h biomass-fired circulating fluid bed

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Kinetics, Catalysis, and Reaction Engineering

NOx reduction in a 130 t/h biomass-fired circulating fluid bed boiler using coupled ozonation and wet absorption technology Jiaming Shao, Chaoqun Xu, Zhihua Wang, Jianping Zhang, Rongtao Wang, Yong He, and Kefa Cen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03355 • Publication Date (Web): 06 Sep 2019 Downloaded from pubs.acs.org on September 6, 2019

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Industrial & Engineering Chemistry Research

NOx reduction in a 130 t/h biomass-fired circulating fluid bed boiler using coupled ozonation and wet absorption technology Jiaming Shao1, Chaoqun Xu1, Zhihua Wang1*, Jianping Zhang2, Rongtao Wang2, Yong He1, Kefa Cen1 1 State

Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, P.R. China 2

China Everbright Greentech Limited, Shenzhen 518040, P.R. China Email: [email protected] Tel: +86-0571-87953162

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract: The low-temperature, high alkali metal and water content flue gas in biomass boilers restrict the application of traditional NOx treatment technology (i.e., SNCR and SCR). In this paper, the coupled ozonation and wet absorption technology was used in a 130 t/h biomass circulating fluid bed boiler. Key parameters, i.e., O3/NO molar ratio, mixing, liquid/gas ratio and O3 residual were investigated with the industrial real case. The higher O3/NO molar ratio achieved the better denitration efficiency, and the O3 residual started to increase once O3/NO molar ratio exceeded 2.1. Mixing uniformity is a key factor for the diffusion of ozone in flue gas, and it would directly influence N2O5 formation and O3 decomposition process. In the slurry, NO3− and SO42− were the major byproducts after NOx and SO2 absorption. With the optimization of key parameters, the NOx emission was controlled below 50 mg/Nm3 under 34.8 kg/h O3 dosage. Keywords: NOx reduction, biomass boiler, ozonation, ultra-low emission, industrial-scale.


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1. Introduction Energy is an indispensable participant in people’s daily life, while the non-renewable energy occupies the overwhelming proportion of current worldwide energy supply.1, 2 Due to the rapid increase of energy demanding and unlimited exploitation, the world are facing serious problems related to depletion of nonrenewable energy resources.3 Meanwhile, the use of non-renewable energy, such as fossil fuels combustion, would cause a series of serious environmental problems (eg., acid rain, photochemical smog, haze and greenhouse effect).4-6 Therefore, many countries are trying to shift their energy consumption structures to renewable energy, which is more environmental friendly.1 For instance, Chinese government starts to advocate low-carbon economy, aiming at increasing non-fossil energy supply up to 15% of total energy consumption by 2020.7 More importantly, renewable energy is generated naturally, which would not consume any natural resource and can be naturally replenished, so it is sustainable energy.1 In worldwide, the most commonly used renewable energies are solar energy, hydrogen, wind power, geothermal and biomass.3 Particularly, biomass is considered as one of substitute for fossil fuels due to its various advantages (eg., carbon neutrality, high fuel flexibility and abundant reserves).8-10 Generally, biomass energy can be utilized directly and indirectly.11 The directly consumption consists of cooking, space heating and industrial process, while the indirectly utilization refers to converting biomass into secondary energy, such as biomass liquefaction and biomass gasification technologies.12-14 Recently, biomass combustion in the boilers for electricity production has been developed intensively, aiming at replacing the existing coal-fired boilers. Although low sulfur species content in biofuels, biomass combustion is still proved to be related to local air quality, especially for the small-scale combustors.15, 16 This is because the NOx emission could not be ignored in the industrial biomass boilers.8,

17

Due to the relatively low temperatures of biomass

combustion, fuel-NOx is the most relevant pathway for NOx emission comparing with thermal-NOx and prompt-NOx. Additionally, the process of primary pyrolysis, tar cracking and char devolatilization are the ACS Paragon Plus Environment


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major routes of fuel NOx formation in biomass combustion.18 Unfortunately, the flue gas characteristics of low temperature, high humidity and high potassium content impede the application of traditional NOx removal technologies (eg., SNCR and SCR).19 More seriously, China has implemented many

strict

environmental regulations for air pollutants control, especially “Ultra-Low Emission (ULE)” for key region (SO22.4, the increased O3/NO seemed to have little contribution on NOx removal, and the maximum ~93% efficiency was obtained. This phenomenon suggests that no further injection is needed when O3/NO approaches 2.0 for plant operating. Notably, the overall NOx removal efficiency detected in the MP 3 is higher than results in Figure.2. It is because the unreacted NOx could react with excess O3 in the wash tower. So N2O5 can be generated continuously in wash tower, and its absorption pushed Eq. 7 moving to the right. In other words, flue gas retention in wash tower would be benefit for NOx removal. 100 90

NOx removal efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70 Initial NO concentration: 90~130 mg/Nm3 O2concentration: 4~6%

60

Flue gas volume:138000~145000 Nm3/h

50 40

NOx romoval

30 0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.6

4.0

Molar ratio of O3/NO

Figure. 3. NOx removal efficiencies with varying O3/NO molar ratios. 3.3 Effect of mixing uniformity The self-designed airbrush arrangement and grid-shaped mixer were installed to accelerate the mixing of flue gas and injected O3. In this project, forty-eight O3 airbrushes were separately distributed in six pipelines with six individual valves, as shown in Figure.4. Well mixing uniformity could avoid local spikes in O3 concentration and lower the ozone decomposition at high temperature. Based on the same total amount of O3, various working conditions (listed in Table 3) were flexibly changed to test the mixing influence. Figure. 5 shows the NOx removal efficiency in these four runs as a function of O3/NO molar ratio. It is ACS Paragon Plus Environment


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obvious that mixing uniformity acts a critical role in the NOx removal process, especially for the gas phase reaction. In these four runs, all opened valves condition (Run 1#) achieved the best NOx removal, following with Run 2# and Run 4#, whose two valves were closed. Run 3# had the worst denitration efficiency with only middle two valves open. When O3/NO1.2. Evidently in Figure. 5, NOx removal efficiencies in Runs 2#~4# were much lower than that of Run 1#. The long reaction time of N2O5 formation allowed Eq.8 taking place, especially in high O3 concentration area. In Run 2#~4#, O3 concentration near the airbrushes is higher than that of Run 1# with the same initial total O3 dosage, so the accumulation of O3 at local area would push the Eq.8 to the right. Therefore, well mixing uniformity between injected O3 and flue gas would slow down O3 decomposition to some extend and achieve better denitration efficiency. 2O3→3O2

(8)

Table 3. Valves working conditions in different runs Valve #1

Valve #2

Valve #3

Valve #4

Valve #5

Valve #6

Run 1#

●

●

●

●

●

●

Run 2#

●

●

●

●

○

○

Run 3#

○

○

●

●

○

○

Run 4#

●

●

○

○

●

●

● means the valve on, ○ means the valve off.


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Figure. 4. Site photo of the O3 injection equipment arrangement. Run 1# Run 2# Run 3# Run 4#

80


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70

60

50

40

30 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4


Figure. 5. NOx removal efficiencies with varying ozone injection values. 3.4 Effect of liquid/gas ratio Liquid/gas ratio would directly influence gas-liquid mass transfer in wash tower. In practice, the change in the liquid/gas ratio is regulated by the amount of opening circulation pump. In this project, two spray layers were installed that equipped with two individual circulation pumps with 543 m3/h flow rate by each. The distance of the two layers is 2 m and the designed flue gas velocity in wash tower is 2.4 m/s. It is calculated that the liquid/gas ratio is 3.6 L/m3 for each spray layer. The height of layer 1 is 7.7 m with ~3.2 s flue gas resistance time. Layer 2 is the higher one equipped inside wash tower, which has ~0.8 s residence time longer than layer 1. Figure. 6 exhibits NOx removal efficiency varying with O3/NO molar ratio under different spray layer. It is obvious that both layer 1 and 2 (L/G-7.2 L/m3, R.T-4.0 s) open ACS Paragon Plus Environment


obtained the best denitration efficiency. Higher liquid/gas ratio allowed the greater mass transfer from gas to liquid phase, which indeed enhanced NOx adsorption. For only one layer open conditions, layer 2 (L/G3.6 L/m3, R.T-4.0 s) had a little higher NOx removal efficiency when O3/NO >1.4 than layer 1 (L/G-3.6 L/m3, R.T-3.2 s). This is because the longer residence time provided longer reaction time between the excess O3 and unreacted NO2, so that more N2O5 was generated in wash tower. More importantly, it indicates that the properly enhancing retention in wash tower could increase NOx removal efficiency in industrial design. 80

70


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60

50

40

L/G-7.2 L/m3 R.T-4.0 s L/G-3.6 L/m3 R.T-3.2 s L/G-3.6 L/m3 R.T-4.0 s

30

20 0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2


Figure. 6. NOx removal efficiencies with varying liquid/gas ratios. 3.5 O3 residual In recent years, the ground-level ozone has been considered as a new air pollutant.31, 32 Theoretically, the stoichiometric ratio of O3/NO is 1.5 for N2O5 formation, which means that O3 would be in excess if O3/NO>1.5. However, in industrial application, O3 dosage is usually greater than this stoichiometric ratio, so the O3 residual must be an issue to evaluate for this ozone deep oxidation technology. A low concentration ozone analyzer continuously monitored O3 residual concentration at the MP 3, and the results are shown in Figure. 7. However, it can not easily to detect O3 residual at MP 3 when O3/NO

h biomass-fired circulating fluid bed

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