Experimental Research on Heterogeneous N₂O Decomposition with Ash and Biomass Gasification Gas

Abstract

In this paper, the promoting effects of ash and biomass gas reburning on N₂O decomposition were investigated based on a fluidized bed reactor, with the assessment of the influence of O₂ on N₂O decomposition with circulating ashes. Experimental results show that different metal oxides contained in ash play distinct roles in the process of N₂O decomposition with biomass gas reburning. Compared with other components in ash, CaO is proven to be very active and has the greatest promoting impact on N₂O decomposition. It is also found that O₂, even in small amounts, can weaken the promoting effect of ash on N₂O decomposition by using biomass gas reburning.

1. Introduction

With the popularization of fluidized bed technology, the nitrous oxide (N₂O) generated in the circulating fluidized bed (CFB) combustion process has aroused more and more attention. The concentration of N₂O emissions from CFB is in the range from 20 to 300 ppm and sometimes 400 ppm [1]. Compared with typical N₂O emissions from conventional pulverized coal-fired boilers (less than 10 ppm), it has becomes a serious problem that may represent an obstacle for the development of CFB boilers [2,3].

Heterogenous N₂O decomposition in CFB boilers indicates that heterogenous reactions must happen between N₂O and some components in solid fuels, ashes or catalysts [4,5]. In CFB power plants the ash consists of various sorts of oxides, such as SiO₂, Al₂O₃, Fe₂O₃ and CaO. The reaction mechanism of the promoting effect on N₂O decomposition with these oxides can be described as adsorbing and catalytic effects. On the one hand, some active radicals exist in the surface of solid oxides, which can absorb both H and OH. The decomposition of N₂O is then strengthened due to reactions such as H + N₂O ↔ N₂ + OH, N₂O + H ↔ NH + NO, N₂O + H ↔ NNH + O, N₂O + H ↔ HNNO and N₂O + OH ↔ N₂ + HO₂. On the other hand, the conversion rate of the heterogenous decomposition of N₂O in the CFB increases owing to the catalytic effect of oxides [6,7,8].

Circulating ash can promote the decomposition of N₂O. The combustion of biomass, solid waste and coal in CFB can produce large amounts of ash. The influences of the different components in ash on N₂O decomposition vary a lot [9,10]. Ca, Fe, Al and other metal oxides in circulating ash are considered as active ingredients. The catalytic effect of circulating ash on N₂O decomposition depends highly on its specific surface area. The larger the specific surface of circulation ash is, the greater benefit for N₂O contacting with circulating ash. With the impact of circulating ash, a small amount of N₂O was converts to nitric oxide (NO) [11].

Shen et al. analyzed the promoting effect of char on N₂O decomposition in the temperature range of 677–977 °C [4], and the results showed that N₂O conversion rate was about 90% under the circumstance with the temperature 900 °C comparing with 60% in bare bed. Meanwhile, the authors analyzed the promoting effect of oxide and sulphate on N₂O decomposition in the temperature range of 677–977 °C, and it was found that the sequence of levels of the promoting effect was: Fe₂O₃ > CaSO₄ > Al₂O₃ > SiO₂ > MgSO₄ > MgO > bare bed, with the conversion of N₂O being 99%, 91%, 81%, 69%, 59%, 47% and 39%, respectively [4]. With respect to the catalytic effect of these oxides, the most important reaction of N₂O was denoted as decomposition reaction, described as 2N₂O ↔ 2N₂ + O₂.

In the low temperature range (usually 200–500 °C), precious metals, zeolite and transition metals (rhodium, etc.) all showed strong catalytic effect on N₂O decomposition [9,12]. N₂O decomposition rates reached 100% with Rh/Na₂O/Al₂O₃ as catalyst at a temperature of 425 °C. Besides, Fe-ZSM-5 or Co-ZSM-5 containing Co-, Cu-, Fe- or ZSM-5 were also beneficial to N₂O decomposition [10,11]. N₂O was decomposed on the surface of catalyst and the decomposition process could be described with following reactions [10]:

N₂O + * → N₂ + O*

(R-1)

N₂O + O* → N₂ + O₂

(R-2)

In order to avoid catalyst destruction due to the fluctuation of N₂O concentration and reduce N₂O emission, reduction gases like H₂, CH₄ and CO were first used at the surface of catalyst [10,13,14]. Debbagh et al. [10] showed that N₂O decomposition with CO at catalyst surface at the temperature range of 200–600 °C was expressed with following reactions:

CO + O* → CO₂ + *

(R-3)

CO + * → CO*

(R-4)

N₂O + CO* → N₂ + CO₂ +*

(R-5)

Gluhoi et al. [13] studied the reactions of H₂ or CO with N₂O in the temperature range of 40–250 °C with the catalysts Au/TiO₂, Au/Al₂O₃, Au/MIO_x/MIIO_x/Al₂O₃, Au/Mox/Al₂O₃ (MI, MII, M = Li, Rb, Mg, Co, Mn, Ce, La, Ti), and proved that the presence of catalysts was beneficial for H₂, CO to reduce N₂O emissions. Kondratenko et al. [14] found that the reduction effect of CH₄ on N₂O was conspicuous with the catalytic effect of iron—silicalite at 450 °C, which could be expressed by the three following reactions:

N₂O + * ↔ N₂ + O*

(R-6)

CH₄ + 3O* ↔ CO + 2H₂O + 3*

(R-7)

CH₄ + 4O* ↔ CO₂ + 2H₂O + 4*

(R-8)

Choosing the temperature range of 22–550 °C, with the effect of Fe-USY zeolite catalyst, and using selective catalytic reduction (SCR) method, Shen and co-workers [15] prepared a variety of mixed gases with CH₄(such as N₂O/CH₄, N₂O/CH₄/NO, N₂O/CH₄/O₂, N₂O/CH₄/H₂O, N₂O/CH₄/NO/O₂, N₂O/CH₄/NO/O₂/H₂O) to study N₂O decomposition, concluding that N₂O conversion rate in N₂O/CH₄ system was much higher than that in other systems.

2. Experimental Devices and Method

The structure of the designed laboratory-scale fluidized bed reactor is shown in Figure 1. The reactor has an inner diameter of 15 mm and a height of 1900 mm and it contains an air distribution plate, vertically positioned in the electrical heating furnace. The bed material of the reactor consisted of screened ash and solid oxides ranging in diameter from 0.3 to 0.425 mm. In order to make the experimental circumstances more similar to the real conditions, circulating ash was used from a CFB power plant in Sichuan Province. Its composition is given in Table 1.

Figure 1. Schematic of the lab-scale quartz fluidized bed reactor.

Figure 1. Schematic of the lab-scale quartz fluidized bed reactor.

Table 1. Compositions of circulating ash from a CFB boiler (%).

	SiO2	Al2O3	Fe2O3	CaO	MgO	TiO2	CaSO4	P2O5	K2O	Na2O
Circulating ash	33.22	13.91	6.66	11.59	1.59	1.97	31.88	0.18	1.01	0.30

Table 1. Compositions of circulating ash from a CFB boiler (%).

	SiO2	Al2O3	Fe2O3	CaO	MgO	TiO2	CaSO4	P2O5	K2O	Na2O
Circulating ash	33.22	13.91	6.66	11.59	1.59	1.97	31.88	0.18	1.01	0.30

Syngas (N₂/N₂O or N₂/N₂O/O₂) with 2000 ppm of N₂O was used as the flue gas and it was injected into the furnace from the bottom of the reactor with the flow rate 5000 mL/min. Another gas stream (H₂, CH₄ or CO) or syn-gas stream (CH₄/CO/H₂/CO₂/N₂ = 4.2/25.9/10.1/5.98/53.82) was used as the reburning gas and injected into the reactor from the side of the reactor, 200 mm above the air distribution plate, with flow rates ranging from 10 mL/min to 50 mL/min.

To discuss the mechanism of heterogenous N₂O decomposition with biomass gas and simulate a real CFB power plant, taking circulating ash as bed material in the CFB boiler, the influence of the main components of the biomass gas (H₂, CH₄ and CO) on heterogenous N₂O decomposition was analyzed. The oxygen contents in the flue gas were 0%, 2.5% and 6%. Reburning experiments were carried out by injecting biomass gas from a nozzle (with a distance of 0.2 m away from air distributor).

The exhaust gas was analyzed on-line by a flue gas analyzer (testo350Pro, Testo, Germany) to determine NO₂ and NO concentrations and collected for further analysis of N₂O content by gas chromatography (Trace DSQ, New York, USA) using a 3 m Porapark Q column. N₂O decomposition, and NO or NO₂ formation are calculated by the following equations:

$${\eta }_{{\mathrm{N}}_2\mathrm{O}}=\left(1-\frac{{\mathrm{N}}_2{\mathrm{O}}_{out}}{{\mathrm{N}}_2{\mathrm{O}}_{in}}\right)\times 100\%$$

(Eq-1)

$${\eta }_{\text{NO}}=\frac{n_{\text{NO},out}}{2n_{{\mathrm{N}}_2\mathrm{O},in}}\times 100\%=\frac{[\text{NO}]\cdot V_c}{22.4\times 2n_{{\mathrm{N}}_2\mathrm{O},in}}\times 100\%$$

(Eq-2)

$${\eta }_{{\text{NO}}_2}=\frac{n_{{\text{NO}}_2,out}}{2n_{{\mathrm{N}}_2\mathrm{O},in}}\times 100\%=\frac{[{\text{NO}}_2]\cdot V_c}{22.4\times 2n_{{\mathrm{N}}_2\mathrm{O},in}}\times 100\%$$

(Eq-3)

$$n_{{\mathrm{N}}_2\mathrm{O},in}=\frac{Q_{{\mathrm{N}}_2\mathrm{O}}\cdot {\tau }_c}{22.4}$$

(Eq-4)

where:

$n_{{\mathrm{N}}_2\mathrm{O},out}$:	mole of N2O at the reactor outlet, mol;
$n_{{\mathrm{N}}_2\mathrm{O},in}$:	mole of N2O at the reactor inlet, mol;
$n_{\text{NO},out}$:	mole of NO at the reactor outlet, mol;
$n_{{\text{NO}}_2,out}$:	mole of NO2 at the reactor outlet, mol;
$Q_{{\mathrm{N}}_2\mathrm{O}}$:	flow of N2O inlet, mL/min;
${\tau }_c$:	time of collecting gas in sample bag, min;
$V_c$:	volume of collecting gas in sample bag, mL;
$[\text{NO}]$:	concentration of NO in exhaust gas, ppm;
$[{\text{NO}}_2]$:	concentration of NO in exhaust gas, ppm.

In research on the reburning character of biomass gasification gas, R represents the reburning gas content and can be calculated by:

$$R=V_b/V_f\times 100\%$$

(Eq-5)

where:

• $V_b$: volume of biomass derived gas, mL;
• $V_f$: volume of flue gas, mL.

3. Results and Discussion

3.1. Influence of Biomass Gas Components without Oxygen in Flue Gas

Figure 2 shows that the N₂O decomposition rate changed with reburning gas content. Comparing the decomposition of N₂O before and after adding circulating ash as bed material, the thermal decomposition proportion of N₂O with bed material is about 5% larger than that seen with the bare bed conditions.

Figure 2. Relationship of N₂O decomposition and reburnig gas: (a) Homogeneous N₂O decomposition with bare bed; (b) Heterogeneous N₂O decomposition with circulating ash as bed material (N₂O = 2000 ppm, N₂O/N₂, 900 °C).

Figure 2. Relationship of N₂O decomposition and reburnig gas: (a) Homogeneous N₂O decomposition with bare bed; (b) Heterogeneous N₂O decomposition with circulating ash as bed material (N₂O = 2000 ppm, N₂O/N₂, 900 °C).

When H₂ is used as reburning gas, the conversion rate of N₂O decomposition with circulating ash is greater than that of the bare bed. When the CO content as reburning gas is less than 0.4%, N₂O decomposition rate with bed material is lower than N₂O that of the bare bed. This is because CO reacts with some components of the bed material, such as Fe₂O₃, which decreases the N₂O decomposition conversion. On the contrary, when the CO content as reburning gas is greater than 0.4%, the conversion of heterogenous decomposition will be higher than the conversion of homogenous decomposition. The effect of CH₄ as reburning gas on N₂O decomposition differs from that of H₂ and CO, as shown in Figure 2. The N₂O homogenous decomposition conversion part with CH₄ reburning was about 70%, but decreased sharply with circulating ash as bed material. However, when the content of CH₄ as reburning gas become greater than 0.6%, only a slight increase occurs. This is the result of the reactions of CH₄ and CaO in the circulating ashes [16]:

CaO + CH₄ → CaOCH₄ → CaOH + CH₃

(R-9)

CaO + CH₄ → OCaCH₄ → HOCaCH₃ → CaOH + CH₃

(R-10)

CaO + CH₄ → CaOCH₄ → CaCH₃OH → Ca + CH₃OH

(R-11)

CaO + CH₄ → OCaCH₄ → HCaOCH₃ → CaOCH₃ + H

(R-12)

CaO + CH₄ → OCaCH₄ → CaCH₃OH → CaOCH₃ + H

(R-13)

Corresponding to Figure 2, NO and NO₂ formation in the N₂O decomposition process are shown in Figure 3 and Figure 4. When the reburning gas content is 0.2%, the NO formation reaches a maximum value. NO formation rate with H₂ as reburning gas is proven to be larger than that of CO and CH₄ with a bare bed. However, NO formation with CH₄ as reburning gas is more than CO and H₂ with circulating ash as the bed material.

Figure 3. Relationship of NO formation and reburning gas content: (a) Homogeneous N₂O decomposition; (b) Heterogeneous N₂O decomposition with circulating ash as bed material (N₂O = 2000 ppm, N₂O/N₂, 900 °C).

Figure 3. Relationship of NO formation and reburning gas content: (a) Homogeneous N₂O decomposition; (b) Heterogeneous N₂O decomposition with circulating ash as bed material (N₂O = 2000 ppm, N₂O/N₂, 900 °C).

Figure 4. Relationship of NO₂ formation and reburning gas content: (a) Homogeneous N₂O decomposition; (b) Heterogeneous N₂O decomposition with circulating ash as bed material (N₂O = 2000 ppm, N₂O/N₂, 900 °C).

Figure 4. Relationship of NO₂ formation and reburning gas content: (a) Homogeneous N₂O decomposition; (b) Heterogeneous N₂O decomposition with circulating ash as bed material (N₂O = 2000 ppm, N₂O/N₂, 900 °C).

3.2. Influence of Biomass Gas Components with Oxygen Content in Flue Gas

The impacts of H₂, CH₄ and CO reburning gases on the rate of heterogenous N₂O decomposition conversion, depended on oxygen content, as shown in Figure 5. When the oxygen content in the flue gas is 2.5%, the N₂O conversion rate increases with the increase of H₂ and CO content (0.4–0.8 mole%), while the dependence of N₂O conversion on the CH₄ content achieves its maximum value at a CH₄ content of 0.6 mole%. However, when the oxygen content is 6%, it is very clear that N₂O conversion rises with increasing contents of H₂, CO and CH₄ in the reburning gas.

Figure 5. Relationship of heterogenous N₂O decomposition, reburning gas content and oxygen content in flue gas: (a) O₂ = 2.5%; (b) O₂ = 6% (N₂O = 2000 ppm, N₂O/N₂/O₂, 900 °C).

Figure 5. Relationship of heterogenous N₂O decomposition, reburning gas content and oxygen content in flue gas: (a) O₂ = 2.5%; (b) O₂ = 6% (N₂O = 2000 ppm, N₂O/N₂/O₂, 900 °C).

Figure 6 and Figure 7 show NO formation and NO₂ formation in the N₂O conversion process (shown in Figure 5). With the circulating ash as bed material, NO and NO₂ formation at an oxygen content of 2.5% is higher than that under the condition of 6% oxygen content. The NO and NO₂ formation proportions with CO as reburning gas is more than that with CH₄ as reburning gas and it reaches its minimum value with H₂ as reburning gas.

Figure 6. Relationship of heterogenous NO formation, reburnig gas and oxygen content in flue gas: (a) O₂ = 2.5%; (b) O₂ = 6% (N₂O = 2000 ppm, N₂O/N₂/O₂, 900 °C).

Figure 6. Relationship of heterogenous NO formation, reburnig gas and oxygen content in flue gas: (a) O₂ = 2.5%; (b) O₂ = 6% (N₂O = 2000 ppm, N₂O/N₂/O₂, 900 °C).

Figure 7. Relationship of heterogenous NO₂ formation, reburning gas and oxygen content in flue gas: (a) O₂ = 2.5%; (b) O₂ = 6% (N₂O = 2000 ppm, N₂O/N₂/O₂, 900 °C).

Figure 7. Relationship of heterogenous NO₂ formation, reburning gas and oxygen content in flue gas: (a) O₂ = 2.5%; (b) O₂ = 6% (N₂O = 2000 ppm, N₂O/N₂/O₂, 900 °C).

3.3. Influence of Ash and Metal Oxides on Heterogenous N₂O Decomposition

In order to analyze the catalytic effect of bed materials on N₂O decomposition, three sets of experimental conditions were compared, as shown in Figure 8, Figure 9 and Figure 10. Thermal decomposition of N₂O with bed material and without biomass gas reburning is the first condition, in which the main conversion of N₂O is thermal decomposition. In the second conditions, considering the influence of biomass gas reburning, the heterogenous experiment was carried out without oxygen in the flue gas, a and a reductive condition was formed. Lastly, the catalytic effect of bed materials on N₂O decomposition is analyzed with biomass gas reburning and oxygen in the flue gas.

Figure 8. Influence of bed material on N₂O decomposition without biomass gas: (a) N₂O = 2000 ppm; (b) N₂O = 2800 ppm (with a height of bed material of 40 mm).

Figure 8. Influence of bed material on N₂O decomposition without biomass gas: (a) N₂O = 2000 ppm; (b) N₂O = 2800 ppm (with a height of bed material of 40 mm).

Figure 8 illustrates the order of the promoting effects of different bed materials on N₂O thermal decomposition without gas reburning. N₂O conversion rate is the highest when CaO is used as bed material at 900 °C. This indicates that CaO has the best promoting effect on N₂O conversion, playing the most significant role as catalyst and consistent with Barišić’s research [17]. According to the study of a four type bed material from the bottom bed of two industrial CFB boilers, it is found that the combustion process of circulating fluidized bed boiler forms CaSO₄ with limestone consumption during the desulfurizing process. Barišić et al. proved that CaO is the most important component of bed material and the production of limestone with catalytic effect on N₂O conversion. In this paper, considering the promoting effect on N₂O conversion, CaO is followed by Fe₂O₃, Al₂O₃ and SiO₂.

The results show that N₂O conversion with CaO bed material and biomass gas reburning is the highest one among the three conditions (Figure 8, Figure 9 and Figure 10). Therefore, given the fact limestone exists in actual CFB boilers as desulfurizer, its ejection together with biomass gas is the most effective way to reduce N₂O emissions.

Figure 9. Influence of biomass gas on heterogenous N₂O decomposition without oxygen content: (a) N₂O = 2000 ppm; (b) N₂O = 2800 ppm (with a height of bed material of 10 mm).

Figure 9. Influence of biomass gas on heterogenous N₂O decomposition without oxygen content: (a) N₂O = 2000 ppm; (b) N₂O = 2800 ppm (with a height of bed material of 10 mm).

Figure 10. Influence of biomass gas on heterogenous N₂O decomposition with oxygen content was 5%: (a) N₂O = 2000 ppm; (b) N₂O = 2800 ppm (with a height of bed material of 10 mm).

Figure 10. Influence of biomass gas on heterogenous N₂O decomposition with oxygen content was 5%: (a) N₂O = 2000 ppm; (b) N₂O = 2800 ppm (with a height of bed material of 10 mm).

4. Conclusions

Using a vertical fluidized bed reactor, reburning process with biomass gasification gas was investigated. Experiments of N₂O decomposition were carried out by changing the reburning gas and the bed material. It is concluded that:

(1)

At the reaction temperature of 900 °C, the promoting effect order of CO, H₂ and CH₄ as biomass gas components on N₂O decomposition is: H₂ > CO > CH₄.

(2)

Under the reductive conditions, the positive effect of H₂ is greater than that of CH₄ and CO on the conversion of heterogenous N₂O with the circulating ash as bed material, whereas under oxidative conditions with the same bed material, the impact of CH₄is proven to be much stronger than that of H₂ or CO.

(3)

With four different typical solid oxides and the circulating ash as bed material, the catalytic effect of CaO on the conversion of N₂O is more effective than that of Fe₂O₃, Al₂O₃ and SiO₂.