Evolution of fuel-N in gas phase during biomass pyrolysis

Renewable and Sustainable Energy Reviews 50 (2015) 408–418

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Evolution of fuel-N in gas phase during biomass pyrolysis Qiangqiang Ren a,n, Changsui Zhao b a b

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China School of Energy and Environment, Southeast University, Nanjing 210096, Jiangsu, China

art ic l e i nf o

a b s t r a c t

Article history: Received 10 November 2014 Received in revised form 28 April 2015 Accepted 12 May 2015

Increasing concerns about fossil fuels availability and environmental risks, like greenhouse gas CO2, pollutants SO2, NOx, heavy metals, are promoting the development of renewable energy. Rising bioenergy share in the power generation sector is strongly promoted all over the world. Biomass combustion has become one of the most promising technologies for large-scale utilization of biomass, which can help to decrease the dependence on fossil fuels and realize CO2 reduction. NOx emission from biomass combustion has direct correlations with haze formation. In order to meet increasingly strict environmental standard, much attention has been focused on NOx reduction with the development of biomass-power generation. Understanding and predicting fuel-nitrogen conversion during biomass pyrolysis is critical and continues to be a challenge for understanding formation and reduction of NOx and N2O during biomass combustion. This review examines the-state-of-the-art of nitrogen transformation mechanism during biomass pyrolysis. Proteins/amino acids, which are the nitrogen functionalities in biomass, decompose dependently of other constituents in biomass. The data obtained from pyrolysis of amino acid with the components in biomass are useful in predicting nitrogen conversion of biomass. Finally, directions for future research are suggested. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Biomass Amino acid Nitrogen transformation Pyrolysis

Contents 1. 2.

3.

4.

5.

6.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 Typical biomass combustion technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 2.1. Stand-alone biomass combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 2.1.1. Grate-firing biomass combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 2.1.2. Circulating fluidized bed combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410 2.2. Co-firing in pulverized coal-fired boilers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410 2.3. NOx and N2O during biomass combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410 Nitrogen transformation from biomass pyrolysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 3.1. Nitrogen transformation from different kinds of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 3.2. Effect of atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 3.3. Effect of mineral matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412 Nitrogen behavior from N-containing model compound. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412 4.1. Nitrogen functionality in biomass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412 4.2. General characteristics of protein/amino acid pyrolysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413 4.3. Nitrogen from the N-containing model compound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413 The dependence of N-containing model compound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414 5.1. Effect of atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414 5.2. Effect of mineral matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414 5.3. Effect of cellulose, hemicelluloses and lignin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415 Future research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

Corresponding author. Tel.: þ 86 10 82543055. E-mail address: [email protected] (Q. Ren).

http://dx.doi.org/10.1016/j.rser.2015.05.043 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

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7. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415 Acknowledgement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

1. Introduction As the environmental and climate change concerns due to the extensive use of fossil fuels was increasingly serious, the world began to pay attention to the development of renewable energy. Biomass has some advantages that it is geographically widely available and it is a storable energy of the alternative renewable supplies. Biomass is a promising CO2-neutral alternative solid fuel due to the decreasing resources of fossil fuels and their effect on global warming. Biomass is a renewable source of energy which will probably play an important role in the transition towards a more sustainable energy supply and in achieving the goals of CO2 reduction [1–5]. The upper estimate of biomass annual availability is about 4500 EJ (220 Gt mass) and is almost ten times the world current energy requirement [6]. It has been concluded that the contribution from biomass could be raised to 200 EJ/year (4.8 Gtoe) by 2050 [7–11]. Biomass storage capacity is rich and is considered to be an important part of future energy. Generally speaking, biomass includes the following aspects [12]:

 Herbaceous biomass, such as cotton stalk, corn stalk, rape straw,

  

wheat straw and rice straw. The annual herbaceous biomasses also can be divided into two sub categories. One is soft straw with high ash content, e.g., wheat straw and rice straw. The other is gray straw with relatively lower ash content and property analogous to woody biomass, e.g., cotton stalk. Woody biomass: pine, poplar, bamboo. The main feature of woody biomass is its low ash content, about 1%. Chaff biomass: rice husk, peanut shell. Animal waste and aquatic plants are also biomass resources.

is affected by some factors including biomass species, mineral composition and content, particle size, heating rate, temperature, residence time, pressure and reactor type. Given the complexity of biomass, understanding the mechanism and process during biomass pyrolysis is difficult. For a better understanding of biomass thermochemical conversion, it is widely accepted that from the view of the individual pyrolysis of its main constituents (i.e. cellulose, hemicelluloses and lignin) helps to understand biomass pyrolysis [16–20]. Nitrogen behavior during pyrolysis is critical for the NOx and N2O formation during biomass combustion. Therefore, in-depth study of nitrogen in the process of straw pyrolysis is important for understanding and control of NOx formation in biomass combustion process. The objective of the present work is to provide a state-of-the-art overview of the current understanding of fuel nitrogen conversion during biomass pyrolysis.

2. Typical biomass combustion technology As will be shown during this review, the scale of a combustion unit, as well as the biomass type and properties, has important impacts in terms of the pollution. Therefore, this section will give an overview of combustion equipment, biomass types and their combustion characterization. The most important advantage of the use of biomass over other sources of renewable energy is the possibility to apply it on the short term. Existing furnaces and conversion techniques originally developed for fossil fuels can operate on biomass fuels with no or only small modifications. 2.1. Stand-alone biomass combustion

There is an expanding market in the use of biomass for supplying both transport fuels and electricity/heat. It was reported that biomass for electricity generation will increase by 4 times [13]. Global biomass resource is enough to meet our energy demand. The supply of biomass energy has no pressure on competing for food with people. Biomass combustion has become one of the most promising technologies for large-scale utilization of biomass, which can help to decrease the dependence on fossil fuels and realize CO2 reduction. Up to 2010, China’s installed capacity of biomass-fired thermal power plants was 5500 MW. According to China National Twelfth-Five Year Plan, the installed capacity of biomass-fired thermal power plants was 20,000 MW. The EU must obtain 20% of its energy needs from renewable sources by 2020 and a 10% share of renewable energy specifically in the transport sector. Biomass contains low sulfur content and relatively high nitrogen content, which can be transferred into NOx and N2O during combustion. In order to meet increasingly strict environmental standard, much attention has been focused on NOx reduction with the development of biomass-power generation. Pyrolysis is a capital step of biomass thermochemical conversion as it is the first step of gasification and combustion. Biomass pyrolysis can be treated recently as the leading conversion platforms for biomass-to-liquid transportation fuels and is usually described using three different regimes based on severity: primary (below 773 K), secondary (973–1123 K) and tertiary (1123–1273 K) [14,15]. Biomass pyrolysis is an especially complicated process and

The two most common types of boilers for biomass combustion are grate-firing system and circulating fluidized bed combustor, both of which have good fuel flexibility and can be fuelled entirely by biomass or co-fired with coal. Key technologies for biomass firing boilers are the fuel pretreatment and supply, high-efficiency combustion, prevention of alkali metal related problems and low NOx emission. 2.1.1. Grate-firing biomass combustion Grate-firing boiler is one of major and popular boiler types to burn biomass for heat and power production in China and European countries [21–23]. Most of these plants are used to combust residues of wood industry in Europe. In China, agricultural residues are the major biomass resources and are the fuel for biomass plants. Grate-firing furnaces can deal with a wide range of biomass fuel types (e.g. sawdust, wood pellets, bark and straw) and are flexible regarding fuel size and moisture content. Basically, grate-firing biomass boilers consist of five key elements: a fuel feeding system, a grate assembly, a secondary air system, an ash discharge system and arrangement of the heaters. The grate has a kind of shaking movement that spreads the fuel evenly. This type of grate has lower maintenance and higher reliability. Carbon burnout efficiency is also further improved. Different types of grate furnaces exist, because the furnace can be optimized for various fuels and operating conditions. Watercooled vibrating grate is used widely in China and Denmark and it can solve ash sintering problems on the grate.

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It can be concluded that grate furnace combustion is a mature combustion technique for which already a range of techniques are available to optimize it for specific types of fuels, good burnout of the exhaust gases and low NOx emissions. Study has been concentrated on biomass combustion and the pollutant in fixed bed reactor as well as the grate furnace [24–45]. However, the decreasing emission limits show that there is a continuous effort of governments to lower the emissions further. Thus, research is needed to develop, optimize and implement emission reduction techniques. Grate furnaces usually require a lower level of investment and entail fewer operating costs and are therefore used principally in smaller power plants. 2.1.2. Circulating fluidized bed combustion Circulating fluidized bed (CFB) combustion technology can be used for a broad variety of fuels. The popularity of fluidized bed combustion is due largely to the technology’s fuel flexibility and the capability of meeting sulfur dioxide and nitrogen oxide emission standards without the need for expensive add-on controls. Basically, Circulating fluidized bed boilers consist of following key elements: a fuel feeding system, the primary air distributing system, the secondary air system, separated solid material circulating system and the arrangement of the heaters. Fuel flexibility is becoming increasingly important since there is an increased need to burn a broad spectrum of biomass and waste derived biomass fuels. Circulating fluidized bed combustion is today a well established technology for biomass combustion for generation of heat and power [41,46–48]. There are a lot of researches concentrated on biomass circulating fluidized bed combustion. For circulating fluidized bed, silicacontaining sand is usually used as the bed material, and potassium in biomass can react with silica to form a low temperature eutectic during combustion, which will damage the operation [49–62] and some possible ways have been used to solve the alkali metal related problems [63–77]. Circulating fluidized bed is well suited for co-firing biomass in existing large coal power boilers [78–82]. In a CO2 constrained future, increasing demands on efficient use of biomass conversion makes it likely that co-firing of biomass with coal becomes an interesting option as part of the bridge to a more sustainable energy system. 2.2. Co-firing in pulverized coal-fired boilers Biomass/coal co-combustion represents a rationally near-term sustainable option that promises in effective CO2 reduction and several societal benefits. Co-firing biomass with coal in traditional coal-fired boilers represents one combination of renewable and fossil energy utilization that derives the greatest benefit from both fuel types. Commercialization of co-firing technologies offers among the best short- and long-term solution to greenhouse gas emissions from power generation. Due to the high steam parameters and technical measures for efficiency improvement available in coal power plants, higher conversion efficiencies than those in dedicated biomass systems can be easily achieved. There has been remarkably rapid progress over the past 10 years in the development of the co-utilization of biomass in coal-fired boiler plants. Several plants have been retrofitted for demonstration purposes. Typical power stations where co-firing is applied are in the range from approximately 50 MWe (a few units are between 5 and 50 MWe) to 700 MWe. The majority are equipped with pulverized coal boilers. Furthermore, bubbling and circulating fluidized bed boilers, cyclone boilers, and stoker boilers are used. It is widely accepted that addition of biomass to a coal-fired boiler does not impact or at worst slightly decrease the overall generation efficiency of a coal-fired power plant. Minor changes in efficiency (either positive or negative) may occur due to more or

Table 1 Nitrogen content in biomass and some fuels [56,88,89]. Fuel

N content/wt%

Agricultural residue Woody biomass Animal waste Aquatic plants Sewage sludge Coal

0.3–3.5 0.1–1.0 0.5–1.0 3.0–7.0 2.5–6.5 0.5–2.5

less energy intensive fuel preparation and handling, while the typically increased moisture content in the fuel will slightly reduce the overall efficiency. The results show that existing roller mills and direct-blowing pulverizing systems in a 300 MW pulverized coal-fired utility boiler can be directly used to grind biomass and to transport pulverized biomass within the limit in the flow rate of the biomass that can be processed [83]. 2.3. NOx and N2O during biomass combustion Table 1 shows typical values for nitrogen content for typical biomass. The nitrogen content in biomass differs from fuel to fuel. Usually, the faster the biomass grows, the higher is the nitrogen content. During biomass gasification and combustion, nitrogen is converted into NOx (NO, NO2) and N2O, which are environmental harmful. NOx causes acid rain and contributes to the formation of photochemical smog, and N2O is a greenhouse gas [84–86]. Typical NOx concentrations resulting from grate furnace and CFB combustion are 100–300 ppm [87]. Emission standards have been developed which have to be met by combustion furnaces. In China, the new National Emission Regulation (NER) issued in early 2012 brings much more stringent challenges on biomass combustion technology. NOx emission must be controlled to less than 100 mg/m3 [90]. Haze is a serious pollution and caused by many sources, such as coal combustion and vehicle exhaust. It was reported that NO3 and NH4þ had direct correlation with the formation of particulate matter and can deteriorate the haze [91–94]. Biomass combustion can cause serious haze, which is due to the NOx emissions [95]. Much attention has been focused on NOx control and reduction with the development of power generation using biomass in order to meet increasingly strict environmental standards. For biomass grate-firing and circulating fluidized bed combustion technologies, the influence of the air supply (amount and distribution), furnace temperature and residence time in the furnace on NOx formation and control were noticeable. In general, a higher proportion of air increased the specific NOx emissions and the Fuel-N conversion ratio. Effective methods to control the NOx emissions level during combustion process were air staging, proper combustion stoichiometry, low temperature and fuel staging. In order to minimize the emission of NOx and N2O, better understanding of the conversion of biomass-N during pyrolysis is essential. There are two sources of nitrogen during combustion process: fuel nitrogen and air nitrogen. The fuel-nitrogen content was observed to be a major factor for determining both NOx and N2O emission. It is released both with the volatiles and during char combustion. The partition of fuel-N between volatiles and char depends on type of biofuel and on combustion conditions (mainly on temperature and particle size and slightly on heating rate and pressure). Roughly, the release of volatile-N is proportional to the release of volatiles, or volatile-C. The chemical form of released volatile-N depends on combustion conditions, particle size, and fuel type. Tar-N, NH3, HCN, HNCO, and N2 are believed to be the major species formed. There are significant discrepancies in literature data regarding the chemical form of released volatile-N. The differences may be due

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Table 2 Nitrogen transformation from pyrolysis of some biomass. Biomass type

Reactor

Tfinal (oC)

Analysis technique

Major N-species

Reference

Chicken manure and kitchen waste Bagasse Sawdust/bagasse Rapeseed Wood bark (birch, pine) Wood bark (pine) Bagasse Cane trash Wheat straw/tobacco Bark pellets

TGA Tube reactor Fluidized bed Fluidized bed Entrained flow reactor TGA Tube reactor Tube reactor TGA Fluidized bed

Sawdust, bark RDF and sawdust Rice straw Wheat straw/corncob

batch reactor Fluidized Bed TGA TGA

900 800 700–900 500–700 800 810–930 800–1000 600–1000 900 700–800 400–900 750 700–900 800 800

FT-IR Chemical Chemical Chemical Chemical Chemical Chemical Chemical FT-IR FT-IR FT-IR FT-IR Chemical FT-IR FT-IR

NH3 HCN NH3 NH3 NH3 NH3 HCN Both HCN and NH3 Both HCN and NH3 It depends NH3 NH3 NH3 HCN Both HCN and NH3

[97] [123] [124] [125] [126] [127] [128] [129] [130] [131] [132] [133] [134] [135] [135]

to the fact that often only a fraction of the primary nitrogen species is measured, and that secondary reactions often take place before analysis. Hence, better knowledge regarding the release of volatileN is needed.

3. Nitrogen transformation from biomass pyrolysis Biomass pyrolysis process is affected by many factors, such as types of biomass, mineral composition, pressure, heating rate and atmosphere [96–122]. Compared with coal, biomass has higher O/N ratios and volatile contents. Higher O/N ratio promotes abundant O and OH radicals and nitrogen is converted mainly into volatile-N. Biomass-nitrogen starts to release at about 200 1C, and the difference exists for different biomass types. This review is focused on NOx and NOx precursors (HCN, NH3 and HNCO) during biomass pyrolysis and to make clear the-state-of-the-art of nitrogen conversion mechanism. 3.1. Nitrogen transformation from different kinds of biomass Due to the heterogeneity of biomass itself and the diversity of the pyrolytic products, the behavior of biomass-nitrogen in pyrolysis process is very complex. The pyrolysis process has great effects on nitrogen conversion. Due to the characteristic of different biomass types, different reactor types adopted and different analytical means in the literature, the current mechanism of nitrogen transformation from biomass is complex and distinctive. In Table 2, it seems that high concentrations of NH3 was observed for some kinds of biomass, while in other studies, HCN was found to dominate. So this section mainly introduces nitrogen conversion during biomass pyrolysis under different conditions. Oxygen and hydrogen in biomass are almost entirely released as volatiles, while carbon, like nitrogen, is released both as volatiles and retained in the char, accompanying multiple phase or heterogeneous reaction of NH3, HCN and HNCO and char-N. The partition of fuel-N between volatiles and char depends on many parameters, such as final temperature, heating rate, particle size, pressure, and fuel type. The fraction of fuel-N released as volatile-N can be expected to be proportional to the fraction of volatiles in the fuel. During the release of volatiles, a great number of decomposition reactions, such as rupture of structures, cross-linking, hydrogen transfer, and substitution reactions take place. Some of the reactions lead to release of volatiles, while others lead to char formation. After the nitrogen is released from the fuel,

absorption absorption absorption absorption absorption absorption absorption

adsorption

further reactions take place in the gas-phase. The reactions are both homogenous and heterogeneous. The structure and composition of biomass is complicated, it is important to find the critical factors affecting biomass-nitrogen conversion. Biomass has high contents of hydrogen and oxygen, HCN/NH3 ratio decreased with increasing fuel-N contents and with decreasing H/N ratio [133,135], the conversion of HCN to NH3 in the interior of the fuel particles is high for fuels with high H/N ratios. For the biomass fuel, the particle size influenced the distribution of N-containing species [136]. As the biomass size increases, the HCN/NH3 and HNCO/HCN molar ratios decrease. Nitrogen is mainly converted to NH3 for biomass of large size during pyrolysis, and nitrogen is mainly converted to HCN and HNCO for small size. Reactor type (heating rate) and particle size have substantial effects on the selectivity of N-conversion due to the selectivity of cracking of cyclic amides and the secondary reaction influencing the formation of NH3, HCN and HNCO. Biomass tends to have a high HCN/NH3 molar ratio at high temperature during pyrolysis [131]. One should note that the ammonia usually is analyzed with acidic titration when chemical adsorption is used. Under these conditions, HNCO is transformed into NH3. The analyzed ammonia yield is then the total yield of NH3 and HNCO. Fourier transform infrared spectrometer (FT-IR) is effective for the quantified analysis of the nitrogen-containing species. 3.2. Effect of atmosphere The atmosphere influences the N-selectivity to HCN, NH3, NO and HNCO [136–139]. The formation of HCN and NH3 during biomass pyrolysis in the presence of oxygen is a result of competition among the opposing effects of oxygen. The presence of oxygen promotes the yields of HCN and HNCO evidently. The introduction of the oxygen makes relatively unstable nitrogen ring structure to break and easily to form HCN and oxygen can promote the nitrogen heterocyclic structure of low-temperature cracking ability. Furthermore, the presence of oxygen in the atmosphere promotes char-N conversion in the late stage. The presence of CO2 reduces HCN formation and suppresses HNCO formation. NH3 seems to be a favorable product from biomass-N in the presence of CO2. In CO2 atmosphere, biomass-CO2 gas–solid reaction can be roughly divided into three steps: first the formation of C–O complex due to adsorption of CO2 in biomass/char surface, and then the C–O complex decomposes on the solid surface. Finally the CO desorbs and escapes. The release of volatile matter is very slow at low temperature during biomass pyrolysis, and the formation of gaseous nitrogen

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compounds is relatively small. Oxygen chemical bond adsorbed on solid surface breaks to form surface oxygen complex C(O), the surface complex first reacts with the relatively unstable molecular structure to the form C(H), C(N), C(O) and (OH) radical groups. These reactive radical groups migrate and collide to form a variety of products on the solid surface, then desorb and release. Tian et al. [129] found that steam in the atmosphere affected nitrogen behavior during cane trash pyrolysis and gasification, during pyrolysis in argon using a fluidized-bed/fixed-bed reactor and a fluidized-bed/tubular reactor, HCN yield is more than NH3 yield, while in the presence of steam, significant NH3 can be formed from the gasification of char-N at 700 1C. Biomass, especially agricultural biomass contains high chlorine content. Ren et al. [138] found that in the presence of HCl, the temperature corresponding to NH3 starting release during biomass pyrolysis increases, and those of HCN and HNCO reduce. HCl in the atmosphere inhibits the conversion of biomass-N into NH3, however, favors the transformation of straw nitrogen into HCN and HNCO. 3.3. Effect of mineral matter Mineral matter is an important part of biomass, and the major mineral matters include silicon, potassium, aluminum, sodium, calcium and iron, etc. Mineral matters have great effects on biomass pyrolysis property and product distribution characteristics [140–147]. It was found that potassium promotes N-conversion to NH3, HCN, NO and HNCO at lower temperature, but decreases the yields of the Ncontaining species at higher temperature during biomass pyrolysis [135]. The effect of calcium on nitrogen conversion is distinctive. The included calcium in biomass inhibits N-conversion to HCN, NH3 and HNCO at lower temperature (oabout 330 1C), while favors the total yields of N-containing species [135] due to the promotion of the yields of N-containing species at higher temperature. In biomassfired boiler, calcium based additives have been widely used for desulfurization and dechlorination during combustion of biomass and waste [148,149]. The addition of CaCO3 has an obvious effect on suppressing the conversion of straw-N into HCN, NH3 and HNCO during straw pyrolysis [135]. The X-ray diffraction (XRD) analysis of K- and Ca-bearing chars reveals that the potassium reacts with the major minerals such as quartz in wheat straw and solid-phase reactions of the K and char-N take place [135]. Iron, aluminum and silicon have different effects on nitrogen conversion compared with potassium and calcium during biomass pyrolysis. The presence of iron, aluminum or silicon decreases conversion of straw-N into NH3 with the sequence of Fe4Si4Al [150]. The iron or silicon addition suppresses N-conversion into HCN and HNCO, and the aluminum addition has no notable influence on HCN emission during pyrolysis. The share of N-conversion to NH3 and HCN increases, but that to HNCO and NO decreases a little in the presence of added iron, aluminum or silicon.

4. Nitrogen behavior from N-containing model compound 4.1. Nitrogen functionality in biomass The speciation and release rate of nitrogen-containing species during pyrolysis conceivably are related to the functional forms of nitrogen in the fuel. In the past, much research regarding release of fuel-N from coal and biomass has been made. Coal has a more aromatic structure than biomass and contains a greater fraction of carbon, and smaller fractions of oxygen and hydrogen. Nitrogen atoms in coal are incorporated in functional groups such as pyridine (six-ring, N-6), pyrrole (five-ring, N-5) and quaternary nitrogen [151–157]. N-5 is the major nitrogen

functionality in coal and its content varies from 50% to 80% and decreases with the increase of coal rank. N-6 content increases with increasing coal rank and its content varies from 20% to 40%. Quaternary nitrogen is independent of coal rank and its content varies from 0 to 20% [158,159]. Many researchers have studied the coal-N conversion into HCN and NH3 from two aspects: coal itself and nitrogen functional groups, and tried to obtain the relationship between the nitrogen functional groups and coal. However, the chemical structure and the elemental composition of coal and biomass are different, so observations made on coal are therefore not necessarily valid for biomass. Composition and structure characteristics of both coal and biomass are complicated, using the nitrogen functional groups as model compounds can avoid the complex influence of pyrolysis products and help to determine nitrogen chemistry and conversion in the biomass. Nitrogen is one of the most important elements in biomass, studies [137,160–172] have shown that the occurrence form of nitrogen in the biomass is not aromatic heterocyclic structures, but mainly exist in the protein (Protein-N) and free amino acids (Amino acid-N), and a small amount of nitrogen in the nucleic acid, chlorophyll, enzymes, vitamins, alkaloids, etc. Protein–nitrogen usually accounts for 80–85% of total nitrogen in biomass. Protein is the basic biomass component and is biological macromolecule made up of amino acids. There is a viewpoint that apart from protein/amino acid, a small quantity of nitrogen in biomass is also found in various heterocycles. Morpholine, dimethylamine, ethylamine and pyrrolidine have been used as the model nitrogen containing biomass [173–177]. The fact that HCN formed from biomass was taken as evidence that nitrogen in biomass could not be in protein or amino acids, but rather in heterocyclic aromatic structures. However, alkylcyanides are formed as direct pyrolysis products from amino acids, while ammonia is formed from bimolecular reactions of primary pyrolysis products. Thus, proteins are the most suitable model compounds for biomass nitrogen [162]. The functionality of nitrogen in biomass and coal differs, and therefore, the release of volatile-N is most likely also different. Protein-N has always been seen as a good representation of biomass-N and further used as a starting point for N-mechanistic studies. As most nitrogen atoms in biomass are bound to proteins, insight into the mechanism of the fuel-N release may be gained by studying the pyrolysis of proteins. Protein consists of 20 kinds of amino acid (glycine, alanine, valine, leucine, isoleucine, methionine, Table 3 Amino acid composition of some biomass (mg g  1). Amino acid

Abbreviation

Wheat straw

Rice straw

Corn cob

Aspartic acid Glutamic acid Leucine Proline Histidine Glycine Threonine Arginine Alanine Tyrosine Cystine Valine Methionine Tryptophan Phenylalanine Isoleucine Lysine Serine

Asp Glu Leu Pro His Gly Thr Arg Ala Tyr Cys Val Met Try Phe Ile Lys Ser

2.118 2.852 1.393 0.973 0.238 1.235 0.745 0.654 1.045 0.457 0.032 0.824 0.163 0.001 0.762 0.599 0.757 1.074

3.314 3.886 2.371 1.205 0.435 1.862 1.153 1.098 1.738 0.741 0.035 1.591 0.297 0.001 1.255 1.109 1.191 1.566

2.049 2.344 1.282 1.607 0.255 1.333 0.738 0.545 1.077 0.418 0.014 0.901 0.127 0.001 0.666 0.601 0.578 0.996

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Fig. 1. The secondary cracking of 2,5-diketopiperazine (DKP) [187].

proline, phenylalanine, tryptophan, asparagine, glutamine, serine, threonine, tyrosine, cysteine, aspartic acid, glutamic acid, lysine, arginine and histidine). The amino acid compositions of protein are different with different features (such as thermal decomposition characteristics, functional structure, etc.). Protein is often used as the model compounds of biomass nitrogen, but as a result of protein composition is very complicated, the protein composition of different biomass is totally different [178], it seems to be difficult to study the characteristics of nitrogen conversion through investigating nitrogen chemistry of protein pyrolysis [132,160,179]. High performance liquid chromatography (HPLC) with Agilent1100 was used to quantitatively determine the composition of amino acids in biomass. Table 3 shows the amino acid composition of some biomass.

The DKP ring will then (secondary reaction) open/break, producing a variety of compounds. Fig. 1 shows the several possible cracking pathways of the DKP decomposition and the relative reaction products under different reaction conditions. In the figure, the number 1, 10 , 2 and 3 represents the different fracture locations of the DKP structure. The cleavage at different locations in the DKP cyclic structure usually contributes to different secondary products. As can be seen from Fig. 1, the secondary reactions which may happen are numerous and pathways are versatile. The main secondary reaction is the crack of cyclic dipeptide which also presents competing pathways. The main products of the secondary reaction are nitriles, imines, pyrroline, hydantoins and α-lactam. Meanwhile, the secondary cracking of amine and imine can form NH3 and nitrogen alkyl diamine [162,181]. The secondary cracking reaction of char-N generates hydantoin, then generates HNCO and HCN [184,185,187]. The formation of HNCO from biomass has been suggested to originate from 2,5diketopiperazine (DKP) and other cyclic amides [161,187–190].

4.2. General characteristics of protein/amino acid pyrolysis NH3 and HCN are not directly formed from the primary reaction of protein/amino acid in the pyrolysis process, but through the secondary reaction of protein/amino acid [165,180,181]. The pyrolysis reaction of protein/amino acid is very complex and many reactions take place, and some lead to volatile release, while other lead to char formation. Generally, the primary pathways of decomposition of protein/ amino acid include dehydration, decarboxylation and deamination reactions. The product of dehydration reaction is cyclic amides, and the main cyclic amide is 2,5-diketopiperazine, referred to as DKP [131,162,166,180,182–186]. An important reaction, leading to volatile release, is the formation of DKP. The product of decarboxylation reaction is amine [166,181]. Compared with the above reactions, deamination reaction is weak [166,181–183]. Moreover, the intermediates of α-lactam [163,181] are formed from the primary reaction. Another important reaction, leading to char formation, is the cross-linking between side groups. Proteins that have reactive side chains can be expected to cross-bind and form char-nitrogen and NH3 [162,187]. The intensity of the reaction depends on the types of protein/amino acids, because the stability of intermediates formed from different amino acids during pyrolysis is dependent.

4.3. Nitrogen from the N-containing model compound Table 4 shows nitrogen transformation from pyrolysis of some amino acids/proteins. Although all the amino acids underwent dehydration, decarboxylation, and deamination reactions, the relative significance of these reactions was different in each case, and the products from amino acids were different in each case due to the distinct structures. The thermal stability is dependent on the amino acid structure, and due to the different structures of amino acids. The behavior of nitrogen conversion is distinctive for different amino acids [191–196]. For some amino acids, HCN is the main N-containing species and its yield is higher, and for some amino acids, NH3 is the main N-containing species. The decomposition of different amino acids is different, and some amino acids involve less secondary reaction. That is caused by their different thermal stability and decomposition temperature. The product distributions appear to depend on the relative volatility and stability of the amino acids and their pyrolysis products. Temperature had a great influence on the conversion of nitrogen into HCN/NH3, HCN and HNCO are mainly formed from the secondary reaction of DKP [161,164].

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Table 4 Nitrogen transformation from pyrolysis of some amino acids/proteins. Model compound

Reactor

Tfinal (oC)

Analysis technique

Comment

Reference

DKP Poly-leucine Polyamide Val, leu Asp Gly Phe/tyr/leu/gly Asp Glu Phe Asp Glu Leu Pro Gly Phe

Fluidized bed Fluidized bed Tubular reactor Tubular reactor Quartz tube Tubular reactor TGA TGA TGA TGA Tubular reactor Tubular reactor Tubular reactor Tubular reactor Tubular reactor Tubular reactor

900–1100 700–800 800–1000 500 700–870 700–1000 800 800 800 800 800 800 800 800 800 800

FT-IR FT-IR FT-IR GC-FTIR-MS GC–MS MS FTIR FT-IR FT-IR FT-IR FT-IR FT-IR FT-IR FT-IR FT-IR FT-IR

DKP forms HCN, HNCO and NH3 HCN, NH3 and HNCO are formed. HCN is the dominant gas. HCN increases with temperature and residence time HNCO was found NH3 is dominant HCN is dominant NH3 is dominant. HNCO is observed HCN and NH3 HCN is dominant HCN and NH3 HCN is dominant HCN is dominant NH3 is dominant HCN is dominant NH3 is dominant NH3 is dominant

[161] [162] [164] [180] [182] [183] [188,189] [193] [193] [193] [195] [195] [195] [195] [195] [195]

Whey protein isolate, soya beans, yellow peas and shea nut meal were also used as the model compounds [131]. The split between HCN and NH3 depends on the compound and temperature. It was found that the HCN/NH3 ratio is very sensitive to temperature and increases with increasing temperature for all compounds. The temperature dependence implies that heating rate and thereby particle size affect the split between HCN and NH3. For whey, soya beans, and yellow peas, HNCO was also quantified. It is suggested that most HCN and HNCO are produced from cracking of cyclic amides formed as primary pyrolysis products. The dependence of the HNCO/HCN ratio on the compound is fairly small, but the temperature dependence of the ratio is substantial, decreasing with increasing temperature. NH3 yields were almost independent of temperature. HCN yield, on the other hand, showed strong temperature dependence and increased with temperature for both of the cyclic amides. HNCO yield decreased with increasing temperature for DKP over the whole temperature interval [161]. The results obtained from model compounds should be looked upon carefully. Even if it is true that nitrogen is found in protein/ amino acids and N-aromatic structures, it is also true that Nstructures are only a part of the biomass matrix (cellulose, hemicellulose and lignin) and therefore N-compounds will not only be in the vicinity but also linked in a variety of ways to many different functional groups (containing N or not), which may significantly influence N-chemistry. This brings limitation to the validity of the results obtained through model compounds studies and their direct transfer to biomass. Model compounds cannot portray fully biomass and its intricate makeup. It calls for the study that the dependence on N-functionalities is critical to biomass-N conversion.

5. The dependence of N-containing model compound In biomass none of the major constituents contains nitrogen. Instead, most nitrogen atoms are bound to proteins, which are a minor constituent of most fuels. Consequently, the fuel-N release is connected to the decomposition of the proteins/amino acids, which in turn may be connected to the decomposition of the major constituents. This part reviews the dependence on Nfunctionalities and the relationship between nitrogen behavior from the model compounds and biomass. 5.1. Effect of atmosphere This part reviews nitrogen behavior from amino acid pyrolysis in the presence of oxygen and CO2 in the atmosphere. Although

the structure of each amino acid is distinctive, the effect of O2 in the atmosphere on the nitrogen transformation of amino acid has something in common [136]. The introduction of O2 promoted the decomposition reaction of the amino acid, which contributes to HCN and HNCO formation. The presence of CO2 in the atmosphere inhibited the conversion of HCN and HNCO from amino acids, especially HNCO is suppressed. The presence of O2 and CO2 changes the selectivity of nitrogen conversion pathways. The presence of O2 increases the HNCO/HCN molar ratio and CO2 inhibits the HNCO/HCN molar ratio. Because of the different properties of O2 and CO2, the secondary reaction pathway and selectivity of the cracking of DKP are influenced and changed. Although the influence of oxygen and CO2 in the atmosphere on nitrogen behavior is different for the amino acids, it is interesting to find some phenomena in common. The presence of oxygen promotes NO and HNCO formation for the amino acids, HCN and HNCO formations are suppressed by introduced CO2 for the amino acids. It helps to understand the N-conversion mechanism from biomass under the same conditions. The presence of oxygen promotes NO and HNCO formation for amino acids, the reason is random bond cleavage occurs readily, the secondary decomposition of DKP becomes intense, and the existence of oxygen enhanced the catalytic thermal cracking of the N-containing char. It is likely that DKP formation is restrained in the presence of CO2. Therefore HCN and HNCO formation are suppressed by introduced CO2 for all the three amino acids. It is in good agreement with the results found in biomass pyrolysis under the same conditions. It is confirmed that the nitrogen behavior for biomass under different atmosphere is due to the amino acid behavior under different atmosphere. It helps to reveal the Nconversion mechanism from biomass during thermal utilization based on a comprehensive consideration of nitrogen behavior from amino acids. 5.2. Effect of mineral matter The yields of HCN and NH3 and nitrogen conversion pathway from amino acid pyrolysis are influenced by the mineral matter [195]. Mineral matter has catalytic effects on the primary decomposition reaction pathway of amino acid as well as the secondary cracking pathway of DKP. For amino acids with reactive side chains, the mineral matter also has catalytic effects on thermal cracking of nitrogen-containing char. The mechanism of the effect of mineral matters on N-conversion from amino acids is likely that mineral matter changes the pathway of the primary decomposition reaction of amino acids and then the secondary cracking

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pathway of DKP is changed. Meanwhile, for the amino acid with reactive side chains, the mineral matters have catalytic effects on thermal cracking of char derived from amino acid, and the nitrogen containing compounds in the char can be converted into HCN. Nitrogen behavior for biomass with mineral matters is due to the catalytic effects of mineral matters on amino acid. It should be noted that synthesis interaction among amino acids, mineral matters and carbohydrates (cellulose, hemicelluloses and lignin) must be considered. 5.3. Effect of cellulose, hemicelluloses and lignin In fact, cellulose, hemicellulose and lignin are the main components and are linked with each other in biomass. Co-pyrolysis of amino acid and cellulose, hemicellulose and lignin helps to understand the role that the three components play part in. The mixtures of amino acid and the carbohydrates undergo solid-state decomposition reactions during co-pyrolysis. HCN and NH3 yields and nitrogen conversion pathway from amino acid pyrolysis are influenced by cellulose, hemicellulose and lignin [194,196]. It is interesting to find some characteristics in common for the role of the components in nitrogen conversion from the aliphatic amino acid and the heterocyclic amino acid. The effects of hemicellulose on NH3 formation from the amino acids are similar, and lignin promotes NH3 formation. This can help to understand why some kinds of biomass produce more NH3 than HCN, and in some cases more HCN than NH3 is produced during pyrolysis. The reactions among cellulose, hemicellulose and lignin have some effects on nitrogen conversion from biomass pyrolysis. The main components in different kinds of straw have not identical effects on N-conversion. Generally, lignin in wheat straw has little effect on N-conversion, while it has a notable effect on nitrogen transfer during rice straw pyrolysis. Cellulose has the biggest effect on nitrogen behavior for wheat straw, and hemicellulose has the biggest effects for rice straw. The reaction between glycine and glucose/fructose shows that nitrogen distribution has been changed in the presence of glucose/ fructose. Glycine might be more likely to react with reducing sugars to form a large number of N-heterocyclic compounds rather than polymerize to form DKP. The decrease of the HCN yield from copyrolysis of glycine and reducing sugars is related to the reduction of DKP and the increase of N-heterocyclic compounds. The contribution of reducing sugar to HCN formation might be more considerable with the increasing content of reducing sugar [197].

6. Future research The ultimate purpose is to predict and further prevent NOx. The partition of biomass-N between volatile, char and tar should be investigated further. The intermediates should be studied and verified during the interaction among the amino acids with the components in biomass, which is critical for the predicting and preventing of NOx and N2O formation. The selectivity of the secondary cracking of the main product peptides from amino acid should be emphasized. Meantime, co-firing biomass with coal has been widely developed, the behavior of nitrogen during co-pyrolysis/combustion should be focused.

7. Concluding remarks Because of the different nitrogen functionality in coal and biomass, nitrogen chemistry for biomass is different from that for coal during pyrolysis. Nitrogen is mainly in the forms of protein/ amino acid in biomass. Different kinds of biomass have distinctive

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selectivity of N-conversion into NH3, HCN and HNCO. Heating rate, particle size, atmosphere and mineral matter have effects on fuel-N conversion during biomass pyrolysis. The distinctive structures of amino acids contribute to different N-containing species formation and N-conversion pathway during pyrolysis. The selectivity of N-conversion from co-pyrolysis of model compounds separately with cellulose, hemicellulose and lignin displays distinctive characteristics. The reactions among amino acids with the components in biomass have great effects on nitrogen conversion from biomass pyrolysis. Cellulose, hemicellulose and lignin influenced the selectivity of N-conversion into HCN and NH3 during amino acid pyrolysis, this is the reason why some kinds of biomass produce more NH3 than HCN, and in some cases more HCN than NH3 is produced during pyrolysis. It is meaningful to predict biomass-N transfer behavior based on a comprehensive consideration of the interaction between the amino acids with the main components in biomass (cellulose, hemicellulose, lignin and mineral matters).

Acknowledgement This work was funded by National Natural Science Foundation of China (nos. 51476169 and 51106157).

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