Estimation of fuel-nitrogen oxide emissions from the element composition of the solid or waste fuel

Fuel 94 (2012) 75–80

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Estimation of fuel-nitrogen oxide emissions from the element composition of the solid or waste fuel Isabel Vermeulen ⇑, Chantal Block, Carlo Vandecasteele University of Leuven, Department of Chemical Engineering, Willem de Croylaan 46, B-3001 Heverlee, Belgium

a r t i c l e

i n f o

Article history: Received 18 August 2010 Received in revised form 25 November 2011 Accepted 29 November 2011 Available online 21 December 2011 Keywords: NOx emission Fuel-N Waste incineration Inventory analysis

a b s t r a c t The formation of NOx during solid fuel or waste combustion is a very complex process, making it particularly difficult to accurately estimate NOx emissions. The main purpose of the present study was to develop a correlation between the composition of a solid fuel or (mixed) waste and the fuel-NOx emissions from its combustion, allowing accurate prediction of NOx emissions from incineration processes, useful for assessing the environmental impact. Literature was scanned for relevant and complete data sets concerning different solid fuel or waste types, incinerated in specific types of incinerators. Previously reported correlations showed a linear relationship between the conversion of fuel-N to NO and the O/N and H/N weight ratio, deduced from the element composition of a solid fuel. The overall fitting of the collected data sets in terms of these ratios is, however, rather poor. Considerations of both the element composition of the fuel, and the functional form of the fuel-N lead to the conclusion that for all data sets a good linear correlation existed between a single weight ratio, the CH/N ratio, and the conversion of fuel-N to NO. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction and objectives Due to the decrease of fossil fuel resources and driven by the European waste policy, the energy use of waste (waste-to-energy) has increased significantly [1]. Nitrogen oxide emissions (NOx) are a major environmental concern associated with all combustion processes, including waste incineration [2–5]: NOx emissions contribute to acidification, photochemical ozone creation and eutrophication [6]. It is commonly assumed that at given operating conditions, emissions of e.g. CO2, SO2, heavy metals, are proportional to the respective input of C, S and heavy metals into the incinerator [7,8]. Conversion is thus only dependent on operating conditions and use is generally made of linear transfer coefficients to calculate the emissions [7,9]. The conversion of fuel-nitrogen to NOx, however, not only depends on the operating conditions and the input of N, but also on the composition and specific characteristics of the input [10–13]. This makes it particularly difficult to estimate NOx emissions when a solid fuel or mixed waste is burnt in an incinerator, and to predict possible variations in NOx emissions with input variation of the incinerator. The formation of NOx during combustion is very complex and is described by hundreds of elementary reactions [10,13]. NOx represents the sum of nitrogen monoxide (NO) and nitrogen dioxide (NO2) [11,12]. Only NO emissions are considered rele-

vant for waste incineration, as NO makes up 95% of NOx emissions [14,15]. The three main sources of NO emissions during combustion are thermal, prompt and fuel-NO [5,10–12], with only fuelNO being relevant at the operating conditions encountered in waste incinerators. Fuel-NO is formed by oxidation of the organically bound nitrogen in the fuel (fuel-N) [11]. The main purpose of this study was to find a strong, but simple correlation between the composition of a solid fuel or waste and the conversion of fuel-N to NO at near-constant operating conditions of an incinerator. This will allow to predict NOx emissions when the used fuel is replaced by another fuel, waste or biomass, and to estimate the corresponding variation in environmental impact. Literature describes that the conversion of fuel-N to NO can be correlated to the fuel weight-ratio of oxygen to nitrogen (O/N ratio) or/and of bound hydrogen to nitrogen (H/N ratio) [16–18]. The first part of the present study assesses the proposed linear regression lines of the conversion of fuel-N to NO in terms of the O/N and H/N weight ratios for different sets of literature data [5,16,18–22]. These correlations proved unsatisfactory for some data sets. A better correlation was obtained using the same data sets, between the conversion of fuel-N to NO and the CH/N ratio. 2. Background information 2.1. Fuel-NO formation mechanisms

⇑ Corresponding author. Tel.: +32 16322344; fax: +32 16322991. E-mail addresses: [email protected] (I. Vermeulen), [email protected] (C. Block), [email protected] (C. Vandecasteele). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.11.071

Most solid fuels contain organically bound nitrogen (fuel-N), whose oxidation leads to the formation of fuel-NO, as illustrated in Fig. 1.

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N2O N2 light gas-N volatile N species

NH3

tar-N

Fuel-N char-N

+NO O,H,OH

NCO NH

+O2

+O2

char surface

HCN

NO

Fig. 1. Most important fuel-N conversion pathways.

During the first combustion phase (devolatilization), fuel-N is split between volatile N species (light gas-N and tar-N) and charN. The distribution of fuel-N between volatiles and char as well as the distribution of the volatile N species depends mainly on the operating conditions of the incinerator (e.g. temperature) and on the properties of the solid fuel. The gas-phase reactions during combustion (volatile-N pathway) lead to the formation of NH3, HCN and HNCO, considered as primary intermediates, and to small amounts of NO, originating from direct oxidation of the fuel-N. In general the formation of HNCO is negligible compared to that of NH3 and HCN [23]. The primary intermediates react with O, H and OH radicals to form principal intermediates, NCO and NH being the most important ones [4,15]. Both the principal intermediates and the char-N are subsequently oxidised to NO or react with NO to produce N2 and N2O. High concentrations of fuel-N will lead to a decreased conversion to NO, as the reactions between two nitrogen-containing species giving N2 and N2O are favoured compared to the formation of NO [12]. The functional form of the fuel-N depends strongly on the carbon content of the solid fuel. High rank solid fuels (with a high carbon content) mainly contain nitrogen bound in heterocyclic ring structures (particularly as pyrrolic-N), while low rank solid fuels contain a significant amount of nitrogen in the form of amines and quaternary-N [24,25]. Volatile-N compounds are mainly released as tarN from high rank fuels, while in low rank fuels they are mainly released as light gas-N, almost directly from the solid fuel [12,24]. The primary intermediate HCN is mainly formed from the heterocyclic structures, present in the tar-N, whereas NH3 is mainly formed from the amines or quaternary-N, present in the light gas-N. In a following step, HCN can be attacked by O, Hand OH radicals to form NCO and NH (reactions (R1) and (R5)). These principal intermediates are either oxidised to NO (reactions (R2) and (R6)) or converted to N2O or N2 by reaction with NO (reactions (R3), (R4), and (R7)) [3,4,10,15,26]:

HCN þ O ! NCO þ H

ðR1Þ

NCO þ O ! NO þ CO

ðR2Þ

NCO þ NO ! N2 þ O þ CO

ðR3Þ



NCO þ NO ! N2 O þ CO 



ðR4Þ

HCN þ O ! NH þ CO

ðR5Þ

NH þ O2 ! NO þ OH

ðR6Þ

NH þ NO ! N2 þ OH

ðR7Þ

At temperatures above 850 °C, a net production of NO is observed [27,28]. At slightly lower temperatures, however, HCN and its principal intermediates further react with NO to produce

N2O and to a lesser extent N2. Especially in the temperature range of the freeboard of a fluidized bed combustor (700–900 °C), HCN is an important precursor for N2O [4,13,26]. At still lower temperatures (200–400 °C) and fuel-lean conditions, typically reached during the flue gas cleaning, N2O can be transformed again in NO [4,13], according to:

N2 O þ O ! 2NO

ðR8Þ 



NH3 on the other hand, can react with H and OH radicals to form NH2 and subsequently NH. Analogous to the reaction paths of HCN, NH is subsequently further oxidised to NO or reacts with NO to form N2. These reactions of NH3 can be summarised as follows [4,10,15]:

NH3 þ O2 ! NO þ H2 O þ H

ðR9Þ

NH3 þ NO ! N2 þ H2 O þ H

ðR10Þ

As for HCN, at temperatures above 850 °C, a net production of NO is observed (reaction (R9)) [27,28]. NH3 is an important reductant of NO at temperatures of approximately 800 °C, a situation encountered and used in the thermal deNOx process [28]. Considering both primary intermediates, NH3 is the strongest reductant for NO under practically all conditions [26,27]. Char-N is in general directly oxidised to NO; the intermediates HCN and NH3 are only formed in small quantities. In analogy with the gas-phase reactions (R3), (R4), (R7), and (R10), NO can also be reduced by reactions with the char-N, according to the reaction mechanism [11,12]:

Char-N þ O2 ! NO

ðR11Þ

Char-N þ NO ! N2 þ . . .

ðR12Þ

The net amount of NO formed from these heterogeneous reactions strongly depends on the intrinsic reactivity and internal surface area of the char [11]. Typically, the overall production of NO from char combustion is less important than the one resulting from the combustion of the volatiles, especially at high temperature and under fuel-lean conditions [2,4,11,15]. 2.2. Correlation between fuel-N to NO conversion and fuel composition It is found difficult to correlate the conversion of fuel-N to NO with the properties of the solid fuel incinerated [4,12,13]. Most literature sources state that it increases linearly with increasing O/N ratio, following the suggestions of Aho et al. [16] and Hämäläinen et al. [17]. Chyang et al. [18] investigated the possible linear relationship, through the origin, between the conversion of fuel-N to NO and the O/N and H/N weight ratios of the solid fuel. Their results showed that a better linear correlation could be obtained with the H/N ratio than with the O/N ratio (correlation coefficient, r2 = 0.97 for H/N and 0.82 for O/N, respectively).

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3. Methods The present paper firstly assesses the linear correlation between the fuel-NO formation and the O/N and H/N ratios of the solid fuel, as suggested by Aho et al. [16], Hämäläinen et al. [17] and Chyang et al. [18], for several sets of literature data. These data sets deal with different types of solid fuel or waste (biomass, refuse derived fuel (RDF), municipal solid waste (MSW), paper, cardboard and different types of coal), burnt in different incinerators (fluidized bed combustor, batch reactor, fixed bed reactor and entrained flow reactor), at a temperature range of minimum 450–750 °C and maximum 973–1123 °C (Table 1). Data on fuel composition (carbon,hydrogen,oxygen and nitrogen concentration), fuel-N to NO conversion and operating conditions of the incinerator were available for these literature sources. Correlation coefficients (r2) were calculated to assess the validity of the proposed linear literature relationships. Secondly, a correlation between the CH/N ratio and the conversion of fuel-N to NO was proposed and investigated, to account for both the element composition of the fuel, and also the functional form of the fuel-N. The same data sets were used to calculate the correlation coefficients for this alternative linear correlation. 4. Results and discussion 4.1. Correlation between fuel-N to NO conversion and O/N and H/N weight ratio Results of Aho et al. [16] and Hämäläinen et al. [17] indicated a linear correlation between the fuel-N to NO conversion and the O/N ratio of the solid fuel. They explained this by indicating that at high oxygen concentration in the solid fuels, generally more phenolic hydroxyls are present, enhancing the conversion of HCN to NH3 and the principal intermediates, leading to the formation of NO. This statement was however contested by Chyang et al.

[18], arguing that AOH groups, present in a carboxyl group (ACOOH), will obstruct the formation of NO rather than enhance it. On the other hand, high concentrations of hydrogen in the solid fuel can be related to the formation of fuel-NO in different ways. With a high hydrogen content in the fuel, the amount of H and OH radicals increases, which contributes to the formation of intermediates. In addition NH3 and HCN, the most important primary intermediates, both contain hydrogen [18]. Moreover a higher concentration of hydrogen, at constant carbon concentration, corresponds in general to more aliphatic and/or volatile structures, which can more easily be degraded to form the primary and principal intermediates. These arguments thus favour a positive correlation between the hydrogen concentration of the solid fuel and the conversion of fuel-N to NO. A high concentration of fuel-N will result in a decreased conversion of fuel-N to NO. Indeed, higher concentrations of fuel-N will lead to more nitrogen containing species (intermediates and oxides) and the reactions between two nitrogen-containing species, leading to N2 or N2O, are favoured compared to the oxidation of nitrogen-containing species, leading to NO [12]. This inverse linear correlation between the concentration of fuel-N and its conversion to NO has also been frequently discussed in literature [2,5,11,12,16,18,29]. The possible correlations between the conversion of fuel-N to NO and the O/N and H/N weight ratio were tested on several sets of data on solid fuel composition and corresponding fuel-N to NO conversion (Table 1). Aho et al. [16] for instance determined the conversion of fuel-N to NO for nine different solid fuels, all incinerated under approximately the same conditions in an entrained flow reactor. Using these data, the linear regression line through the origin of the conversion of fuel-N to NO of the different solid fuels on their H/N and O/N weight ratio was calculated (Fig. 2). From the correlation coefficients it is clear that for the H/N ratio (r2 = 0.81) the regression line through the origin provides a better fit than for the O/N ratio (r2 = 0.56).

Table 1 Literature sources, concerning the conversion of fuel-N to NO for different solid fuel or waste types, incinerated in different incinerators, at a fixed temperature range. Incinerator

Exp. conditions: temperature

Solid fuel or waste type

Average elemental analysis

Reference

Entrained flow reactor

800 °C

Columbian coal, lignite, birch bark, fir bark and different types of peat

Aho et al. [16]

Fluidized bed combustor

Bed temperature: 700–900 °C

Spruce wood, Beech wood, Alder wood, straw and malt waste

Fixed bed reactor

Freeboard temperature: 973–1123 °C

Newspaper, cardboard, glossy paper, LDPE and PVC

Fluidized bed combustor

Bed temperature: 800–1000 °C

Different mixtures of lignite and municipal solid waste (MSW) (10%, 20%, 30% and 40% of MSW)

Vortexing fluidized bed combustor

Bed temperature: 800 °C

Rice husk, bituminous coal, low sulphur subbituminous coal, high sulphur subbituminous coal and soybean

Fluidized bed combustor

Bed temperature: 850–900 °C

Different types of refuse derived fuels

Batch reactor

450–750 °C

Solid biomass: sawdust, bark, waste wood and medium-density fibreboard

C:61.3 wt% H:5.9 wt% O:31.0 wt% N: 1.5 wt% C:51.3 wt% H:5.8 wt% O:41.2 wt% N: 1.5 wt% C:54.7 wt% H:7.3 wt% O:28.4 wt% N: 0.1 wt% C:62.1 wt% H:6.1 wt% O:25.4 wt% N: 1.9 wt% C:44.8 wt% H:4.8 wt% O:24.7 wt% N: 2.4 wt% C:65.2 wt% H:7.8 wt% O:23.1 wt% N: 1.6 wt% C:48.3 wt% H:6.2 wt% O:41.5 wt% N: 2.0 wt%

Winter et al. [19]

Sorum et al. [20]

Suksankraisorn et al. [21]

Chyang et al. [18]

Hernandez-Anatol et al. [22]

Stubenberger et al. [5]

I. Vermeulen et al. / Fuel 94 (2012) 75–80 100 90 80 70

r2(O/N) = 0.56

r2(H/N) = 0.81

60 50 40 30 20

O/N H/N

10 0 0

20

40

60

80

100

120

140

160

conversion of fuel-N to NO [%]

conversion of fuel-N to NO [%]

78

y = 6.84x + 13.26 r2(CH/N) = 0.96

100 90 80 60 50 40 30

O/N H/N CH/N

20 10 0 0

H/N, O/N

y = 0.58x + 19.56 r2(O/N) = 0.80

y = 3.84x + 14.81 r2(H/N) = 0.94

70

20

40

60

80

100

120

140

160

CH/N, H/N, O/N

Fig. 2. Conversion of fuel-N to NO (%) as a function of the H/N and O/N weight ratio (based on data from Aho et al. [16]).

Table 2 Correlation coefficient (r2) of the linear regression line through the origin of the conversion of fuel-N to NO (%) on the H/N, O/N and CH/N weight ratio for the different data sets. Data set

r2(H/N)

r2(O/N)

r2(CH/N)

Aho et al. [16] Winter et al. [19] Sorum et al. [20] Suksankraisorn et al. [21] Chyang et al. [18] Hernandez-Anatol et al. [22] Stubenberger et al. [5] Average

0.81 0.83 0.76 0.30 0.97 0.97 0.79 0.77

0.56 0.84 / 0.24 0.82 0.62 0.80 0.55

0.86 0.84 0.88 0.81 0.95 0.99 0.79 0.87

These linear correlations through the origin were similarly verified for the other data sets; the corresponding correlation coefficients are summarised in Table 2, which shows again that the conversion of fuel-N to NO is better correlated to the H/N ratio than to the O/N ratio, in agreement with Chyang et al. [18]. 4.2. Correlation between the fuel-N to NO conversion and CH/N weight ratio It is generally accepted that a good correlation corresponds to a correlation coefficient of 0.81 for r2 or 0.9 for r [30]. The previous calculations (Table 2) demonstrated a rather poor regression of the conversion of fuel-N to NO on the H/N weight ratio for data of Stubenberger et al. [5], Sorum et al. [20] and Suksankraisorn et al. [21]. Similarly, the correlation on the O/N weight ratio, was even poorer. The use of the CH/N ratio, was therefore introduced and tested by the present authors. In practically all cases a better or similar fit was obtained, the correlation coefficients exceeded 0.81 for all data sets (except for Stubenberger et al. [5]: r2 = 0.79) and averaged 0.87 (Table 2). Even the data of Suksankraisorn et al. [21], with a poor correlation for the O/N and H/N methods, was fitted fairly well. The improved correlation can be attributed to the fact that when the CH/N ratio is considered, the rank of the solid fuel and consequently also the functional form of the fuel-N, is taken into account (Section 2.1). As the carbon content increases, nitrogen is preferably bound in heterocyclic structures leading to the formation of HCN, rather than being present in the form of amines or quaternary-N structures, leading to the formation of NH3 [24]. NH3 is a stronger reductant of NO than HCN, as HCN tends to reduce NO to N2O, rather than to N2 [26,27]. Taking into account that N2O can degrade again to NO in fuel lean conditions and at low temperature (reaction (R8)), it can thus be explained that higher amounts of carbon correspond to a higher net conversion of fuelN to NO.

Fig. 3. Conversion of fuel-N to NO (%) as a function of the H/N, O/N and CH/N weight ratio (based on data from Aho et al. [16]).

Although in literature, a linear correlation through the origin is assumed for these composition-determined parameters, it must be noticed that for the incineration of biomass [5,16,19], the best fitting regression line of the conversion of fuel-N to NO on O/N and H/ N does not pass through the origin (Fig. 3). Also for the correlation with CH/N, the regression line of biomass crosses the ordinate axis at a conversion between 5% and 15%, indicating that a certain percentage of the conversion of fuel-N to NO is independent of the CH/N ratio. Stubenberger et al. [5] distinguished two separate formation mechanisms of fuel-NO emissions during the combustion of biomass fuels: one due to the ‘volatile pathway’ and a second due to the oxidation of char-N. The latter formation mechanism was found to be typical for biomass fuels and amounted to 7–30% of the total NO emissions. This way, at least two experiments should be conducted to estimate the conversion of fuel-N to NO for biomass fuels in order to determine this correlation with a regression line not forced through the origin. 4.3. Application The correlation above relates the fuel-NO emissions at constant process conditions, to the element composition of a solid fuel. If for one single fuel composition the NO emission is measured, the conversion of fuel-N to NO can be estimated for other solid fuel compositions, incinerated in the same incinerator and under the same conditions. This was verified, using the different data sets from literature (Table 1). Starting from the CH/N ratio and the corresponding conversion of fuel-N to NO for one reference solid fuel, the conversions

Table 3 Average relative deviation and range of relative deviations of the different data sets for a reference fuel. Data set

Reference solid fuel

Average relative deviation (%)

Range of relative deviations (%)

Aho et al. [16]

LS-peat and CB-peat B-wood and malt waste Cardboard 20% MSW

3.5

[29.1;8.4]

2.6

[22.6;12.0]

Bituminous coal RDF1

7.1

Bark and MDF board

0.8

Winter et al. [19] Sorum et al. [20] Suksankraisorn et al. [21] Chyang et al. [18] Hernandez-Anatol et al. [22] Stubenberger et al. [5]

0.90 1.3

0.1

[27.6;46.6] [1.3;4.3] [19.6;7.3] [4.4;4.6] [12.0;19.5]

I. Vermeulen et al. / Fuel 94 (2012) 75–80

of all other solid fuels were estimated. For data sets concerning biomasses two reference fuels were selected ([5,16,19], Table 3). In Table 3 the average relative deviation and the range of relative deviations between the estimated and the actual conversions of fuel-N to NO are summarised for a given reference fuel. A solid fuel with a moderate conversion of fuel-N to NO was selected as reference fuel, but similar results could be found using other solid fuels as reference, as long as their conversion was not too close to 0% or 100%. A negative relative deviation indicates an underestimation of the conversion, a positive relative deviation, an overestimation. Overall, the obtained estimates were acceptable: only one relative deviation amounted to 46.6%, the others were all much smaller. The average relative deviations ranged from 7.1% to 1.3% for the different data sets.

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this correlation takes both the element composition, and the functional form of the fuel into account. The regression line for biomass fuels does not pass through the origin: at CH/N  0, the conversion of fuel-N to NO ranged from 5% to 15%. The proposed correlation allows to estimate the conversion of fuel-N to NO using the element composition of the solid fuel (CH/N ratio), from conversion data for one particular CH/N. This way of estimating was verified, using different data sets from literature and an acceptable fitting was obtained. Besides, it was shown that controlling the fuel-NOx emissions by air staging or by implementing a deNOx installation does not affect the validity of the proposed correlation, it will merely alter the slope of the correlation curve. References

4.4. Control of NOx emissions during solid or waste fuel combustion So far, near-constant operating conditions of the incinerator were assumed. However, combustion modifications, such as staging of the combustion air, are increasingly applied to control NOx emissions [28,31,32]. The basic idea of air-staging is controlling the NOx formation by controlling the oxygen availability, as NOx emissions are positively correlated to the excess air [26,31,33– 35]. Staging or splitting of the combustion air limits the oxygen availability in the flame, thus creating an oxygen lean primary combustion zone in which a considerable part of the fuel-N is released as N2 [34,36]. The secondary combustion zone is oxygen rich in order to achieve complete combustion of the gases, but the oxygen excess is kept to a strict minimum, function of the reference threshold O2-limit as fixed by the prevailing legislation (11 vol.% for MSW incinerators, 3 vol.% for standard boilers, etc.). By applying air staging, a reduction of the NOx emissions by 30–60% can typically be achieved, compared to single-stage air injection [28,21]. Recent studies [5,21] have identified a linear correlation between the conversion of fuel-N to NO and the oxygen availability in the primary combustion zone. Therefore, air staging will not influence the validity of the proposed linear correlation between the conversion of fuel-N to NO and the CH/N ratio, it merely reduces the slope of the correlation curve. Besides possible combustion modifications, in many incinerators nowadays a deNOx installation (SNCR or SCR) was implemented, enabling the reduction of the (fuel-N) NOx emissions with 50–80% [28,37–39]. Such a deNOx installation enables the reduction of (fuel-N) NOx by addition of NH3, similar to reaction (R10). Mahmoudi et al. [28] found that a maximum reduction of the NOx emissions by about 84% could be achieved for an NH3/NO-ratio of 0.9–1.2, independent of the original NOx concentration. Further increasing the amount of NH3, did not increase the reduction of NOx emissions due to oxidation of the excess NH3 and/or NH3 slip [28]. It appears thus that roughly the same percentage of fuel-NO will be reduced by a deNOx installation, regardless of the solid fuel type and the respective fuel-NOx formation. This way, just like air staging, implementation of a deNOx installation will only influence the slope of the correlation curve between the CH/N ratio and the conversion of fuel-N to NO, but will not affect the correlation itself. 5. Conclusions Linear correlations from literature for the conversion of fuel-N to NO in terms of the O/N and H/N weight ratio, show a rather poor overall regression coefficient (r2avg of 0.77 for H/N and 0.55 for O/N). For the same data sets a better linear correlation through the origin between the conversion of fuel-N to NO and the CH/N ratio was obtained ðr2avg ¼ 0:87Þ. This improvement is the result of the fact that

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