# Impact of the type of particle boards on the nitrogen fate during their pyrolysis and combustion

Thomas Bertus, Jérôme Lemonon, Francisco Javier Escudero Sanz, Sylvain Salvador

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Thomas Bertus, Jérôme Lemonon, Francisco Javier Escudero Sanz, Sylvain Salvador. Impact of the type of particle boards on the nitrogen fate during their pyrolysis and combustion. Fuel Processing Technology, 2025, 277, pp.108316. 10.1016/j.fuproc.2025.108316. hal-05264673

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Research article

# Impact of the type of particle boards on the nitrogen fate during their pyrolysis and combustion

Thomas Bertus<sup>a,b,*</sup>, Jérôme Lémonon<sup>b</sup>, F. Javier Escudero Sanz<sup>b</sup>, Sylvain Salvador<sup>b</sup>

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

**Keywords:**

Particle board

Combustion

NOx

NOx precursors

Nitrogen fate

## ABSTRACT

Particle boards, wastes made out of wood particles bonded with nitrogen-rich adhesives, produce high NOx emissions during combustion, requiring control in biomass grate furnaces. However, the diversity of particle board feedstocks has often been overlooked, and the specific effects of different types have not been studied, despite accounting for over 10 % of the total volume.

This work analyzes nitrogen behavior during combustion of standard, moisture-resistant, and fire-retardant particle boards. The combustion process was investigated as a whole, but also by proceeding separately to pyrolysis and char oxidation experiments. Thermogravimetric analysis and experiments conducted in a cross-fired fixed bed reactor were performed under both air and inert (N2) atmospheres. The nitrogen content in various combustion products (incondensable gases, condensates, and residual solids) was quantified to assess the impact of chemical treatments on nitrogen fate.

Results showed that standard and moisture-resistant particle boards showed comparable combustion behaviors. Notable differences emerged during the combustion of fire-retardant particle boards, likely due to the influence of fire-retardant agents. In these cases, a slower heating rate within the bed and reduced hydrogen cyanide (HCN) emissions were observed compared to the other two types. Across all experiments, most of the nitrogen released was found in condensates

## 1. Introduction

In recent years, there has been a significant rise in wood-based panels production, with an approximate increase of 223 % between 2000 and 2022 [1]. This growth trend is projected to continue in the coming years [2]. Particle boards, despite no longer being the predominant wood-based panel in terms of production anymore, still account for 26.5 % of the global production [1]. To ensure sustainable practices, the industry is increasingly utilizing recycled wood to minimize the need for tree cutting in particle board manufacturing processes. The growing utilization of recycled wood and recycled particle boards requires the usage of a greater quantity of adhesives whose nature depends on the specificities of the particle boards. The most common adhesive is urea formaldehyde (UF); 85 % of particle board manufacturing is bonded with UF [2]. Other adhesives are phenol-formaldehyde (PF), melamine-urea-formaldehyde (MUF) and polymeric diphenylmethane diisocyanate (pMDI).

After recycling, particle boards' mechanical properties deteriorate,

meaning that it is not possible yet to recycle them indefinitely [3,4]. When particle boards cannot be recycled, the most common way to dispose of them is to burn them in large scale biomass grate furnaces or municipal solid waste incinerators, thus permitting to recover heat from their combustion. However, most particle boards have a high nitrogen content due to the presence of nitrogen in great quantities in UF and MUF. The successive recycling of particle boards and adhesive adding results in particle boards containing increasing amounts of nitrogen over the years.

This increasing nitrogen content is especially problematic when the particle board is burned. In this process, nitrogen present in the panels is first converted during pyrolysis into various compounds such as ammonia (NH₃), hydrogen cyanide (HCN), and isocyanic acid (HNCO). These compounds subsequently undergo either oxidation in the gas phase, resulting in the formation of nitrogen oxides (NOx) or reduction to produce N₂. Some nitrogen also remains in the char during the pyrolysis stage and is converted into NOx or N₂ during the char oxidation stage. NOx are pollutant gases that represent a hazard for the

<footnote><sup>a</sup> Compte.R, 4 rue de l'Industrie, Arlanc, Auvergne-Rhône-Alpes, FR 63220, France</footnote>

<footnote><sup>b</sup> IMT Mines Albi-Carmaux, Centre RAPSODEE, UMR CNRS 5302, Campus Jarlard, Albi, Occitanie, FR 81013, France</footnote>

<footnote>* Corresponding author at: Compte.R, 4 rue de l'Industrie, Arlanc, Auvergne-Rhône-Alpes, FR 63220, France.
E-mail address: thomas.bertus@mines-albi.fr (T. Bertus).</footnote>
environment. From a health perspective, they can cause respiratory issues [5]. From an environmental standpoint, they can cause acidic rains and act as greenhouse gases [6,7]. Therefore, it is imperative to reduce their emissions.

For that purpose, it is necessary to study the combustion of particle boards, with a focus on nitrogen species. As shown in Fig. 1, nitrogen fate during thermal treatment of wood-based panels has been studied numerous times [8–18]. Most of these papers concern the medium density fiberboard (MDF), but the thermal behavior of MDF seems to be similar to that of particle boards [19,20]. Most of these studies were focused on wood-based panels pyrolysis, either for decontamination purposes [8,10] or to produce high energy density products such as bio-oils, syngas and char [12,16,20]. Only three of these papers studied the gaseous emissions of nitrogen species during combustion of wood-based panels [9,11,21]. Risholm-Sundman et al. [11] quantified NH₃ and HCN with wet chemical analysis, thus giving no insight on these species' emissions as a function of time. The works of Widmann et al. [21] and Stubenberger et al. [9] studied the combustion of MDF board in a lab-scale pot furnace and provided continuous measurements of the gas species including NH₃, HCN and NO. The work of Stubenberger et al. [9] added the measures of NO₂ and N₂O.

All these studies have been conducted on standard particle board or MDF. However, about 10% of the European production of particle board is either moisture-resistant or fire-retardant [22]. The treatments applied to these particle boards may have an influence on nitrogen fate during their combustion and ultimately on the NOₓ emissions. Their combustion characteristics must therefore be studied.

The aim of this work is to assess the potential influence of these treatments on nitrogen fate during the combustion of particle board. To do so, different types of particle board were combusted in a semi-pilot scale furnace. Pyrolysis and char oxidation experiments were also carried out in order to separate the different stages of combustion and to gain a better understanding of the combustion process.

## 2. Experimental setup

### 2.1. Fuel elemental composition

In this study, four different fuels are used: a standard particle board (PB), a moisture-resistant particle board (MPB), a fire-retardant particle

board (FPB), and wood chips (WC) as a reference fuel. The three particle boards were purchased from a retailer and were chosen for their similar thickness of 1.8 cm. All three particle boards were cut into 3 × 3 × 1.8 cm pieces so that they could form a homogeneous bed in the reactor that will be presented later. From visual observation of colors in a cut, the moisture-resisting treatment was seemingly applied mainly on the outer layers of the particle board, while the fire-retarding one seems to be only present in its core. Proximate and ultimate analysis of the different fuels is shown in Table 1. The three particle boards have similar nitrogen contents of around 6.3 %. Ash forming elements are known to influence nitrogen fate during combustion [23,24]. To this end, analysis of inorganic elements present in the different fuels was carried out. FPB presents the highest quantity of inorganic elements (33,013 mg/kg), followed by PB (20,846 g/kg), MPB (17,439 g/kg) and finally WC (1570 g/kg). This analysis also revealed that FPB had a significantly higher phosphorus content (21,700 mg/kg) compared to PB (340 mg/kg) and MPB (638 mg/kg). Phosphorus is indeed commonly used in fire-retardant treatments [25].

### 2.2. TGA conditions

Thermogravimetric analysis (TGA) was conducted on the particle board samples to reveal their mass loss. The samples were first ground using a cryogenic grinder. Then, 200 mg of it is heated at 30 °C and maintained at this temperature for 20 min. After that, the temperature increases to 110 °C at a rate of 10 °C/min and is maintained at 110 °C for 20 min. Finally, the temperature goes up to 800 °C at a rate of 10 °C/min and is kept at this temperature for 2 h. One experiment is conducted under air to reproduce combustion conditions. To better separate the pyrolysis stage and the char oxidation stage, a second experiment is conducted under N₂ for 1 h before switching to air. Each experiment was carried out two times to ensure reproducibility.

### 2.3. Crossed-fired fixed bed experiments

Batch pilot scale experiments were conducted in a crossed-fired fixed bed reactor, enabling the collection and analysis of products. The experimental setup is represented in Fig. 2. The reactor is a 60 cm high cylinder with a 27 cm inner diameter. It is heated via surrounding thermal resistances. A thermocouple is placed between the heating

<figure><img src="image_3.png" /><figcaption>Fig. 1. Nitrogen content of fuels and operating temperatures of the experiments in the literature on nitrogen fate during thermal treatment of wood-based panels [8–18].</figcaption></figure>
<table><caption>**Table 1**

Proximate and ultimate analysis of the different fuels studied.</caption><thead><tr><th rowspan="2"></th><th colspan="4">Proximate analysis (wt% wet basis)</th><th colspan="5">Ultimate analysis (wt% dry basis)</th></tr><tr><th>MC</th><th>VM</th><th>FC</th><th>Ash</th><th>C</th><th>H</th><th>N</th><th>S</th><th>O</th></tr></thead><tbody><tr><td>PB</td><td>7.51</td><td>67.29</td><td>22.46</td><td>2.75</td><td>46.80</td><td>5.94</td><td>5.89</td><td>0.19</td><td>38.14</td></tr><tr><td>MPB</td><td>7.07</td><td>68.01</td><td>22.07</td><td>2.85</td><td>46.80</td><td>6.00</td><td>6.63</td><td>0.16</td><td>37.30</td></tr><tr><td>FPB</td><td>7.10</td><td>64.22</td><td>23.37</td><td>5.30</td><td>44.20</td><td>5.91</td><td>6.34</td><td>0.15</td><td>37.66</td></tr><tr><td>WC</td><td>10.44</td><td>70.52</td><td>19.55</td><td>0.27</td><td>51.10</td><td>6.16</td><td>0.09</td><td>&lt;0.02</td><td>42.31</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Scheme of the experimental setup.</figcaption></figure>

resistances and the outer wall of the reactor for temperature regulation. Additionally, ten other thermocouples are placed inside the reactor at various heights to measure the temperature inside the fuel bed and the freeboard. Radially, temperature measurement takes place at approximately 1/6 radius from the axis. The biomass forms a bed placed over a grid at the bottom of the reactor. It is heated from above, mostly through radiation. The atmosphere gas comes from under the grate and crosses the biomass bed. Its flow is controlled by a mass flow controller. After exiting the reactor, the flue gas goes through a heated line at 350 °C and then through two condensers. The first condenser's cooling fluid temperature is set at 5 °C and the second one at -20 °C. Jars are placed under each condenser to collect the condensates, which consist mainly of water and tar. The rest of the gas goes through a charcoal filter before going to the gas evacuation. Part of it is extracted using pumps to the analysis section before the charcoal filter.

The sampled gas is continuously analyzed by different techniques that need specific gas pre-treatments. CH₄, CO, CO₂, H₂, N₂ and O₂ are quantified with an Agilent 490 micro gas chromatographer. In order to protect the chromatograph, desiccant and cellulose filters are placed upstream to capture remaining water and particles. On another branch, NH₃ and HCN are quantified with an Avatar 360 FTIR analyzer equipped with a gas cell with 2 m optical length. NO and CO₂ are also quantified

with NDIR and O₂ is quantified with a paramagnetic sensor. The chromatograph and the infrared analyzers are separated because the desiccant used to dry the gas before the chromatograph also adsorbs NH₃. H₂O emission profile in time is not measured during the experiments since water vapor condenses in the condensers. Cumulated condensates retrieved in the jars are analyzed with various techniques. Total nitrogen content is determined via TOC-N analysis and water content is determined via Karl Fischer titration. Since the condensates are mainly composed of water, in which NH₃ is highly soluble, ion chromatography was also conducted in order to quantify NH₄⁺ ions. NH₃ found in the condensates will be accounted for as a gas in the rest of the study. HCN is also very soluble in water as CN⁻ but could not be quantified due to material limitations. The elemental composition of the solid residues is determined with a FLASH 2000 organic elemental analyzer.

Three types of batch experiments are conducted at the pilot scale. The combustion experiments consist in carrying the entire experiment under air. The pyrolysis experiments consist in conducting the experiment under N₂, and char oxidation experiments consist in heating the char obtained via pyrolysis under air. Combustion and pyrolysis initial sample mass is 1 kg of dry biomass. Given the fact that the different fuels do not have the same density, the height of the fuel bed varies depending on the fuel. For particle boards, the bed is approximately 6 cm high,
while for WC it is 10 cm high. For all three experiment types, the fuel is initially placed in the reactor at room temperature and is then heated to 800 °C with a heating ramp of 20 °C/min. The atmosphere gas flow rate is set at 15 NL/min, corresponding to a speed of 0.0044 m/s in an empty reactor at 20 °C. An experiment is considered over when the composition of the flue gas is the same as the atmosphere gas. The condensates are collected twice: a first time at the end of the experiment, and a second time when the condensers are back to room temperature, typically after the night. Ash (after combustion and char oxidation experiments) and chars (after pyrolysis experiments) are retrieved once the reactor is back to room temperature.

During preliminary experiments, it was noted that from one experience to another, residual ammonia could desorb from the condensers. To prevent this from happening, the condensers are rinsed with ethanol after each experiment.

To evaluate the reproducibility of the experiments, each combustion experiment was conducted at least 2 times. The coefficients of variation (ratio of standard deviation to the mean value) of the cumulative quantities of the different gas produced are represented in Table 2. For all major gases, coefficients of variations are good, the highest is the one of CH₄ for PB at 17.04 % and most of them are below 10 %. Concerning nitrogenous gases, NH₃ and NO have acceptable coefficients of variation. The one of NH₃ for WC is higher than the others because only very small quantities were measured. Thus, even a small absolute variation of the quantification results in a great relative dispersion.

HCN measurement is the least reproducible. This can be attributed to two reasons. First, the overall quantities of HCN were small, meaning that a small absolute variation of the measurement would result in an important relative variation. Then, HCN is highly soluble in water as the ion CN⁻, but CN⁻ in aqueous phase couldn't be quantified in this work. The reproducibility of HCN measurements would be significantly better by taking into account CN⁻ in the HCN balance.

A carbon mass balance was also calculated and is presented in Table 3. 80 to 93 % of the carbon present in the fuel was found in the products, which attests to good quality experiments. The missing carbon in the balance may be explained as follows. Firstly, some residual tars remain in the condensers at the conclusion of the experiment and could not be analyzed. Secondly, a small amount of tars and particles captured within the various filters is also not available for analysis.

# 3. Results and discussion

## 3.1. TGA results

TGA results are reported in Fig. 3. In subfigure 3.a), it can be seen that WC pyrolysis starts at the same temperature as PB and MPB. However, its char yield is lower, which was expected given the lower quantity of inorganic elements in WC. The char oxidation step is faster than for the particle boards. An explanation could be the lower mineral content of WC. It has indeed been observed that the presence of some inorganic matter slows down char oxidation [26].

PB and MPB show very similar mass loss time evolutions, suggesting that the moisture-resisting treatment has no impact on thermal degradation. As expected, the FPB mass loss time evolution is different from

<caption>**Table 2**</caption>

Coefficients of variation of the different gas produced during combustion experiments (%).

<table><caption>**Table 3**</caption><thead><tr><th></th><th>Fuel</th><th>PB</th><th>MPB</th><th>FPB</th><th>WC</th></tr></thead><tbody><tr><td rowspan="4">Nitrogenous gases</td><td>NH<sub>3</sub></td><td>12.04</td><td>14.29</td><td>5.05</td><td>42.87</td></tr><tr><td>HCN</td><td>103.91</td><td>141.42</td><td>62.89</td><td>-</td></tr><tr><td>NO</td><td>9.33</td><td>1.32</td><td>15.33</td><td>14.63</td></tr><tr><td>CO</td><td>12.33</td><td>4.94</td><td>7.68</td><td>10.51</td></tr><tr><td rowspan="4">Major gases</td><td>CO<sub>2</sub></td><td>2.2</td><td>1.52</td><td>2.52</td><td>1.62</td></tr><tr><td>CH<sub>4</sub></td><td>17.04</td><td>0.2</td><td>5.05</td><td>12.26</td></tr><tr><td>H<sub>2</sub></td><td>7.33</td><td>1.26</td><td>1.79</td><td>5.53</td></tr></tbody></table>

Percentage of fuel C found in the products of each experiment (wt%).

<table><thead><tr><th>Fuel</th><th>PB</th><th>MPB</th><th>FPB</th><th>WC</th></tr></thead><tbody><tr><td>Combustion</td><td>88.68 ±<br>1.03</td><td>87.70 ±<br>0.22</td><td>92.35 ±<br>0.95</td><td>81.84 ±<br>0.43</td></tr><tr><td>Pyrolysis + Char<br>Ox.</td><td>79.05</td><td>79.28</td><td>83.96</td><td>79.79</td></tr></tbody></table>

the other two. At the end of the pyrolysis stage, the char yield of FPB is approximately 35 % while that of PB is 25 %. This could be due to the effect of the phosphorus-based fire retardant. Indeed, thermal degradation of phosphorus-based fire retardants are known to create thick layers of solid residue that blocks heat and mass transfers [25,27]. It can also be observed that the devolatilization stage starts at a lower temperature for FPB. This could also be caused by the presence of phosphorus, as observed during TGA of red phosphorus impregnated glass fiber [28] and various phosphate doped particleboards [29]. These differences between the particle boards can also be observed on subfigure 3.b) where the whole process is carried out under air.

## 3.2. Total fuel N conversion yields

During the fixed bed reactor experiments, nitrogen species were quantified in the different products (gas, condensates, char, and ash) of combustion, pyrolysis, and char oxidation. The results of these quantifications are reported in Fig. 4. The majority of the nitrogen in the products quantified during the experiments was found in the condensates, when present. No nitrogen is present in the ash and no condensates were formed during the char oxidation as expected. It should also be noted that during FPB combustion and char oxidation, the solid residue was found in the form of vitrified ash. This could be due to the presence of phosphorus, which is known to melt and form solid residues at 800 °C [30,31].

Total fuel N conversion yields are presented in Fig. 4. For char oxidation experiments, N distribution is calculated with char-N content as a basis. Total fuel N conversion yields range between 34 % for FPB and 58 % for WC during combustion, and between 55 % for PB and 59 % for MPB during pyrolysis. In the literature, values range between 27 % and 58 % for wood-based panels combustion in a fixed bed reactor [9,17,21]. Only the quantified nitrogen is represented in Fig. 4. A fraction of the missing nitrogen remains in the condensers, either in tars or in adsorbed NH₃. Nitrogen could also be present in unquantified gases such as HNCO or N₂O. Finally, it could also have been reduced to N₂. Even though N₂ is quantified by the µ-GC, the fraction of N₂ coming from the fuel N is too small compared to the overall N₂ flow rate and could not be included in the nitrogen distribution. For char oxidation experiments, we can assume that all the missing nitrogen is N₂. Indeed, no tar is produced during char oxidation experiments and N₂O concentrations are reported to be negligible [32,33].

It must be noted that the measured NO/char-N ratios are lower than those available in the literature, which are around 20–40 % [32,33]. This difference can be attributed to several factors, including the height of the char bed. It has indeed already been observed that a higher char bed favors the reduction of NO by the char [32], as well as the large size of the char particles [34] and the presence of oxygen in the bed [35].

The main difference between WC and particle boards lies in the fraction of the nitrogen quantified in the condensates. This difference can be attributed to the form of the nitrogen in the fuels. In WC, nitrogen is mostly present in the form of amino acids [36] while in particle boards it is present in UF. UF thermal degradation happens at lower temperature and produces tars composed of less stable heterocyclic compounds compared to WC pyrolysis [37]. Thus, whereas the nitrogenous tars from particle boards thermal degradation crack into NH₃ and HCN in our experiments, tars from WC remain as such and are found in the condensates.
<figure><img src="image_5.png" /><figcaption>Fig. 3. TG analysis of PB, MPB, FPB and WC under a) N₂ + air and b) air only.</figcaption></figure>

We can also see that for combustion experiments, WC showed the highest nitrogen elemental balance. An explanation could be that, for particle boards, more fuel-N is converted to here unquantified N₂. Indeed, we can see on subfigure 4.b.3) that the char-N to NO conversion is lower for particle boards compared to WC. Also, nitrogenous tars from the pyrolysis of PB crack more easily, mostly into NH₃ that can be oxidized to N₂ in a reaction catalyzed by the char [38].

The lower char-N to NO conversion for particle boards can be attributed to their higher char-N content and the amount of inorganic elements. Indeed, both parameters are known to enhance the NO reduction reaction by the char [23,33,39].

Concerning the particle boards, PB and MPB show similar nitrogen repartition for all experiments. Two major differences can be noted between these particle boards and FPB. First, more nitrogen remains in the char after the pyrolysis experiment. Even by considering the fact that the char yield of FPB is higher than the other two, nitrogen concentration in the char remains the highest (3.88 % for FPB versus 2.99 % for MPB and 2.62 % for PB). This higher retaining of nitrogen in the char is attributed to the flame retarding compound. Second, the nitrogen conversion rate of FPB is lower than the other two for combustion and char oxidation experiments. An explanation could be the higher amount of nitrogen in the char associated with a higher char mass. It has indeed already been observed that these two parameters influence the NO/char-N ratio during char oxidation experiments [32,33]. The formed vitrified ash may also have encapsulated some nitrogen.

Regarding the distribution of nitrogen in the different gases, it can be seen that PB and MPB have similar ones during combustion experiments. During pyrolysis experiments, however, less HCN is quantified for PB than for MPB. HCN production is also the main difference between FPB and the other two particle boards. It is lower during combustion experiments but higher during pyrolysis experiments. To explain this differences, time evolution profiles of the temperature inside the bed as well as the different gaseous species are needed.

### 3.3. Time evolution of major combustion gases flow rates

Before investigating the time evolution of nitrogenous gases flow rates, the major combustion gases ones will be discussed for the different fuels, as they provide a good understanding of the different stages of combustion. They are presented in Fig. 5. The time evolutions are expressed as flow rates STP (Standard Temperature and Pressure: 25 °C and 1 bar) to prevent misinterpretation when using concentration values that are affected by the variation of the total flue gas flow rate.

An oxidizing pyrolysis stage (Pyro.) and a char oxidation stage (Char

Oxidation) can be identified and are delimited by vertical dotted lines. The delimitation between these two stages is placed at the moment when CH₄ is no more produced. This choice was made because CH₄ is only produced by the pyrolysis of the biomass. The char oxidation stage is considered over once the O₂ level is back to its original value. It can be seen that most of the gas species are produced during the oxidizing pyrolysis stage. Only CO₂ and CO are produced during the oxidation of the char. Throughout the oxidizing pyrolysis and at the beginning of the char oxidation, the O₂ present in the air is fully consumed, meaning that the reactor is in air lean conditions during this period.

Oxidizing pyrolysis stage has similar duration for all fuels. FPB oxidizing pyrolysis has a slightly higher duration. Regarding the produced gases, they are also similar for all the fuels except for CO for WC that is much higher than that of the particle boards. The main differences between the different fuels are observed during the char oxidation stage. It is shorter for WC than for the particle boards, as observed during the TGA experiments. PB and MPB have similar durations, while FPB has a much longer one, presumably due to the effect of the fire-retardant. Indeed, phosphorus-based fire-retardants are known to form a protective layer around the char, slowing down its oxidation.

### 3.4. Time evolutions of nitrogenous gases flow rates

The flow rates of the different gaseous nitrogen species emitted during the combustion of the different fuels as well as O₂ and the temperature inside the bed are shown in Fig. 6. Vertical dotted lines of the same color as the one associated to each fuel mark the end of the oxidizing pyrolysis stage and the end of the char oxidation for all experiments. Since the duration of these stages are really close for PB and MPB, only the lines of PB are represented. The setpoint temperature is represented as a black dotted line. The represented temperature is the temperature 5 cm above the grate, which is 1 cm below the top of the fuel bed for the particle boards and 5 cm below the top of the fuel bed for WC.

During the heating phase, the temperature profiles of PB and WPB are very close. The temperature profile of WC is slightly lower than that of PB and WPB. This is due to the difference in height between the fuel beds. Since the WC bed is higher, the thermocouple is further away from the top of the bed where the ignition takes place, thus causing a delay. FPB heating rate is much lower than the others. This can be attributed to the fire-retarding compound, as it limits heat and mass transfer within the bed, preventing exothermic reactions. It can be noted that, due to exothermic reactions happening in the bed, the setpoint temperature is exceeded for every fuel during the char oxidation stage.
<figure><img src="image_6.png" /><figcaption>Fig. 4. a) Conversion yields of the fuel N in the products of: (a.1) Combustion, (a.2) Pyrolysis and (a.3) Char oxidation, and b) Conversion yields of the fuel N into the gaseous species during (b.1) Combustion, (b.2) Pyrolysis and (b.3) Char oxidation.</figcaption></figure>

Neither NH₃ nor HCN were found in the gas phase during combustion of WC, as observed in Fig. 4. For all particle boards, NH₃ and HCN emissions mostly take place during the oxidizing pyrolysis stage. Their emission profiles show two peaks, one appearing at the very beginning of the pyrolysis stage and a more important one in a second time. The NH₃ emission profile also indicates a later stage where NH₃ release increases due to the desorption of previously adsorbed NH₃ from the condensers.

There are two distinct stages for NO emissions. A first peak is present during the oxidizing pyrolysis stage, and a large production peak at the

end of the char oxidation phase. For WC, only the first peak can be seen. This behavior was already observed in the literature [9,17,18]. The first peak is likely due to the oxidation of the NOx precursors in the gas phase. The second peak is due to the oxidation of char-N. As can be seen in Fig. 6, this peak starts when the oxygen concentration starts to rise again and stops when the oxygen flow rate is back to its original value. Indeed, there is no O₂ coming out of the bed as long as there is a sufficient amount of char to reduce the produced NO. Some O₂ comes out of the bed when there is not enough char to consume all of it, and coincidentally, to reduce all the produced NO. Finally, the O₂ flow rate is back to its
<figure><img src="image_7.png" /><figcaption>Fig. 5. Time evolution of flow rates of the major gaseous species during the combustion of a) PB, b) MPB, c) FPB and d) WC.</figcaption></figure>

original value when there is no more char in the bed and so, no more
source of NO.

Nitrogen species emissions and temperature profiles are very similar for PB and MPB, whereas some differences can be seen for FPB, as it was expected when looking at the conversion yields in Fig. 4. During the oxidizing pyrolysis stage, we can see that FPB produces more NH₃ and less HCN compared to the other two particle boards. This is likely due to

the lower heating rate, which is known to promote the formation of NH₃ during tar cracking at the expense of HCN [15,40]. We can also see that the first peak of NO is much higher for FPB. This can be explained by the fact that more NH₃ is produced, and so, more NO can be produced by NH₃ oxidation.

During the char oxidation stage, it can be seen that the NO peak of FPB is broader and its maximum height is lower than that of the other
<figure><img src="image_8.png" /><figcaption>Fig. 6. Time evolution of flow rates of a) NH₃, b) HCN, c) NO, d) O₂, and e) temperatures 5 cm above the grate during combustion of – PB, – MPB, – FPB and ††† WC.</figcaption></figure>

two particle boards. The broader aspect is attributed to the fire retardant that slows down char oxidation and, as such, the NO release. As for the maximum height, it can be attributed to the higher nitrogen content and amount of inorganic elements of this char, both of them enhancing NO reduction by the char [33].

The results of the pyrolysis experiments for the different fuels are shown in Fig. 7. Flow rates of nitrogen species and temperature profiles are represented. Compared to combustion experiments, the temperature time evolutions are closer. This confirms that the slower heating rate of FPB during combustion can be mainly attributed to the hindering of exothermic oxidation reactions. It can also be noted that the setpoint temperature is not exceeded for any fuel.

During the pyrolysis of WC, no nitrogen species were quantified in the gas phase. The two peaks of NH₃ observed during combustion

remain visible but a third one has appeared. The reason for the presence of this third peak is uncertain. For the three panels, no great differences are observed between the NH₃ emission profiles, apart from the fact that the first peak is still higher for FPB. The first peak of HCN emissions observed during combustion is no longer present during pyrolysis. The maximum HCN flow rate is way lower for PB and MPB, whereas it remains of the same amplitude for FPB. This can once again be explained by the heating rate in the fuel bed. It is lower for PB and MPB during pyrolysis since the heat generated by the oxidation reactions is not present anymore. The HCN emission profiles of FPB during pyrolysis is very close to the one observed during combustion. The temperature of the bed during FPB pyrolysis is the same as the one during its combustion, which could explain this similarity.

Only one NO peak can be observed for all feedstocks pyrolysis. FPB
<figure><img src="image_9.png" /><figcaption>Fig. 7. Time evolution of flow rates of a) NH₃, b) HCN, c) NO, and d) temperatures 5 cm above the grate during pyrolysis of – PB, – MPB, – FPB and ··· WC.</figcaption></figure>

is, as in combustion, the particle board leading to the highest NO emissions. Pyrolysis NO peak is higher than all of combustion NO peaks for all feedstocks.

The results of the char oxidation experiments for the different fuels are shown in Fig. 8. During those, neither NH₃ nor HCN were quantified in the gas for the four fuels. NO and O₂ flow rates as well as temperature profiles are presented. The heating rates inside the bed for all fuels are very similar. This indicates that the pyrolysis experiment altered the properties of the fire retardant, which doesn't seem to prevent the oxidation reaction as much as before. Another consequence of this is that the char oxidation now lasts almost the same time for all three particle boards.

Same as in combustion experiments, two peaks of NO can be observed. The first one is high and fine while the second one is much broader. Contrary to the combustion experiments, NO flow rate does not reach 0 between these two peaks. This can be linked to the fact that the atmosphere is fuel lean in the bed during the entirety of the experiment. For WC char oxidation, only the second peak is present.

The height of the first peak is very similar for all particle boards. Concerning the second peak, the one of FPB is lower than the other two, presumably because of the higher amount of inorganic elements and nitrogen content of its char.

## 4. Conclusions

In this work, the products of combustion, pyrolysis, and char oxidation of three particle boards and wood chips were studied with a

focus on nitrogen fate. Selected feedstocks are a standard particle board (PB), a moisture-resistant particle board (MPB) and a fire-retardant particle board (FPB). Experiments have been conducted at a semi pilot scale in a crossed-fired fixed bed reactor. Nitrogen was quantified in the different products (solid residue, condensates, and gas phase) of each experiment. These quantifications made it possible to present the nitrogen distribution in the different products as well as the time evolution of NH₃, HCN and NO flow rates. TGA of all three particle boards and WC were also conducted to study the mass loss of the different fuels during heating.

TG analysis showed that PB and MPB have similar thermal degradation curves, while the degradation of FPB occurs sooner and produces more char. These differences could be due to the formation of a protective layer around the char by the fire-retardant present in the FPB, supposedly phosphorus-based.

The study of the time evolution of nitrogenous gases flow rates has shed light on the stages during which they were emitted. NH₃ and HCN were produced exclusively during the pyrolysis stage and were found to be very sensitive to the heating rate of the bed.

NO emission during combustion shows two stages. The first one is characterized by a fine peak appearing during the pyrolysis stage, followed by a broader peak during the char oxidation stage, coincidently to some O₂ coming out of the bed.

PB and MPB showed similar behaviors during combustion and char oxidation, and only minor differences during pyrolysis. FPB showed stronger differences, possibly due to the fire-retardant action lowering the temperature inside the bed and thus hindering HCN emissions
<figure><img src="image_10.png" /><figcaption>Fig. 8. Time evolution of flow rates of a) NO, b) O₂ and c) Temperature during oxidation of char from – PB, – MPB, – FPB and ⋯ WC.</figcaption></figure>

during combustion. Apart from this, no clear influence of the chemical treatment on nitrogen fate was pointed out. Numerically modeling the combustion and pyrolysis of the fuel bed could help us better understand the extent to which the fire-retardant influence the temperature inside the bed, and ultimately, the fate of nitrogen.

A significant proportion of the nitrogen released during pyrolysis and combustion was found in the condensates. In this study, only NH₄⁺ was identified. In future works, measurement of other ions formed by the dissolution of nitrogenous gases like CN⁻ and NO₂⁻ is necessary to strengthen the understanding of nitrogen fate during biomass combustion experiments. Determination of nitrogen containing tars is also necessary, as they are known to be a major precursor to HCN and NH₃.

### CRediT authorship contribution statement

**Thomas Bertus:** Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Jérôme Lémonon:** Methodology, Investigation, Conceptualization. **F. Javier Escudero Sanz:** Writing – review & editing. **Sylvain Salvador:** Writing – review & editing, Supervision.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

This material is based upon work supported by the COMPTE.R company and the ANRT (Association nationale de la recherche et de la technologie) with a CIFRE fellowship granted to Thomas BERTUS. Louise ROQUESI and Jean-Marie SABATHIER are gratefully

acknowledged for their valuable contribution regarding the condensates and solid phase residue characterization.

### Data availability

The authors do not have permission to share data.

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