Article pubs.acs.org/EF
Experimental Study on the NO and N2O Formation Characteristics during Biomass Combustion Jisong Bai, Chunjiang Yu,* Lianming Li, Peng Wu, Zhongyang Luo, and Mingjiang Ni State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: In the present study, the NO and N2O formation characteristics during combustion of five biomass fuels (rice straw, wheat straw, corn stalk, sugarcane leaf, and eucalyptus bark) and one bituminous coal, as a comparison, were investigated in a horizontal fixed-bed reactor. It showed that there was still a considerable degree of N2O conversion for biomass fuels, although NO was formed in a much larger amount. Most of NO and N2O were formed during the devolatilization stage for biomass fuels; therefore, optimizing the supply of air and fuel during biomass combustion in actual boilers can be expected to achieve ultralow emissions of nitrogen oxides. There was no clear correlation between the NO and N2O yields and fuel N content, which indicated that the nitrogen functionality and other components, such as inherent mineral matter, present in biomass must have great influence on the nitrogen transformation during thermal processing. When rice straw was co-fired with eucalyptus bark, nonlinear behavior could be seen and the fuel N conversion to NO was largely enhanced. In the temperature range of 700−900 °C, the fuel N conversion to NO first increased and then decreased slightly, while the conversion to N2O showed a continuous decreasing trend. The fuel N conversion to NO and N2O showed a general increasing trend with the increase of the inlet oxygen concentration, and this phenomenon was more obvious at higher temperatures. The results presented in this study will help to gain some insight onto the fundamental mechanism of fuel N conversion during biomass combustion.
1. INTRODUCTION Among various biomass use technologies, combustion for the production of electricity is the most developed and widely applied because of its low costs and high reliability.1−3 While biomass fuels have the advantage of being CO2-neutral, there are still potential problems concerned with the environmental pollution in the thermal use process. The emission of nitrogen oxides (mainly NO and N2O) is one of the most important challenges in the field.4−7 Generally, the nitrogen content in biomass is relatively lower compared to coal and other fossil fuels, but considering the lower heating values of biomass, the NO and N2O emissions may be even higher based on heating values. Moreover, biomass combustion is usually performed in relatively low-temperature conditions for inhibiting or alleviating the alkali-related issues and thereby facilitating the N2O formation. To develop efficient NO and N2O reduction measures, it is important to obtain a deep understanding of the nitrogen transformation during the biomass thermal conversion process. The NO and N2O formation characteristics during coal combustion have been studied extensively,8−11 and the general mechanism of the nitrogen oxides formation is fairly wellestablished; however, some open issues are still present. Few detailed studies focused on biomass combustion can be found in the available literature. It is generally believed that nitrogen in biomass is mainly bound in amino acids and proteins,12−15 whereas coal N mainly exists in the form of pyrrolic and pyridinic structures.8,16 The difference in the N functionalities would greatly affect the selective conversion of fuel N during biomass and coal combustion. The release of fuel N during solid fuel combustion is a very complex process. When the combustion process begins, fuel N is distributed between volatiles and the char matrix during the © 2012 American Chemical Society
devolatilization stage. Such distribution and the type of volatile N compounds are essential for the nitrogen oxides formation, mainly because of the different conversion mechanisms of volatile N and char N. Therefore, in previous studies, much attention has been paid to the fate of fuel N during biomass pyrolysis. The main nitrogen-containing products of biomass pyrolysis are NH3 and HCN, and a minor amount of HCNO formation is also reported in some recent studies.13,17−19 Those precursors could either be oxidized to nitrogen oxides or converted to N2 during subsequent combustion, which mainly depend upon the local stoichiometry. Recently, there have been a number of investigations focusing on the NO and N2O emissions during biomass combustion in a large-scale fluidized-bed or grate combustor.5,7,20−22 Generally, quite a low N2O concentration was detected, and the NO emission appeared to be strongly influenced by the nitrogen content of biomass fuels rather than the operating conditions. However, in those studies, because of the coupling effect of many factors, it is hard to deduce the exact mechanism of nitrogen migration. In the present study, the formation characteristics of NO and N2O were studied in a horizontal fixed-bed reactor, which could sharply reduce the occurrence of secondary reactions, especially the gas−solid reactions. The biomass fuels (rice straw, wheat straw, corn stalk, sugarcane leaf, and eucalyptus bark) selected are commonly used in the biomass power plants in China. To begin with, the combustion and emission characteristics of biomass fuels were analyzed in-depth based on the comparison to bituminous coal. Received: August 21, 2012 Revised: November 25, 2012 Published: November 28, 2012 515
dx.doi.org/10.1021/ef301383g | Energy Fuels 2013, 27, 515−522
Energy & Fuels
Article
Table 1. Ultimate and Proximate Analyses of Biomass Samples (As Received) proximate analysis (wt %)
a
ultimate analysis (wt %)
sample
moisture
ash
volatile matter
fixed carbon
C
H
N
S
Oa
rice straw wheat straw corn stalk sugarcane leaf eucalyptus bark bituminous coal
2.39 11.63 10.34 9.82 10.79 3.29
7.98 5.11 6.18 4.24 4.68 22.37
71.38 66.37 67.39 68.79 66.19 27.81
18.25 16.89 16.09 17.15 18.34 46.53
42.73 39.61 40.22 42.24 40.69 59.28
5.32 4.92 5.12 5.30 4.72 4.05
1.06 0.43 0.87 0.69 0.24 0.92
0.16 0.13 0.11 0.17 0.06 0.91
40.36 38.17 37.16 37.54 38.80 9.18
By difference.
Table 2. Results of Ash Analysis ash components (wt %) sample
Al2O3
CaO
Na2O
MgO
K2O
Fe2O3
SiO2
P2O5
rice straw wheat straw corn stalk sugarcane leaf eucalyptus bark
0.82 1.56 7.53 5.67 3.32
3.23 2.98 6.07 10.01 45.23
1.21 1.73 1.03 0.53 1.01
3.02 3.45 6.43 2.69 8.68
21.32 18.81 9.78 5.00 7.76
0.62 0.51 3.32 3.91 1.35
46.25 47.03 44.35 66.46 16.26
1.34 2.76 0.87 3.01 2.21
Then, the effects of biomass fuel type, fuel blends, temperature, and oxygen concentration on the NO and N2O formations were investigated.
2. EXPERIMENTAL SECTION 2.1. Fuels. Five different biomass samples, rice straw (RS), wheat straw (WS), corn stalk (CS), sugarcane leaf (SL), and eucalyptus bark (EB), and one bituminous coal (BC) were used in this study. All of the fuel samples were milled and sieved to the size of
