Article pubs.acs.org/EF
Experimental Study of the Path of Nitrogen in Chemical Looping Combustion Using a Nickel-Based Oxygen Carrier Stefan Penthor,*,† Karl Mayer,† Tobias Pröll,‡ and Hermann Hofbauer† †
Institute of Chemical Engineering, Vienna University of Technology, 1060 Vienna, Austria Institute for Chemical and Energy Engineering, University of Natural Resources and Life Sciences Vienna, 1190 Vienna, Austria
‡
ABSTRACT: The path of nitrogen and the emissions of nitrogen oxides, such as NO, NO2, and N2O (NyOx), have been evaluated via experiments at a 120 kW dual-circulating fluidized-bed chemical looping pilot plant. Natural gas has been used as fuel, and its fuel nitrogen content has been varied by adding ammonia (NH3) as a model NyOx precursor up to a total fuel N content of 1.36 wt %. A nickel-based oxygen carrier with an active NiO content of 40 wt % has been used. While NH3 conversion was practically complete with below 8 ppmv [dry basis (db)] NH3 in the fuel reactor exhaust gas, the concentration of NO was always below 6 ppmv (db) in the air reactor and no NO was detected in the fuel reactor exhaust gas. Concentrations of NO2 and N2O were below the detection limit of the equipment used (99%) for all operating points (see Figure 5). The remaining
Figure 3. Overall CLC performance with increasing N content in the FR feed.
detection limits of the used gas-analyzing equipment, i.e., below 2 ppmv. Thus, NO2 and N2O concentrations are not included in the following discussion. The concentration of nitrogen species in the FR feed for all operating points is summarized in Table 5. N2 from natural gas does not add up to fuel N values. Figure 5. NH3 conversion and NH3 and N2 concentrations in FR exhaust gas with increasing N content in fuel.
Table 5. Nitrogen Species in FR Feeda operating point OP OP OP OP a
1 2 3 4
N2 (vol %)
NH3 (vol %)
fuel N (wt %)
0.94 0.93 0.93 0.92
0 0.41 0.81 1.60
0 0.35 0.69 1.36
NH3 in the FR [5−8 ppmv dry basis (db)] was most likely a result of a gas slip through the reactor. Values for NH3 conversion have been calculated on the basis of the measured NH3 concentration in the FR exhaust gas. Post-processing of the experimental data revealed that the raw NO and N2O concentrations obtained from the FR exhaust gas [17 ppmv (db) and 60 ppmv (db)] during the final operating point are not fully reliable because of cross-sensitivity between NO with high concentrations of CO2 and between N2O and CO.39 Table 6 illustrates the range by comparing
Fuel N only refers to N from NH3.
Nitrogen emissions in the AR exhaust gas were measured for the sake of completeness and for closing the nitrogen balance. This is of importance to determine the amount of nitrogen species transported from the AR to the FR via gas leakage through the ULS. Figure 4 shows the NO concentrations in the AR exhaust gas for different nitrogen contents in the fuel feed. The NO concentration does not change with increasing N content in the fuel and is well below current emission regulation limits. The secondary y axis in Figure 4 shows the corresponding values for the NO emissions related to the energy input, representing an alternative method for
Table 6. Cross-Sensitivity between CO2/NO and CO/N2O
gas 1 gas 2
CO2 (vol %)
CO (vol %)
response NO (ppmv)
response N2O (ppmv)
35.0 33.97
2.99 34.97
12.2 12.3
38.6 148.7
CO/CO2 concentrations for two different gas compositions and the NO/N2O responses. The two gases used to check this cross-sensitivity were calibration gases containing no NO or N2O. This cross-sensitivity has to be taken into account when evaluating NO emissions in the FR exhaust gas; i.e., correcting the NO value suggests that no NO is present in the FR exhaust gas. Because of the high cross-sensitivity between N2O and CO, N2O concentrations have not been considered in the mass balance. Figure 6 illustrates the mass balance for nitrogen components for AR and FR. The scaling for the two reactors is different because of the much larger amount of nitrogen entering the AR via the combustion air. It can be seen that NH3 is nearly fully reduced to N2 (see also increasing N2 concentration in Figure 5). The mass balance also shows that the largest part of nitrogen in the FR exhaust gas actually comes from gas leakage from AR to FR via the upper loop seal.
Figure 4. NO concentration in the AR exhaust gas dependent upon the N content in the fuel. NOAR is the amount of NO related to the AR exhaust gas stream, and NOAR,total is the amount of NO related to the sum of AR and FR exhaust gas streams. The secondary y axis shows the NO emission related to energy input. 6607
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Figure 6. Mass balance for nitrogen components for both reactors (N content fuel of 1.28 wt %). Scaling for AR and FR is different.
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CONCLUSION AND OUTLOOK The pathway of nitrogen and the formation of N y O x components in CLC has been evaluated in an experimental study at a 120 kW CLC pilot plant for gaseous fuels using a Nibased OC. For that purpose, ammonia (NH3) acting as a model precursor for NyOx components has been added to the fuel. In this way, the fuel N content of the fuel has been varied between 0 and 1.36 wt %. The results show that NH3 conversion in the FR is above 99% and that ammonia is almost fully reduced to N2. Measured NO concentrations have always been below 6 ppmv (db) in the AR exhaust gas, and no NO concentrations have been detected in the FR exhaust gas. When it comes to evaluation of emissions in CLC, they have to be evaluated individually for the AR and the FR exhaust gases. Because of the character of this novel combustion technology, it has to be discussed if there are better ways to quantify emissions, e.g., relating emissions to energy input instead of concentrations. Nevertheless, measured values are well below all limits in current regulations of NOx emissions and available recommendations of NOx concentrations for CO2 purity in CCS. Thus, CLC has the potential not only to be an energy-efficient carbon capture technology but also to act as a highly efficient primary measure against NOx emissions. Further work should focus on evaluating nitrogen emissions in CLC with other OCs, e.g., copper- or manganese-based OCs. Reaction mechanisms indicate that they may cause higher NOx emissions. This is especially interesting for so-called chemical looping with oxygen uncoupling (CLOU) OCs, where gaseous oxygen is present in the FR.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +43-1-58801-166367. Fax: +43-1-58801-16699. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was part of the Austrian Government’s Climate and Energy Fund Project BioCLC 829982 coordinated by ANDRITZ Energy and Environment. Financial support given by the Climate and Energy Fund and ANDRITZ Energy and Environment is gratefully acknowledged. 6608
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