaptation to fuels of varying compositions and calorific values of the fuel. During typical operation, the EGR ratio varies from 0 (no EGR) to 0.3 (oxidizer contains 30% exhaust gases and 70% air). The recirculation ratio R is defined as the mass ratio of the recirculated flue gases and the oxidizer. The average composition of exhaust gases, based on measurements in an industrial plant,2 is as follows (mass fractions): N2 = 0.68, O2 = 0.03, CO2 = 0.25 and H2O = 0.04. In the operational conditions of the feather utilization plant, the value of R is up to 0.3, so the reduction in the oxygen mass fraction can be significant, almost 20%. In Figure 4, it is shown that the increasing R lowers Tmax and the temperature difference ΔT. For realistic values of R up to 0.3, the difference can be as high as 200 K. Because the recirculated flue gases are relatively cold, the transitions between combustion regimes are observed only with very high recirculation ratios. Thus, the external EGR has only a minor impact on the survey of the combustion regimes. 3.4. Scalar Dissipation Rate. The SDR links diffusion in the MF space with mixing in the physical space as described in section 2.2. A higher value of maximum SDR (N0), meaning H


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Figure 8. Three different configurations of the combustion chamber operationally tested in the feather gasification plant in Olsztyn: (a, d and g) coflow set up with additional air inlets;31 (b, e and h) cross jets;32 and (c, f and i) cyclonic cross jets. Pictures of the interior of the combustion chamber: taken from the upper (d, e and f), bottom (g), or middle peer window (h and i).

3.5. Profiles of OH and CO. As presented in Figure 6, the OH profiles and production rates are generally higher in SG1 than in SG7. The production is sensitive to the initial temperature. In SG1, the peaks are highest and widest in the HTAC regime (case F), whereas in SG7, they are highest and widest in the MILD regime (cases E, F, and D). In SG1, the peaks are at stoichiometry, whereas in SG7, they are systematically shifted toward lower ξ. The production rate moves toward higher ξ as the initial temperature is increased (cases of MILD combustion). The peaks of OH consumption are correlated with peaks of heat release rate presented in Figure 2(c and d). Note the difference in peaks location between syngases and their methane equivalent, remarkable for SG1 but still visible in SG7. The diagrams for carbon monoxide (Figure 7) show marked differences between the two gases. The difference in the slope is determined by the boundary condition at ξ = 1i.e., by the mass fraction of CO in the fuel. That mass fraction is nearly ten times higher in SG1 than in SG7 (see Table 1). In SG1, the mass fraction profile is monotonic for all cases. In SG7, however, they are monotonic only in the pilot-assisted regime, but MILD combustion produces a broad peak in the fuel-rich region (see the rescaled plot in the inset in Figure 7b). The net consumption of CO in both gases is obviously dictated by the net flux coming in from the fuel side, which is given by the slope of the mass fraction at ξ = 1. In SG7, this is more than ten times less than in SG1. The production rate

curves, however, are not rescaled in proportion. In fact, the peak production in SG7 is higher than it is in SG1. This peak is responsible for the large value of the CO mass fraction above stoichiometry, which, in the MILD combustion cases, exceeds the mass fraction in the fuel (the dash-dot lines in Figure 7b are above the value at ξ = 1). As for other production rates, in the MILD regime, the peaks are most shifted toward higher ξ. In SG7 combustion, the consumption takes place mostly below stoichiometry, whereas the production occurs above it. In the case of SG1, the consumption has its maximum at ξstoich and is somewhat symmetric, whereas the production occurs far into the fuel-rich region.

4. DISCUSSION An important feature that allows for quickly distinguishing between combustion regimes with low-temperature reactants (Tin > Tign) and high-temperature reactants (Tin < Tign) is the visibility of flames. When flames are visible, the combustion is conventional or pilot-assisted. When flames are invisible, MILD combustion14,15 or HTAC7 is achieved. This well-documented feature can be used for a qualitative contrast of the theoretically predicted regime with the combustion mode achieved in industrial gasification plants where gases are continuously burning. In the case of the Szepietowo wood chip gasification plant, the flames are clearly visible in the combustion chamber via the inspection window. The temperature is constantly measured by several thermocouples located at the top and the I


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combustion at Szepietowo is predominantly conventional, whereas Mild combustion is achieved at Olsztyn. In fact, as summarized in Table 3, MILD combustion is actually achievable at Olsztyn with a temperature of the residualderived gases of approximately 1000 K, instead of the 1400 K predicted theoretically. This occurs only when the combustion chamber is heated. This observation suggests the importance of heat transferred from the walls of the combustion chamber in achieving MILD combustion.

outlet of the combustion chamber. An additional thermocouple is occasionally used to determine the temperature in the area where the flames are visible (this measurement cannot be constant owing to the short lifetime of the thermocouple in this condition). The results can be as high as 1900 K. The clearly visible flames and relatively high maximum temperature confirm the theoretical prediction that the wood-derived gases are burning in the conventional (classic) regime. During the stable operation of the organic residuals gasification plant in Olsztyn, the flames are usually barely visible via the inspection windows2,32 as in the case of MILD combustion, FLOX, or HTAC.7,9−11,15,42,49 During the 6 y working period, three different geometric configurations of the combustion chamber were tested long-term during normal industrial operation. The configurations included a coflow setup31 presented in Figure 8a, cross-jet32 shown in Figure 8b, and cyclonic configuration presented in Figure 8c. In each case, the flameless mode was achieved as shown in Figure 8d−i. All configurations are summarized in Table 3. Logs from environmental monitoring show that the TOC and CO

5. CONCLUSIONS Numerical simulations of laminar non-premixed flames in the combustion of different types of naturally preheated lowcalorific-value gasification gases have been performed. The heat release rate in biomass syngas are different from those in the diluted methane equivalent where a pyrolysis zone is typically visible and the maximum heat release better correlates with the stoichiometry. The structures of OH and CO also indicates differences. This so-called methane equivalent cannot be used as an analog of real low-calorific-value gases. The analysis shows that achieving of MILD combustion regime is possible only when fuel is highly diluted and preheated. In fact gases derived from feathers gasification (equivalent to 0.04 CH4 mixed with N2) are sufficiently diluted while gases derived from wood (equivalent to 0.8 CH4 mixed with N2) are not. Consequently, syngas obtained from gasifying wood chips is burned mostly in the conventional regime, as predicted theoretically and confirmed by observation from the plant. Combustion can be driven toward HTAC by increasing the temperature of either the fuel or the oxidizer, but MILD combustion is difficult to achieve for this fuel, even with a high value of SDR and large EGR. It is much easier to achieve the MILD combustion regime with the more diluted syngas coming from a gasifier loaded with poultry feathers. Both theoretical predictions and observations from the industrial plant are in agreement that for hot syngas from feathers MILD combustion can be achieved even with a cold oxidizer. The long-term observation from the gasification plant showed that MILD combustion is not necessarily related to the specific geometric setup but can be achieved for different configurations. The proposed simplified approach based on mixture-fraction analysis, rather than specific geometric setup, is then useful for a prompt overview of the combustion regimes of noncaloric gases derived from biomass gasification.

Table 3. Summary of Three Different Configurations of Combustion Chamber Operationally Tested in a Feather Gasification Plant in Olsztyn coflow31

configuration fuel inlet air inlet EGR temp exhaust gasesa upper tempb middle tempc auxiliary burner EGR content NOx emissiond TOC and CO emission visibility of flameless theoretical regime achieved regime

cross-jets32

Temperatures 1000 ± 50 K 1000 ± 50 K 295 ± 15 K 295 ± 15 K 400 ± 20 K 400 ± 20 K 1250 ± 80 K 1300 ± 80 K 1200 ± 100 K 1250 ± 50 K NA 1300 ± 50 K switched off switched off 20%

cyclonic cross-jets 1000 ± 50 K 295 ± 15 K 400 ± 20 K 1250 ± 50 K 1350 ± 50 K 1300 ± 50 K switched off

100 ± 30 mg/kg

optional (less than 5%) 120 ± 30 mg/kg

optional (less than 5%) 70 ± 30 mg/kg

nearly zero

below limit

nearly zero

occasionally

rarely

rarely

pilot-assisted

pilot-assisted

pilot-assisted

MILD combustion

MILD combustion

MILD combustion

a Temperature at the outlet of the combustion chamber. bTemperature measured by the thermocouple located at the top of the chamber. c Temperature measured by the thermocouple located in the middle of the chamber. dEnvironmental limit for this installation is 200 mg/kg.

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AUTHOR INFORMATION

Corresponding Authors

emissions were usually negligible, and the NOx emission did not exceed the stringent environmental norm.2 The stability and self-sustainability (no need for an auxiliary burner when the combustion chamber is heated up), barely visible flames, and low NOx emission indirectly confirm that MILD combustion was achieved. When the fuel is not sufficiently hot, the flames became clearly visible and localized in the upper part of the combustion chamber. This occurs when the installation is coincident with the switching on or off. In these phases, the auxiliary pilot burner must be switched on as in the pilot-assisted combustion regime. These long-term qualitative observations support the results of the survey on combustion regimes, indicating that

*E-mail: [email protected] (K.K.). *E-mail: [email protected] (E.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

K.K. thanks Konrad Bajer (1956−2014) for his invaluable supervision and support. K.K. thanks Marek Dudyński for extensive discussions. This work was supported by the Ministerstwo Nauki i Szkolnictwa Wyższego (Poland), grant Iuventus Plus IP2014 024373. J


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Regimes of Nonpremixed Combustion of Hot Low-Calorific-Value

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