TG-FTIR characterization of coal and biomass single fuels and blends under slow heating rate conditions: Partitioning of the fuel-bound nitrogen

a e f Tec Coal Biomass Thermogravimetry FTIR analysis Fuel-bound nitrogen partitioning r o wa co-pyrolysis was studied using TG-FTIR characterization with the focus on the Fuel Processing Technology 91 (2010) 103–115 Contents lists available at ScienceDirect Fuel Processing j ourna l homepage: www.e ls has become more attractive in recent years as a consequence of concerns for the climate (global warming) and increased fossil fuel prices against facing decreased easy availability. Co-firing of these novel fuels is already used to mitigate CO2 emissions in an econo- mically attractiveway. For an overview of the technology see e.g. [1,2]. This use of suchwaste biomaterials is considered as an acceptable way to dispose them while they are thus even exploited as energy source. MBM was banned by the EU as cattle feed since 1994 to prevent the spread of BSE (bovine spongiform encephalopathy). gasification for IGCC or syngas production purpose can benefit from this. An improved understanding is particularly needed for the formation of NOx precursors. NOx emission is one of the major concerns for co-firing technology, as the co-fired fuels often contain substantial amounts of fuel-bound nitrogen. The release of the related nitrogen species during the initial particle pyrolysis stage has not been completely elucidated yet. Especially the interaction between old fuels and diverse younger co-fired materials has got less research attention in this respect. A number of researchers have studied the co- ⁎ Corresponding author. E-mail address: [email protected] (W. de Jo 1 Present address: Shell Global Solutions International Waste Services, Badhuisweg 3, NL-1031 CM Amsterdam 2 Present address: TU Munich, Lehrstuhl für therm Garching, Germany. 0378-3820/$ – see front matter © 2009 Elsevier B.V. Al doi:10.1016/j.fuproc.2009.09.001 ap secondary fuels, like M), in power generation reduce harmful emissions, more detailed insight is needed into combustion (sub)processes, especially in the initial pyrolysis stage of particle conversion. Also more advanced applications like co- The application of biomass and other che poultry waste and meat and bone meal (MB 1. Introduction between 300 and 380 °C. Woody and agricultural biomass materials show higher devolatilization rates than animal waste. When comparing different fuels, the percentage of fuel-bound nitrogen converted to volatile bound-N species (NH3, HCN, HNCO) does not correlate with the initial fuel-N content. Biomass pyrolysis resulted in higher volatile-N yields than coal, which potentially indicates that NOx control during co-firing might be favored. No significant interactions occurred during the pyrolysis of coal/biomass blends at conditions typical of TG analysis (slow heating rate). Evolved gas analysis of volatile species confirmed the absence of mutual interactions during woody biomass co-pyrolysis. However, non-additive behavior of selected gas species was found during slaughter and poultry litter co-pyrolysis. Higher CH4 yields between 450 and 750 °C and higher ammonia and CO yields between 550 and 900 °C were measured. Such a result is likely to be attributed to catalytic effects of alkali and alkaline earth metals present in high quantity in animal waste ash. The fact that the co-pyrolysis of woody and agricultural biomass is well modeled by simple addition of the individual behavior of its components permits to predict the mixture's behavior based on experimental data available for single fuels. On the other hand, animal waste co-pyrolysis presented in some cases synergistic effects in gas products although additive behavior occurred for the solid phase. © 2009 Elsevier B.V. All rights reserved. In order to further improve the co-firing process with the aim to pyrolysis using a coal/biomass fue results obtained reporting signifi view of the exist The aim of th the pyrolysis be coal–biomass m ng). B.V., Energy, Utility, Water and , The Netherlands. ische Kraftanlagen, D 85748 l rights reserved. similar pyrolysis behavior, with a maximum weight loss Keywords: release patterns and quantitative analysis of the gaseous bound nitrogen species. It was shown that all investigated biomass fuels present more or less Received in revised form 31 August 2009 Accepted 4 September 2009 pyrolysis as well as during TG-FTIR characterization of coal and biom heating rate conditions: Partitioning of th G. Di Nola 1, W. de Jong ⁎, H. Spliethoff 2 Energy Technology Section, Process & Energy Department, Faculty 3me, Delft University o a b s t r a c ta r t i c l e i n f o Article history: Received 28 October 2008 The devolatilization behavio power sector for co-firing ss single fuels and blends under slow fuel-bound nitrogen hnology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands f a bituminous coal and different biomass fuels currently applied in the Dutch s experimentally investigated. The volatile composition during single fuel Technology ev ie r.com/ locate / fuproc wide range of pyrolysis conditions, reactor types and ls. Given such a wide range of variables involved, the by different groups are sometimes conflicting, some cant synergies, others additive behaviors. An over- ing literature is summarized in Table 1. e research described in this paper is to characterize havior of single biomass fuels and to study whether ixtures follow their parent fuel behavior during Table 1 Co-pyrolysis studies of coal/biomass blends. 1st author Exp.d HR/temp [°C/min//°C] Fuelsb,c Blending [w/w] Gas analysis Ref Rüdiger DTF –/800–1100 Coal-ss-st 25–50–75 GC [4] Pan TGA 100/900 Bl-lq-pc 20–80 None [5] Moliner Pyr n.r.a/900 lv-hsb-pr 30 GC–MS [6] Collot FB-FDB 10/850–1000 Dm-bw 50 None [7] Suelves Pyr n.r.a/900 lv-hsb-pr 30–40–60 GC–MS [8] Biagini TGA 20/900 hv-lv-sd-ss 15 to 60 None [9] Kastanaki TGA 10/850 Li-oc-fr-cr 5–10–20 None [10] Meesri HF 10–30–50/1000 Coal-sd 5–75 GC [11] Meesri DTF ~104/900–1400 Coal-sd 5–10 GC [11] Moghtaderi HF 10–30–50/1000 Coal-sd 5–75 GC [12] Moghtaderi DTF ~104/900–1400 Coal-sd 5–10 GC [12] Vamvuka TGA 10–100/850 Li-oc-fr-cr 5–10–20 none [13] Vuthaluru TGA 20/1250 Sub-ww-ws 10–50 none [14] Jones Py-GC-MS 1/600 Bit-hvb-li-sd 25–50–75 GC–MS [15] Jones TGA 25/900 Bit-hvb-li-sd 25–50–75 GC–MS [15] a Not reported. b Coals: bit, Bituminous; bl, Black; dm, DawMill; hsb, High vol. sub-bituminous; hvb, High vol. bituminous; hv, High volatile; li, Lignite; lq, Low quality; pr, Petroleum residue; sub, Sub-bituminous. c Biomass: cr, Cotton residue; fr, Forest residue; oc, Olive cake; pc, Pine chips; pr, Palm k d tor; 104 G. Di Nola et al. / Fuel Processing Technology 91 (2010) 103–115 decomposition at well controlled slow heating rates, or that syner- gistic effects take place. Especially, nitrogen species quantification and concerning fuels, coal and slaughter residues have been scarcely addressed in this respect. 2. Experimental 2.1. Materials Several biomass materials, ranging from woody and agricultural biomass to slaughter and poultry residues, and a coal blend have been selected for analysis of their devolatilization behavior in this work. They were supplied by Dutch power operators (E.On Benelux, Essent Energie), currently employing them on commercial projects in their power stations. Table 2 summarizes the standard fuel analyses. Ash analyses for both coal and biomass fuels are reported in Table 3. In a deposition directed characterization study, Tortosa et al. reported the particle size distribution for these fuels [3]. Coal/biomass blends pyrolysis experiments have subsequently been carried out at 10 °C/min. The procedure that was followed during the thermogravimetric experiments is similar to that used for Equipment: DTF, Drop tube furnace; FB, Fixed Bed reactor; FDB, Fluidized-Bed reac the single fuels. The influence of the biofuel share on the mixture was analyzed, from 5 to 20% share on thermal basis. Table 4 gives the Table 2 Proximate, ultimatea analyses (on wt. % dry basis) and lower heating values of the fuels. Coal CLb MBMb B-wood Corn residue Olive cake Palm kernel Moisturec 9.15 8.7 2.74 9.08 7.44 7.3 7.01 Ash 12.5 24.3 17.1 1.8 7.6 7.7 5.5 Volatiles 35.4 71.0 80.1 76.5 73.1 70.9 76.9 Fixed C 52.1 4.6 2.8 21.6 19.3 21.5 17.6 C 71.0 37.1 43.1 50.3 44.7 50.2 47.6 H 4.9 4.2 6.0 8.0 5.9 8.0 6.9 N 1.5 3.8 9.2 1.0 0.6 1.3 2.7 S 0.7 0.7 1.2 – 0.1 – – Od 9.5 29.4 22.5 39.9 41.1 33.7 38.1 Cl 0.001 0.5 0.8 – – – – LHVc,e 25.03 8.78 16.1 16.7 15.0 18.9 17.9 a Performed at ECN on a CE CHNO analyzer Flash EA 1112. b Biofuels were received in dried and sterilized form; MBM=meat and bone meal; CL=Chicken Litter. c As received basis. d Calculated by difference. e Expressed in (MJ/kg). overview of the conversion from thermal to mass basis for the biomass fuels. Depending on the calorific value of the biomass fuel, values span between 6.5 and 47.8% on mass basis. 2.2. Apparatus A thermobalance (TGA) of the type SDT 2960 was used and coupled with an FTIR spectrometer in order to identify and quantify relevant gas species. A scheme of the TG-FTIR setup is shown elsewhere [16]. The coupling was realized via a heated transfer line, kept isothermal at 150 °C. Heating rates of 10, 30, and 100 °C/min were chosen in the current work, which facilitates future kinetics parameter derivation which is out of the scope of this paper. FTIR gas analysis was carried out using a resolution of 0.25 cm−1, 12 scans per sampling, for a sampling interval of 33 s. Details on the FTIR calibration have also been described in [16]. 2.3. Methods A quantity of 5±0.5 mg of sample was used for every experiment. The balance was continuously purged with 100 ml/min of Helium ernel; sd, Saw dust; ss, Sewage sludge; st, Straw; ws, Wheat straw; ww, Waste wood. HF, Horizontal furnace; Pyr, Pyrolyzer; TGA, Thermogravimetric analyser. (chosen for its excellent heat transfer properties to reduce temper- ature inhomogeneities in the reaction zone [17]) to sweep the pyrolysis gases and prevent secondary reactions and tar redeposition Table 3 Ash analysis of the biomass fuels (in wt. %, dry basis). Coal CL MBM B-wood Corn Olive Palm SiO2 53.49 4.5 on the sample. An isothermal step of 45 min at an ambient temperature was applied to remove the remaining air from the oven. The sample was heated at constant heating rate conditions to 130 °C to evaporate the moisture. After an isothermal step of 10 min, the heating ramp continued to the final temperature of 900 °C. A 20 min isothermal step assured a complete pyrolysis, thus the volatile content was recorded. Purge gas was then switched to air for 20 min to burn the residual char andmeasure in this way the fixed carbon and ash content of the sample. During the entire experiment, pyrolysis gases were swept out of the oven by the Helium flow and transported to the spectrometer gas cell. A series of IR spectra was acquired. Experiments were carried out at least twice to ensure repeatability, more times in case some variability was observed. Dimensional analysis performed in an earlier work on the same equipment by Heikkinen [18] addressed possible limitations in heat and mass transfer during experiments similar to those carried out in this work with the thermobalance. Biot number (Bi) and external pyrolysis number (Py') were calculated, as described by Pyle and Zaror [19]. It was found that, for heating rates at least up to 20 °C/min, the pyrolysis rate was only limited by the reaction kinetics. 3. Results and discussion 3.1. Weight loss and derivative weight loss Woody materials and agricultural biomass weight loss curves are reported in Fig. 1(b). Woody biomass pyrolysis clearly highlighted the thermal degradation of their main constituents (cellulose, hemicel- lulose, and lignin, whose weight loss curves are reported as reference in Fig. 1(a)). Their decomposition can be divided into three stages. In the first (T 106 G. Di Nola et al. / Fuel Processing Technology 91 (2010) 103–115 The agricultural residues present similar decomposition behavior. However, peaks and shoulders are to a certain extent shifted due to mutual differences in the cellulose, hemicellulose and lignin relative content. For all the woody/agricultural fuels, the volatiles represent the major pyrolysis product with yields >80% (w/w). Moreover, pyrolysis rates are similar, with the only exception of olive cake yielding lower volatiles at an overall lower rate. Fig. 1(c) shows the slow heating rate pyrolysis of meat and bone meal (MBM) and chicken litter (CL). Both materials decompose between 180 °C and 450 °C. MBM shows highest devolatilization rate at 313 °C and a shoulder at 360 °C. Chicken litter presents multiple decomposition peaks (295, 321, 413 °C), due to its heterogeneous nature. A further peak at high temperatures (≥650 °C) is associated with the presence of carbonate. Maximum pyrolysis rates of both materials are in the same range (0.5%wt/°C) and are lower than those of woody biomass fuels. Weight loss data presented here are in good agreement with previous results from Annamalai et al. [24] for chicken litter and Conesa et al. [25] and Senneca [26] for MBM. Thermogravimetric analysis of the high volatile bituminous coal blend is reported in Fig. 1(d). The decomposition peak occurs at 450 °C, then coal pyrolysis proceeds in the form of a tail as the complex polycyclic aromatic compounds are slowly decomposed until the final temperature of 900 °C. The global weight loss at the final temperature is 40.5% on a dry, ash free basis. 3.2. Influence of the heating rate The influence of pyrolysis parameters such as temperature, heating rate and residence time have been shown to determine the yield and composition of the derived products [27]. Besides the Fig. 2. Influence of the heating rate on the rate of weight loss for selected fuels; Onset grap experiments at 10 °C/min, already discussed in the previous section, also thermogravimetric runs at 30 and 100 °C/min were performed in order to investigate the influence of the heating rate. Relevant results are reported in Fig. 2 (from (a) to (d)). Table 5 summarizes the h represents weight loss curves (—×— 10 °C/min, —△— 30 °C/min, ⋯□⋯ 100 °C/min). Table 5 Temperatures of maximum pyrolysis rate and residue at final temperature as functions of the heating rate (10–100 °C/min). Fuel Heating rate [°C/min] Tpeak,1 [°C] Tpeak,2 [°C] Tpeak,3 [°C] Tpeak,4 [°C] Residue at 900 °C [wt.% (daf)] B-wood 10 359 11.30 30 376 11.36 100 399 12.00 Corn residue 10 331 9.26 30 350 11.75 100 372 10.52 Olive cake 10 309 11.58 30 326 13.03 100 348 17.64 Palm kernel 10 285 13.36 30 298 11.97 100 323 15.17 Chicken litter 10 295 321 413 649 7.18 30 313 – 449 695 4.81 100 334 – 459 712 2.97 MBM 10 319 5.58 30 337 5.15 100 355 3.47 Coal blend 10 451 64.67 30 469 64.26 100 459 62.37 CaCO3 10 – – – 655 temperatures of maximum pyrolysis rate and the char residual content at 900 °C. From all tested samples, a lateral shift can be noticed in the thermograms towards higher temperatures. This is explained with the occurrence of an increased thermal lag as the heating rate increases [28,29]. The previously discussed heteroge- neous nature of chicken litter in particular is shown by its peculiar multiple pyrolysis peak behavior. As a reference, the CaCO3 decom- position peak is reported at the bottom of the table. In general, heating rate does not have a significant influence in these measurements, i.e. char and gas yields do not show major changes within the measured range. Corn residue, B-wood and olive cakeweight loss curves show little or no change. Coal presents slightly lower char and higher gas yields as the heating rate increases. This finding that the final char yield is independent on the heating rate within the range of heating rates used can be due to the fact that either cross linking reactions are not relevant within those pyrolysis conditions, or that themechanism of cleavage-bond breaking (leading to formation of gas products) and repolymerization and cross linking (leading to char formation) are in equilibrium. 3.3. FTIR analysis 3.3.1. Woody and agricultural biomass The fate of main gas products from the pyrolysis of B-wood is shown in Fig. 3, as a representation of the family of woody and agricultural biomass fuels. The nitrogenous species — NH3, HCN, and HNCO — released during their pyrolysis do in most cases not play a relevant role due to their low fuel-bound nitrogen content (0.6–1.3% on dry basis). The thermal lag in the gas products evolution with increasing heating rate is clearly visible. The main peaks of CO and CO2 release shift from 359→448→530–587 °C as the heating rate increases. Further, the peak resolution worsens as the influence of thermal lag increases. As a matter of fact, the shoulder on the left hand side of the main pyrolysis peak (decomposition of hemicellulose part of the material) gradually disappears as we proceed from 10 to 100 °C/min. Similarly to CO, CO2, the CH4 release presents a consistent dependence on the heating rate. Its relative concentration in the pyrolysis products is, however, lower by a factor six compared to the other species. The high yields of CO2 and CO measured during woody biomass pyrolysis are a consequence of the high oxygen content of these biomass fuels and underline the fact that oxygen-containing functional groups are abundant in their molecular structure. Likewise B-wood, also the pyrolysis gas evolution from the other fuels experiments are mainly characterized by a “shoulder-peak” release of CO2 and evolution of CH4 at higher temperature. Also for these fuels the resolution of the primary devolatilization peak worsens as the heating rate increases, as the shoulder on the left side of the peak gradually disappears. CO is less sensitive to variations of the heating rate compared to CO2 and CH4. 3.3.2. Slaughter and poultry residue Evolution of MBM and CL pyrolysis gas products is described in Figs. 4 and 5. Slaughter and poultry residue pyrolysis presents some similarities as well as differences in comparison with woody and agricultural biomass. As those, CO2 represents the main volatile product. For MBM, CO2 release comes as a major peak followed by two minor ones in a tailing section. As to chicken litter, a low temperature 107G. Di Nola et al. / Fuel Processing Technology 91 (2010) 103–115 Fig. 3. Gas products release during B-wood pyrolysis at three heat ing rates (—×— 10 °C/min, —△— 30 °C/min, ⋯⁎⋯ 100 °C/min). 108 G. Di Nola et al. / Fuel Processing Technology 91 (2010) 103–115 peak and a high temperature one of similar intensity (the latter due to CaCO3 degradation) are reported. CO is formed with much lower yields and CH4 shows similar decomposing behavior for both materials between 350 and 650 °C (at 10 °C/min) with comparable overall yields. Unlike woody and agricultural biomass, slaughter and poultry residues are characterized by high nitrogen content (3.8–9.2% on dry basis). Thus, the evolution of N-compounds plays a major role for these materials, which is neither trivial nor a-priori yet predictable. As shown in Fig. 4(b), the volatile part of theMBM fuel-bound nitrogen is released in a single step as NH3 during the main volatile weight loss, HCN and HNCO release being negligible. On the other hand, the volatile-N content of chicken litter occurs to be roughly equally divided between NH3–HCN–HNCO as presented in Fig. 5(b) and (c). Ammonia is apparently formed by a multi-step mechanism, as three different peaks can be clearly separated across the whole temperature range. This decomposition behavior is most likely to be attributed to the intrinsic nature of the poultry manure, constituted by both proteins and urea. HCN and HNCO are released as a single peak. The discussion on the partitioning of the fuel-bound nitrogen will be further expanded in Section 3.4. 3.3.3. Coal The results of the evolution of main volatile gas species from the pyrolysis of our high volatile bituminous coal are presented in Fig. 6. Coal pyrolysis starts around 350–400 °C. Release of CO2 occurs in the first stage due to the cracking of the bridge bonds with the lowest bonding energy, such as –O from ether bridges and –OH radicals (also Fig. 4. Gas products release during Meat and bone meal pyrolysis at three heating rates (—×— 10 °C/min, —△— 30 °C/min, ⋯⁎⋯ 100 °C/min). CH2 fragments might be released in this first stage, together with larger ring systems containing free radicals). Then, small aliphatic gas molecules, mainly CH4, are formed from the rapid recombination reactions among smaller radicals and their reaction with hydrogen. The release of tar could not be measured during our TG-FTIR mea- surements. With increasing temperature, hydrogenation and hydro- cracking of the aromatic clusters become significant, showing a relatively low but constant release of CO that competes with recom- bination and condensation in the charmatrix.With increasing heating rates, from 10 to 100 °C/min, a clear shift to higher temperatures occurs due to thermal lag, with main peaks shifting from ≈450 °C to 650–750 °C. In particular, CO and CH4 show higher dependence as the heating rate increases compared with the CO2 evolution pattern. As to the release of nitrogenous species, at the slow heating rate conditions typical of thermal analysis, the high volatile bituminous coal releases its fuel-bound nitrogen mainly as NH3, in agreement with earlier findings [30-32]. HNCO releasewas considerably lower andwithin the noise level. 3.4. Partitioning of the fuel-bound nitrogen during pyrolysis An overview of the yields of nitrogenous species obtained at the three different heating rates of the TGA experiments is reported in Table 6. A higher percentage of the fuel-bound nitrogen is converted into volatiles (NH3, HCN, HNCO) for biomass pyrolysis compared with coal pyrolysis. However, this amount does not correlate with the initial fuel-N content, as shown in Fig. 7, nor the O/N ratio, as has been suggested in literature by Aho et al. [33]. Unfortunately, no infor- mation is available on both the tar-N, char-N and molecular nitrogen formed in pyrolysis, since they could not be quantified during our experiments. Increasing the heating rate from 10 to 100 °C/min, yields a reduction of the volatile gaseous nitrogen, although this trend is not always consistent. This observation can be explained as follows: increasing the heating rate in the TGA brings along heat and mass transfer limitations, thus the volatiles may not have enough time to be released under those conditions. Results of nitrogen partitioning pre- sented in Table 6 show that NH3 is the main N-product measured in the evolved volatiles. This finding agrees with earlier results of Leppälahti et al. [34] who found that different coal and biomass fuels formed more NH3 than HCN at slow heating rate. The mechanism by which the nitrogen in coal and biomass is converted into the NOx precursors during pyrolysis is still imperfectly understood at present, and several issues are disputed in the scientific community. This is why our study addresses a broad range of fuels, showing quantified release of in particular the light nitrogen gaseous species. It is generally agreed that the yield of HCN increases with increasing temperature [35-42]. However, the trend in the yield of NH3 is still not very clear. While Kambara et al. [41] showed that the yield of NH3 increased monotonically with increasing temperature in a pyroprobe, the data from a fluidized-bed reactor often showed maxima in the NH3 yield at around 800 °C [37-39]. The results seem to depend strongly on the configurations of the reactors used. Even for the same set of coals, in fact, entirely different trends have been observed for the formation of HCN and NH3 when the coals were pyrolyzed in different types of reactors. When a suite of rank-ordered American coals was pyrolyzed in an entrained-flow reactor, very little NH3 was detected in the product gas [43]. However, when a similar suite of rank-ordered coals was pyrolyzed in a TGA, NH3 was found to be one of the most important nitrogen-containing products from the nitrogen in coals [30]. It should, however, be pointed out that the yields of NH3 are often complicated by possible non-desired reactions, such that NH3 can be lost to N2 through interaction with materials as stainless steel and quartz [44]. Even for the pyrolysis of simple model compounds such as pyrrole and pyridine, no agreement has been reached in the literature. For example, while no NH3 was detected when pyrrole and pyridine were pyrolyzed in a shock-tube reactor 109G. Di Nola et al. / Fuel Processing Technology 91 (2010) 103–115 [45], NH3 was found to be an important product when the same compounds were pyrolyzed in an entrained-flow reactor [46]. It is also believed that, if the experimental procedures and the reactor configuration allow for holding the nascent char particles at high temperature for long time as in the case of thermogravimetric analysis, significant amounts of NH3 may be detected [47-50]. In particular, NH3 from the pyrolysis of coals cannot bemainly attributed to the presence of amino groups, because these exist in coals in negligible concentrations [40,41,51,52]. Therefore, the direct and/or indirect hydrogenation of the nitrogen in the heteroaromatic structures is the main source of NH3 from the pyrolyzing solid particles. The indirect routes, either through the hydrogenation of HCN on the char surface [30] or through the hydrogenation of HCN in the gas phase [35], cannot account for the observed NH3 yields from the pyrolysis of a high rank coal, which is depleted in hydrogen. It is believed that the direct hydrogenation of the nitrogen in the pyrolyzing coal/char particles is the main route for the formation of NH3 and that the active hydrogen required for the hydrogenation of the nitrogen in heteroaromatic ring systems is generated from the thermal cracking reactions taking place inside the pyrolyzing solid coal/char particle [49]. Concluding, the partitioning of the fuel nitrogen during devolati- lization is very important and it influences the NOx/N2O formation in combustion. In combustion systems, it is highly desirable if the fuel-N is released with the volatiles rather than retained in the char. Volatile nitrogen can be easily reduced up to 80% by primary low-NOx strategies such as air partitioning and fuel reburning [53], whereas NOx reduction from the char-N is more difficult to achieve. Therefore, Fig. 5. Gas products release during Chicken litter pyrolysis at three h the fact that biomass pyrolysis results in higher vol-N yields, may mean that NOx control is easier for biomass combustion than for coal and that anyway, biomass co-firing has an intrinsic NOx reduction potential as technique. 3.5. DTG analysis of the coal–biomass blends Experimental results are compared with calculated curves based on a weighted sum of the parent fuels pyrolysis curves obtained before, according to Eq. (1). As weighting coefficient xbiomass, the actual measured share of the biomass fuel present in each experiment was utilized: dm dT � � mixture = xcoal⋅ dm dT � � coal + xbiomass⋅ dm dT � � biomass : ð1Þ When no interaction occurs between coal and biomass fuels, the release of volatiles (i.e. the char production) during co-pyrolysis is expected to increase linearly with the wt.% biomass share. This implies the absence of chemical interaction, or synergistic effects between the fuels composing the mixture. Fig. 8 indicates that additive behavior was reported in the volatile yields for all the coal/biomass blends studied. The fitting of all curves, not reported in the figure, is linear with an accuracy ≥98%. Therefore it can be stated that, under the experimental conditions used in these measurements (10 °C/min), for main products class formation (total volatiles, char), coal pyrolysis is not affected by the presence of eating rates (—×— 10 °C/min, —△— 30 °C/min, ⋯⁎⋯ 100 °C/min). 110 G. Di Nola et al. / Fuel Processing T oxygenated products released during biomass devolatilization. These results are in good agreementwith earlier findings on the co-pyrolysis behavior of coal/biomass fuels (mostly though woody materials) [5,7,9–15] reported in Table 1. Results of thermogravimetric experiments of mixtures of coal with corn residue and chicken litter are shown in Figs. 9 and 10, respectively. Hereby, experimental results are presented directly next to modeled curves calculated using the weighted sum method according to Eq. (1). Two major observations can be drawn: on one hand the fuels behave in an additive manner as already depicted by the compact representation of Fig. 8. Furthermore, it is interesting to notice how the relative importance of the first peak (representing biomass devolatilization) increases at the expense of the second peak (coal pyrolysis) as the biomass share increases in the mixture. The third Fig. 6. Gas products release during Coal pyrolysis at three heatin Table 6 Partitioning of the fuel-bound nitrogen as a function of the heating rate (10–100 °C/ min). Fuel Heating rate [°C/min] NH3 HCN HNCO [wt.% (daf)] Coal 10 13.95 11.5 0.0 30 10.43 9.65 2.33 100 10.53 5.49 2.94 Chicken litter 10 21.75 22.42 17.35 30 23.12 15.42 10.03 100 15.03 11.29 4.71 Meat and bone meal 10 26.48 6.54 1.41 30 23.30 5.69 1.08 100 19.82 5.61 0.38 echnology 91 (2010) 103–115 peak present during co-pyrolysis of chicken litter is associated with the presence of CaCO3 within the heterogeneous structure of those biofuels. In these cases a sharp release of CO2 occurs at temperatures of 650–700 °C. In the above described TGA experiments, a low heating rate of 10 °C/min was used. The observed additive behavior of the parent fuels and the consequent lack of synergistic effects might be a consequence of such experimental condition. The decomposition of single components originating from biomass and coal occurs on different time scales (kinetics of thermal degradation processes is slower than heat/mass transfer), thus without mutual overlapping. As the heating rate increases, the intrinsic devolatilization becomes gradually slower compared with the heating up time of the sample and the devolatilization regions of different fuels might overlap. In such situation, biomass devolatilization could influence coal pyrolysis to a different extent, eventually causing non-additivity behavior. 3.6. FTIR analysis during co-pyrolysis Similar to what was described for the volatile/char yield, also the gas composition of the volatiles evolved during co-pyrolysis of biomass fuels was analyzed during TGA measurements at 10 °C/min. The weighted sum method was applied singularly for each molecular species quantified in order to studywhether the coal/biomassmixture followed additively the parent fuels and eventually if any selective behavior for specific gas species could be highlighted. Applied to each molecular species, the weighted sum gives: ½X�i;mixture = xcoal⋅½X�i;coal + xbiomass⋅½X�i;biomass ð2Þ g rates (—×— 10 °C/min, —△— 30 °C/min, ⋯⁎⋯ 100 °C/min). where [X] represents the concentration of the generic molecular species and the subscript i simply indicates that the same equation can be similarly written for each molecular specie quantified during the spectrometer calibration. The mixtures consist of 5, 10 and 20% biomass share on thermal basis of corn residue, chicken litter and MBM. Olive cake and palm kernel were mixed with 10 and 20%. Main pyrolysis gas products and nitrogenous species are shown in graphs plotted against the pyrolysis temperature. Fig. 7. Influence of the heating rate on the partitioning of the fuel-bound nitrogen in pyrolysis volatile species. 111G. Di Nola et al. / Fuel Processing Technology 91 (2010) 103–115 Fig. 8. Effect of the coal/biomass blending ratio on the evolution of volatiles and char during co-pyrolysis by TGA. 3.6.1. Woody and agricultural biomass co-pyrolysis Fig. 11 presents the evolution of CO during corn residue co- pyrolysis. The overall CO evolution can be divided into three different regions, with increasing temperature. The low temperature region is characterized by corn residue degradation in a shape of a sharp peak centered around 350 °C with total concentration proportional to the corn share in the mixture. In the intermediate temperature range (400–700 °C) coal decomposition dominates. Above 700 °C secondary CO evolution occurs, whichmainly originates from the corn fraction as can be seen by the single fuel FTIR analysis. The measured concentration increases to a lower extent compared to the calculated value. As to other main pyrolysis products, CO2 and CH4, their evolution can be calculated by those of parent fuel with a high degree Fig. 9. Influence of the biomass share on the co-pyrolysis behavior of coal/corn residue mixtures. 112 G. Di Nola et al. / Fuel Processing Technology 91 (2010) 103–115 of accuracy. Due to the low fuel-N bound content, the release of NH3 is practically not detectable, as well as for the other nitrogenous species. Palm kernel and olive cake co-pyrolysis presents additive behavior also in the gas products evolution, no synergistic effects could be observed. This finding is in line with the work of Meesri et al. [11] and Moghtaderi et al. [12] as well as other researchers [13–15] for woody biomass co-pyrolysis. However, olive residue and in particular palm derived material has been much less addressed than wood residues, in particular gas phase nitrogen species quantification has only very scarcely been addressed. Evolution of CO and CH4 from palm kernel co- pyrolysis is reported in Figs. 12 and 13. Nomajor differences are visible between10 and 20%biomass share. Slightly higher CO release in the low temperature region (palm kernel degradation) and lower concentra- Fig. 10. Influence of the biomass share on the co-pyrolysis behavior of coal/chicken litter mixtures. tions of CO andCH4 atT≥500 °C are due to a proportional lower amount of coal in the mixture. Results for the weight loss behavior during pyrolysis of olive cakeare similar and in agreementwith the results from Vamvuka et al. [13]. More details are reported in [16]. Overall, the fact that the devolatilization behavior of the mixture is sufficiently well predicted by simple summation of the individual behavior of its components has significant implications. It can in fact permit satisfactory prediction of a mixture's behavior based on the experimental data for each fuel. However, the observed additive behavior of the parent fuels might also be a consequence of the Fig. 11. Influence of the biomass share on CO evolution during co-pyrolysis of coal/corn residue at 10 °C/min. 113G. Di Nola et al. / Fuel Processing Technology 91 (2010) 103–115 specific experimental conditions. At such slow heating rates the decomposition of single components originating from biomass and coal occurs with different time scales, possibly resulting in lack of mutual overlapping. 3.6.2. Slaughter and poultry residue co-pyrolysis Measurements of gas products from co-pyrolysis of chicken litter and meat and bone meal resulted in overall reasonable agreement with weighted sum calculated curves. All measured gaseous species evolution could in fact be well estimated by their parent fuel thermal behavior. However, a few exceptions were reported. For a 10% thermal share of chicken litter, evolution of NH3, CO and CH4 was clearly deviating from calculated values. Fig. 14 shows their evolution. Ammonia evolution is characterized by a multiple peak release obtained by overlapping single fuel curves (see Fig. 5(b)). The low temperature peak is in these measurements negligible, due to the low relative presence of the biomass fuel in the mixture. For temperatures >550 °C, the actual measured NH3 yield is ~2.5 times higher than the calculated one. However, absolute concentration values are rather small. Similar trends occur for the other species reported in figure, with experimental yields of CO and CH4 higher than expected. Again NH3 evolution, this time originated by coal/MBMmixture, is plotted in Fig. 15 for an increasing share of MBM in the mixture. Results show that also in this case gas products evolution from co- pyrolysis of slaughter residue can be satisfactory predicted by analyzing the single fuels. Compared to coal and woody and agricultural wastes, slaughter and poultry residues are characterized by high ash content. Other Fig. 12. Influence of the biomass share on CO evolution during co-pyrolysis of coal/palm kernel. differences are also present, such as the organic elemental content distribution. However, in this discussion wewill merely focus on their ash content to pinpoint some possible interactions. Ash analysis was performed for all the fuels, results were summarized in Table 3. Coal ash is mainly composed of silicates and aluminium oxides, while biomass fuel ash contains large amounts of oxidised alkali and alkaline earth metals. Namely, chicken litter ash is characterized by high Ca content and significant LOI (Loss On Ignition). MBM ash contains a comparable Ca fraction (mainly from the bone part) next to a high phosphorus content. Presence of alkali metals is known to lead to various undesirable reactions in combustion furnaces and power boilers [54,55]. Bed agglomeration during fluidized-bed combustion [56,57], slagging and fouling on heat transfer surfaces in PF boilers [55,58,59] together with high temperature corrosion [60,61] are only some examples of negative consequences of the alkali chemistry in combustion. Catalytic effects of alkali species and phosphorus in the combustion flue gases with particular emphasis on their impact on the efficiency and lifespan of SCR catalyst were widely investigated by Beck et al. [62-64]. However, rather than on their impact in combustion applications, in this discussion we are focusing on their influence in pyrolysis processes. In particular, on the possibility that high amounts of Ca and P, as present in chicken litter and MBM ashes, could catalyze and enhance the release of some gaseous products. Alkali evolution during pyrolysis has been investigated in the last years [65,66], to study the influence of pyrolysis parameters (temperature, heating rate, and residence time) on the release of volatile alkali as well as to assess the importance of fuel washing techniques to reduce their formation. Ohtsuka et al. [31,67] reported Fig. 13. Influence of the biomass share on CH4 evolution during co-pyrolysis of coal/ palm kernel. 114 G. Di Nola et al. / Fuel Processing Technology 91 (2010) 103–115 that the presence of Na, K and Ca as hydroxides enhanced the formation of both N2 and ammonia from HCN and tar-N during coal pyrolysis for temperatures >400 °C. Moreover, Ca(OH)2 remarkably promoted CO formation in the region of 400–700 °C, which is in linewith our observations reported in Figs. 14 and 15(a). Ohtsuka [31] in relation to this observation proposed the following conversion route: tar � N þ H2O→NH3 þ CO þ H2 ð3Þ Therefore, it is concluded that the non-additive behavior of selected volatile species observed in few cases during the co-pyrolysis of slaughter and poultry litter might be caused by catalytic effects Fig. 14. Gas products evolution during co-pyrolysis of coal/Chicken litter (90/10%th). of alkali and alkaline earthmetals present in high quantities in the ash of these biofuels, contrary to the low-ash woody fuels. This needs further investigation in the future via model mineral compound blending. 4. Conclusions Devolatilization characteristics of woody/agricultural residues, slaughter/poultry residues and a high volatile bituminous coal were investigated by thermogravimetric analysis. Experiments were car- ried out at the final temperature of 900 °C and at three heating rates, between 10 and 100 °C/min. Volatile products have been analyzed by Fig. 15. Influence of the biomass share on ammonia evolution during co-pyrolysis of coal/MBM. [5] Y.G. Pan, E. Velo, L. Puigjaner, Fuel 75 (1996) 412–418. [6] R. Moliner, I. Suelves, M.J. Lazaro, Energy Fuels 12 (1998) 963–968. [7] A.G. Collot, Y. Zhuo, D.R. Dugwell, R. Kandiyoti, Fuel 78 (1999) 667–679. [8] I. Suelves, R. Moliner, M.J. Lazaro, J. Anal. Appl. Pyrol. 55 (2000) 29–41. [9] E. Biagini, F. Lippi, L. Petarca, L. Tognotti, Fuel 81 (2002) 1041–1050. 115G. Di Nola et al. / Fuel Processing Technology 91 (2010) 103–115 FTIR gas analysis, major pyrolysis species as well as nitrogen com- pounds were quantified. Thermal degradation of woody/agricultural biomass starts around 300 °C and presents slight differences in the peak position/height due to mutual differences in the relative content of the main constituents. However, in all cases most of the weight loss occurs between 400 and 600 °C and the main pyrolysis product is represented by volatiles, with yields ≥80% (w/w). As to slaughter/poultry pyrolysis behavior, their devolatilization occurs in a similar temperature region, with lower pyrolysis rates in the range of 0.5%wt/°C. Chicken litter is characterized by multiple decomposition peaks due to its heteroge- neous nature. The coal blend decomposes at higher temperature compared to biomass, with a peak around 450 °C and an overall sensibly lower pyrolysis rate. Increasing the heating rate from 10 to 100 °C/min caused the volatile release to shift towards higher temperatures due to increasing thermal lag. However, no significant influence is reported on the final gas/char yields within the measured range. The main volatile species evolved from woody/agricultural bio- mass pyrolysis are CO and CO2, the CH4 yield is sensibly lower. High CO yields are measured for T>700 °C, possibly attributed to secondary tar cracking reactions. The release of N-products is in general negligible, given the very low fuel-bound nitrogen content. Also slaughter and poultry litter pyrolysis behavior presents high CO2 release and increasing yields of CO for T>700 °C. However, their main characteristics lie in the nitrogen species evolution. NH3 is the prin- cipal N species evolved during MBM pyrolysis, whereas HCN–NH3– HNCO are formed during chicken litter degradation with different onset temperatures, indicating the presence of several N-functional- ities characterized by different binding energies. The percentage of fuel-bound nitrogen converted to volatile-N species (NH3, HCN, and HNCO) does not correlate with the initial fuel- N content. Biomass pyrolysis results in higher volatile yields (and high volatile-N yields) than coal, which potentially means that NOx control during biomass co-firing might be easier compared with coal combustion. Thermogravimetric analysis of coal/biomass blends shows that no significant interactions occur in the solid phase during pyrolysis. The shape of the curves and the position of the peaks remain basically unaltered. FTIR analysis of evolved volatile species during woody biomass co- pyrolysis confirms the absence of mutual interactions between the fuels. The volatile species behavior of the blend can be sufficiently well predicted by adding the individual contributions of the parent fuels. The non-additive behavior of selected volatile species (namely NH3, CO and CH4) reported in some cases during the co-pyrolysis of slaughter and poultry litter might be attributed to catalytic effects of alkali and alkaline earth metals which are present in high quantity in the ash of those biofuels. 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Introduction Experimental Materials Apparatus Methods Results and discussion Weight loss and derivative weight loss Influence of the heating rate FTIR analysis Woody and agricultural biomass Slaughter and poultry residue Coal Partitioning of the fuel-bound nitrogen during pyrolysis DTG analysis of the coal–biomass blends Outline placeholder FTIR analysis during co-pyrolysis Woody and agricultural biomass co-pyrolysis Slaughter and poultry residue co-pyrolysis Conclusions Acknowledgements References