Introduction

Biogas plants are alternative sources for renewable energy, biomass waste treatment and organic fertilizers (digestion waste, i.e. digestate). However, an accumulation of biogas plants in certain regions might lead to an oversupply of digestate1, causing many environmental concerns (odor control, transportation cost, pathogen, heavy metal (loid) contamination)2. Digestate pyrolysis can be an environmentally and energy recovery solution for digestate disposal3.Typically, this process is performed between 400 and 700 °C at near atmospheric pressure or below, in the absence of oxygen. Bio-oil produced from biomass pyrolysis has the potential to become an alternative renewable fuel and raw material for fine chemicals production generating thus bio-based high added value products4.

In general, pyrolysis oil contains hundreds of organic compounds including hydrocarbons and oxygenated compounds (i.e. organic acids, aldehydes, ketones and phenolics)5. The presents of these compounds cause bio-oil to have low heating value, low solubility in fuels such as diesel/gasoline, poor thermal and chemical stability and high acidity, high viscosity and high corrosiveness than petroleum6,7. However, pyrolysis oil can be converted to transportation fuel by catalytic treatment. The catalysts and conditions used are very similar to those used in petroleum hydrodesulfurization, hydrotreating, and hydrocracking processes, more generally described as hydroprocessing8. A promising upgrading technology is considered to be catalytic hydrotreatment of bio-oil, which involves treatment of pyrolysis oil with hydrogen in the presence of a heterogeneous catalyst leading to gasoline or diesel like products9.

Different hydrotreating catalysts have been investigated, such as Ni/SiO2, Ni/ZrO2, Ni/CeO2, Ni/Al2O3, Ni-Cu/SiO2, Ru/C, Pt/C, Pd/C, Pd/SiO2, Pt-Ni/SiO2, Pd–Ni/SiO2 and Cu/SBA-1510. Wildschut J. et al.,11 compared the results of typical hydrotreatment catalysts (sulfide NiMo/Al2O3 and CoMo/Al2O3) with heterogeneous noble-metal catalysts (Ru/C, Ru/TiO2, Ru/Al2O3, Pt/C, and Pd/C). The Ru/C catalyst was found to be superior to the classical hydrotreating catalysts with respect to oil yield (up to 60 wt %) and deoxygenation level (up to 90 wt %). However, due to the high cost and low availability of noble metals, their industrial applications are more difficult12. Moreover, efforts on increasing the economic performance of bio-oil hydrotreating technology lies on the application of non-noble metal catalysts, mild reaction condition (low temperature and hydrogen pressure) and high selectivity towards desired products. Nevertheless, the design of highly active and stable catalysts remains as the key challenge which plays a significant role in the implementation of biomass in the future biorefinery schemes13. In recent years, non-noble metal catalysts based on less-expensive metals such as Ni, Co and Mo have been reported to exhibit high activity and selectivity for decarboxylation/decarbonylation process14. Among them, NiMo and CoMo sulfides catalyst have been widely used in bio-oil hydrotreating. However, the main disadvantage of these catalysts is the requirement of the presence of sulfur in the processed feeds to maintain their activity and selectivity and, hence, end products is inevitably contaminated by sulfur13,15. Also, sulphided catalysts are rapidly deactivated by water9 and coke formation16. In order to solve these issues, non-sulfided NiMo or MoCo catalysts on various supports have been studied17. However, many challenges remain to improve the catalyst activity, stability, and selectivity at minimum amounts of hidrogen (i.e., low hidrogen pressures), and low temperatures, because high hydrogen pressure and temperatures, favor coke formation and thus leading to reactor fouling and product deterioration18,19. Significant amounts of coke could form during the hydrotreating at elevated temperature due to the insufficient active hydrogen supply, which cannot match the demand of the broken bonds. Furthermore, coke formation is a complicated process that is poorly studied20. Although the commercial CoMo and NiMo catalysts are commonly used at high temperatures of above 350 °C, they are sufficiently active at lower temperatures15,17,21. Nevertheless, the information regarding the application of non-noble catalysts such as CoMo at low temperatures and pressure is rather limited15. In this context, the objective of this paper is to study, the catalytic upgrading of digestat pyrolysis oil via mild hydrotreating process of over CoMo /γ-Al2O3-HMS. The influence of temperature and pressure on bio-oil conversion and deoxygenation efficiency was investigated. In addition, a chemical modelling algorithm was developed in order to evaluate the harmful emissions resulted from combustion of hydrotreating gaseous phase. This calculation algorithm can be applied to any pressurized or atmospheric combustion engine fueled with different mixture of combustible gases such as: natural gas, hydrogen-enriched natural gas and oxy-fuel.

Results and discussion

Catalyst characterization

The textural properties, such as specific surface area, pore size distribution and pore volume of the γ-Al2O3-HMS support and corresponding CoMo catalyst, were evaluated by nitrogen adsorption–desorption isotherms using Brunauer-Emmet-Teller (BET) method (Table 1). The γ-Al2O3-HMS support has a high specific surface area of 308.37 m2/g, a mean pore diameter of 5.49 nm and a total pore volume of 0.631 cm3/g. The addition of Co and Mo to γ-Al2O3-HMS support leads to a decrease in textural properties. The pore size and pore volume reflect a decrease of about 30% after impregnation indicating that impregnation may block the pores (Table 1).

Table 1 Textural properties of the support and of the catalyst.
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The N2 adsorption–desorption isotherm of the support and its catalyst is attributed to IV type isotherm (Figs. 1, 2) and the nitrogen isotherm reveals a hysteresis loop of H2 and H3-type respectively. The alteration of hysteresis loop shape for CoMo/γ-Al2O3-HMS catalyst also indicate that there is plugging of mesopores and there are changes in the support structure after impregnation.

Figure 1
figure 1

Pore size distribution and adsorption–desorption isotherm of the γ-Al2O3 –HMS support.

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Figure 2
figure 2

Pore size distribution and adsorption–desorption isotherm of the CoMo/γ-Al2O3 –HMS catalyst.

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Scanning Electron Microscopy (SEM) images of the synthesized CoMo/γ-Al2O3-HMS catalyst are shown in Fig. 3 with different magnifications (the left-side image scale bar is 100 µm and the right-side 5 µm) and show the homogeneity of the sample.

Figure 3
figure 3

SEM images of the catalyst CoMo/γ-Al2O3-HMS a100 µm and b 5 µm image scale bar.

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The Fourier transform infrared (FT-IR) spectrum of the catalyst (Fig. 4) shows broad bands around 3420 cm-1 and 1630 cm-1, which were assigned to stretching and bending modes of –OH group of water molecules on the surface of the solid22. The peaks in the region of 450–800 cm-1 were assigned to the oxide structures. The accurate assignment of the bands to specific compounds was difficult because the different compound bands overlap. The Si–O–Si specific bands (from HMS) are assigned at 1090 cm-1, 950 and 800 cm−123.

Figure 4
figure 4

IR spectrum of CoMo/γ-Al2O3-HMS catalyst.

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The acidic properties of CoMo/γ-Al2O3-HMS catalys were evaluated through the FT-IR analysis of pyridine adsorption (Fig. 5). The pyridine-adsorbed FT-IR spectrum showed features in the region of 1400–1600 cm−1 due to the stretching vibrations of M–N (metal–nitrogen) and N–H (pyridinium ion). The band assigned to pyridine adsorbed onto Lewis-type acid sites is recorded at 1445 cm−1, the band at 1543 cm−1 is due to pyridine adsorbed on Brönsted-type acid sites and the band around wavenumber 1488 cm−1 is due to physisorbed pyridine24,25. The calculated results are 97.41 μmol/cm for Lewis-type acid sites and 18.83 μmol/cm for Brönsted-type acid sites.The acid site density of 1.64 × 1018 sites/m2 and total acidity of 4.59 × 1020 sites/g of the catalyst were calculated according to A.I. Osman et al.26 (Figure S1).

Figure 5
figure 5

Pyridine-adsorbed FT-IR spectrum of CoMo/γ-Al2O3-HMS catalyst.

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The surface components and chemical states of metal species on the catalysts were studied by X-Ray Photoelectron Spectroscopy (XPS) method. Results obtained by this technique are shown in Fig. 7 and Table 2. XPS signals were assigned according to the literature and the official web page of National Institute of Standards and Technology (NIST)27. Binding energies in the ranges of 780–783 eV and 232 eV are ascribed to Co2p and Mo3d spectra and 154 eV to Si 2 s. The characteristic Mo 3d5/2 peak at 232.12 eV indicate the presence of Mo6+ which can be attributed to MoO328,29. Co is present as Co3+ and Co2+ species. The binding energy of Co3+, with the Co 2p3/2 profile, is 782.27 eV which could be ascribed to Co3O4 and CoOOH30,31 Co2+ may be present as CoO and CoAl2O432 with Co 2p3/2 peak at 780.68 eV. From the calcination step of the catalyst preparation it could also be formed Co and Mo aluminates or double oxides containing Co and Mo. According to Fig. 6 and Table 2, Si (from HMS mesoporous silica) occurs in the Si 2 s energy range of 154.23 eV, assigned to Si4+ (SiO2).

Table 2 XPS binding energies and assigned species for Co 2p, Mo 3d and Si 2 s.
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Figure 6
figure 6

XPS Co 2p, Mo 3d and Si 2 s spectra of CoMo/γ-Al2O3-HMS catalyst.

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Chemical composition of pyrolysis bio-oil

The chemical composition of pyrolysis bio-oil is shown in Table S1. The main compounds identified were unsaturated organic compounds, carbonyl compounds, furan derivatives, phenols, lower carboxylic acids and unsaturated fatty acids. These compounds have a different polarity and a wide range of boiling point. Compounds with high reactivity containing olefin groups conjugated with carbonyl groups (i.e. cyclopenten-1-one derivatives) were also been identified. These compounds can deactivate the catalyst at high temperatures due to the formation of oligomers.

The physical characteristics of the pyrolysis bio-oil (Table 3) show relatively high water content (over 25 w.t % ), density about 0.9956 g / mL due to the presence of water and short carboxylic acids (acetic, propionic and butyric ) in relatively high concentrations. The presence of short carboxylic acids and fatty acids justifies the high total acid number and saponification number of almost 135 and respectively, 224.12 mg KOH /g.

Table 3 The physical characteristics of the pyrolysis bio-oil.
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Catalysts activity evaluation

The catalytic activity of CoMo /γ-Al2O3-HMS for the bio-oil hydrotreating was evaluated in the temperature range of 250–320 °C, pressure between 20–40 bar, and constant LHSV of 3 h−1.

The effect of temperature over bio-oil yield is presented in Fig. 7. The results showed that the bio-oil conversion increases gradually with temperatures, from 62.86% to over 77.42% at maxim temperature of 320 °C. This behavior is due to the different reactivity of the various classes of oxygenated compounds present in the bio-oil. The phenolic as well as the carbonyl compounds are reactive in the hydrotreating process from lower temperature values of 250 °C. Meanwhile, the carboxylic compounds have a lower reactivity in the deoxygenation at temperature value of 250 °C, thus the hydrotreating process of these compounds begins at temperatures above 300 °C.

Figure 7
figure 7

The influence of temperature over bio-oil conversion.

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Figure 8 presents the yields of products composition of liquid organic phase resulted after bio-oil hydrotreating process. The yields of C6-C8 hydrocarbon and furan fractions have similar values at temperatures up to 275 °C, after which the yield of furans decreased slightly with temperature reaching about 10% at 320 °C, due to their lower reactivity in the hydrodeoxygenation process. The yield of fatty acid composition decreased with increasing temperature reaching the lowest value about 5% at 320 °C. The yield of C15-C18 hydrocarbon fraction increases with increasing temperature, perhaps due to the hydro-decarboxylation reaction of the fatty acids from the waste vegetable oil fraction. As expected, the total acid number of hydrotreated bio-oil decreases with temperature with almost 89%, from 135.9 to almost 15 mg KOH/g.

Figure 8
figure 8

The influence of temperature over the yields in products composition of liquid organic phase.

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Minimizing the consumption of hydrogen is an important issue for hydrotreating process of bio-oil due to the possibility of obtaining ring saturated products via unwanted hydrogenation process under high hydrogen pressure. These compounds decrease the octane number of the upgraded product which could hinder its direct utilization as fuel or blending with conventional oil or in the forthcoming bio-refineries33. The latest results of Mo-based catalysts show great potential for converting lignin-derived bio-oil into aromatic hydrocarbons at low hydrogen pressure13. In this regard, we study the influence of pressure over bio-oil conversion during hydrotrating process in the range of 20–40 bar at constant temperature of 320 °C. As can be observed in Fig. 9, the conversion of bio-oil increases with pressure, the increase being more pronounced in the range of 30–40 bar. It is observed that low pressures (20–30 bar) favors preferentially the hydrodeoxygenation reaction of carbonyl and phenolic compounds34,35 while higher pressures hydrodeoxygenate the carboxylic compounds21,36. Therefore, at 20 bar the bio-oil conversion was of 77.42 and increased to 87.23% at 40 bar.

Figure 9
figure 9

The influence of temperature over bio-oil conversion.

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Regarding the yields of products composition, high pressure favors the decarboxylation of the lipids and fatty acids, so that the yield of the C15-C18 hydrocarbon fraction increases from 41.64 at 20 bar to maximum value of 50.79 at 40 bar. In contrast, the yield of C6-C8 hydrocarbon fraction, resulting mainly from the hydrotreating of phenols and carbonyl compounds, is not significantly influenced by the pressure variation, the growth being insignificantly higher. The yield of furans does not change practically on the studied pressure range, the efficiency of the catalyst studied in the hydrotreating process of furans being relatively low at these pressure values (Fig. 10). The total acid number of the hydrotreated bio-oil decreases with the increase of the pressure, behavior explained by the increase activity of the Co-Mo catalyst, at pressure over 30 bar, for hydro-decarboxylation process.

Figure 10
figure 10

The influence of temperature over the yields in products composition of liquid organic phase.

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Evaluation of main noxious emissions of bio-oil hydrotreating gaseous phase

After each experiment, the gas-phase composition was analyzed, and the most representative data are given in Table 4. The main component of gaseous phase is unreacted hydrogen, followed by CO and C1-C4 hydrocarbons in different percent depending on the experimental parameters used.

Table 4 The mole composition in the flow section “0”.
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Unlike natural gas which has been intensively studied in the combustion process, synthesis gas fuel mixtures have not been widely investigated. Therefore, an evaluation of main noxious emissions was performed through a pure chemical modeling algorithm developed. This chemical modeling algorithm allows to chemically analyze the constant pressure combustion for any gaseous mixture fuels. The chemical model chose the minimum and necessary chemical equations in order to quantify the air based combustion noxious.

The evaluation of main noxious emissions was performed through a pure chemical modeling algorithm developed by authors at necessity. This chemical modeling algorithm allows to chemically analyze the constant pressure combustion for any gaseous mixture fuels. The chemical model chose the minimum and necessary chemical equations in order to quantify the air based combustion noxious. The main combustion noxious emissions, i.e. CO2, CO and NOx, for the gaseous mixture fuel were evaluated through a combustion chemistry model involving the energy and mass balance equations applied to three types of chemical reactions:

  • primary oxidation of inlet chemical species of gaseous fuel,

  • secondary dissociation chemical reactions and,

  • tertiary recombination chemical reactions.

The combustion was thought to be made in two successive fictitious steps complying with energy and mass balance laws, see Fig. 11.

  • the first step is an isothermal and constant pressure combustion at the standard temperature T0 = 298 K and at an imposed pressure p ≥ p0, with p0 = 0.1 MPa; this step is performed inside an isothermal fictitious combustion space, IFCS, and it is conceived without dissociation;

  • the second step is heating with dissociation of flue gases produced in the first step; this process takes place inside the second fictitious heating and dissociation space, FHDS, and here it is consuming the heat released in the first step, i.e. the higher heating value HHV.

Figure 11
figure 11

The scheme of the combustion process.

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The combustion inside IFCS

The mass balance for IFCS considers chemical oxidation reactions without dissociation of the gaseous fuel. The mass balance allowed the computation of inlet and outlet mole fractions of all chemical species. The Table 4 includes the mole fractions in the flow section “0” and the Table 5 includes the mole fractions in the flow section “1”.

Table 5 The mole composition in the flow section ”1”.
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The energy balance for IFCS computed the value of the combustion higher heating value, HHV, released by the constant temperature and constant pressure combustion of the fuel.

$$\sum\limits_{outlet} {N_{i} \cdot h_{f,i}^{0} } - \sum\limits_{inlet} {N_{j} \cdot h_{f,j}^{0} } = HHV\quad \left[ {\text{kJ/kmole}} \right]$$

Ni and \(h_{f,i}^{0}\) are the kmole number and the standard enthalpy of formation for each outlet chemical species, Nj and \(h_{f,j}^{0}\) are the kmole number and the standard enthalpy of formation for each inlet chemical species. Thus it is yielding, see Table 6:

$$\begin{aligned} HHV & = y_{CO2} \cdot h_{f,CO2}^{0} + z_{H2O} \cdot h_{f,H2O}^{0} - n_{CO} \cdot h_{f,CO}^{0} - n_{CH4} \cdot h_{f,CH4}^{0} - n_{C2H4} \cdot h_{f,C2H4}^{0} \\ & - n_{C2H6} \cdot h_{f,C2H6}^{0} - n_{C3H6} \cdot h_{f,C3H6}^{0} - n_{C3H8} \cdot h_{f,C3H8}^{0} - n_{C4H10} \cdot h_{f,C4H10}^{0} = - 5.15403 \cdot 10^{5} \quad \left[ {\text{kJ/kmole}} \right] \\ \end{aligned}$$
(1)
Table 6 The enthalpy of formation of chemical species in the flow sections “0”, “1”.
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The flue gases heating with dissociation inside FHDS

The non-dissociated flue gases leaving the IFCS enter the FHDS where two simultaneous processes take place:

  • chemical reactions of dissociation and recombination controlled by chemical equilibrium constants and

  • constant pressure heating of all chemical species, non or dissociated ones.

The both processes are completed by consuming the HHV. The flow scheme required by the mass and energy balance laws is presented in the Fig. 12. The below chemical reactions of dissociation and recombination were considerated.

$$2^{.} {\text{CO}}_{{2}} \to 2^{.} {\text{CO}} + {\text{O}}_{{2}}$$
(2)
$$2^{.} {\text{H}}_{{2}} {\text{O}} \to 2^{.} {\text{H}}_{{2}} + {\text{O}}_{{2}}$$
(3)
$$2^{.} {\text{H}}_{{2}} {\text{O}} \to 2^{.} {\text{OH}} + {\text{H}}_{{2}}$$
(4)
$${\text{N}}_{2} + {\text{O}}_{2} \to {\text{2NO}}$$
(5)
$${\text{N}}_{{2}} + \, 2^{.} {\text{O}}_{{2}} \to 2^{.} {\text{NO}}_{2}$$
(6)
Figure 12
figure 12

The inlet/outlet flow scheme of chemical species for FHDS.

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The mass balance for FHDS gives the following four dependences:

$${\text{a}} = {\text{r}}_{{{\text{N2}}}} {-} \, 0.5^{.} {\text{f}}{-}0.5^{.} {\text{i}}$$
(7)
$${\text{b}}: = {\text{r}}_{{{\text{O2}}}} {-} \, 0.5^{.} {\text{f }}{-}{\text{ i}} + 0.5^{.} {\text{e }} + \, 0.5^{.} {\text{g}}$$
(8)
$${\text{c}} = {\text{y}}_{{{\text{CO2}}}} {-}{\text{e}}$$
(9)
$${\text{d}} = {\text{z}}_{{{\text{H2O}}}} {-}{\text{g}}{-}{\text{h}}$$
(10)

The mass balance is completed trough chemical equilibrium constants K.

$$\nu_{{\text{A}}} \cdot {\text{A }} + \nu_{{\text{B}}} \cdot {\text{B}} \to \nu_{{\text{C}}} \cdot {\text{C }} + \nu_{{\text{D}}} \cdot {\text{D}} \Rightarrow K = \frac{{C^{{\nu_{C} }} \cdot D^{{\nu_{D} }} }}{{A^{{\nu_{A} }} \cdot B^{{\nu_{B} }} }} \cdot \left( {\frac{p}{{p_{0} }}} \right)^{{\nu_{C} + \nu_{D} - \nu_{A} - \nu_{B} }}$$

Therefore they are yielding:

  • dissociation of CO2

    $$K1 = \frac{{e^{2} \cdot b}}{{c^{2} }} \cdot \left( {\frac{p}{{p_{0} }}} \right)^{2 + 1 - 2} = \frac{{e^{2} \cdot b}}{{c^{2} }} \cdot \left( {\frac{p}{{p_{0} }}} \right) \Rightarrow e = \left( {\frac{{K1 \cdot c^{2} }}{b} \cdot \frac{{p_{0} }}{p}} \right)^{0.5}$$
    (11)

  • dissociation of H2O giving OH and H2

    $$K2 = \frac{{g^{2} \cdot b}}{{d^{2} }} \cdot \left( {\frac{p}{{p_{0} }}} \right)^{2 + 1 - 2} = \frac{{g^{2} \cdot b}}{{d^{2} }} \cdot \left( {\frac{p}{{p_{0} }}} \right) \Rightarrow g = \left( {\frac{{K2 \cdot d^{2} \cdot p_{0} }}{b \cdot p}} \right)^{0.5}$$
    (12)

  • dissociation of H2O giving H2 and O2

    $$K3 = \frac{{h^{2} \cdot g}}{{d^{2} }} \cdot \left( {\frac{p}{{p_{0} }}} \right)^{2 + 1 - 2} = \frac{{h^{2} \cdot g}}{{d^{2} }} \cdot \left( {\frac{p}{{p_{0} }}} \right) \Rightarrow h = \left( {\frac{{K3 \cdot d^{2} \cdot p_{0} }}{g \cdot p}} \right)^{0.5}$$
    (13)

  • recombination of N2 and O2 giving NO

    $$K4 = \frac{{f^{2} }}{a \cdot b} \cdot \left( {\frac{p}{{p_{0} }}} \right)^{2 - 1 - 1} \Rightarrow f = \left( {K4 \cdot a \cdot b} \right)^{0.5}$$
    (14)

  • recombination of N2 and O2 giving NO2

    $$K5 = \frac{{i^{2} }}{{a \cdot b^{2} }} \cdot \left( {\frac{p}{{p_{0} }}} \right)^{2 - 1 - 2} = \frac{{i^{2} }}{{a \cdot b^{2} }} \cdot \left( {\frac{{p_{0} }}{p}} \right) \Rightarrow i = \left( {\frac{{K5 \cdot a \cdot b^{2} \cdot p}}{{p_{0} }}} \right)^{0.5}$$
    (15)

The Eqs. (7 to 15) could be solved if there are known the flue gases temperature, Tfg, in the flow section “2” and the excess air, x, in the flow section “1”.

The energy balance gives the extra dependence between unknown mole composition of flue gases leaving FHDS and Tfg and x.

$$\begin{aligned} & a \cdot \int_{{T_{0} }}^{{T_{fg} }} {c_{p,N2} } \cdot dT + b \cdot \int_{{T_{0} }}^{{T_{fg} }} {c_{p,O2} } \cdot dT + c \cdot \left( {h_{f,CO2}^{0} + \int_{{T_{0} }}^{{T_{fg} }} {c_{p,CO2} } \cdot dT} \right) + d \cdot \left( {h_{f,H2O}^{0} + \int_{{T_{0} }}^{{T_{fg} }} {c_{p,H2O} } \cdot dT} \right) \\ & \quad + e \cdot \left( {h_{f,CO}^{0} + \int_{{T_{0} }}^{{T_{fg} }} {c_{p,CO} } \cdot dT} \right) + f \cdot \left( {h_{f,NO}^{0} + \int_{{T_{0} }}^{{T_{fg} }} {c_{p,NO} } \cdot dT} \right) + \left( {g + 0.5 \cdot h} \right) \cdot \int_{{T_{0} }}^{{T_{fg} }} {c_{p,H2} } \cdot dT \\ & \quad + h \cdot \left( {h_{f,OH}^{0} + \int_{{T_{0} }}^{{T_{fg} }} {c_{p,OH} } \cdot dT} \right) + i \cdot \left( {h_{f,NO2}^{0} + \int_{{T_{0} }}^{{T_{fg} }} {c_{p,NO2} } \cdot dT} \right) - y_{CO2} \cdot h_{f,CO2}^{0} - z_{H2O} \cdot h_{f,H2O}^{0} = \left| {HHV} \right| \\ \end{aligned}$$
(16)

The specific heat capacities, cp, were approximated by polynomial dependences on temperature. The enthalpy of formation of new chemical species is included in the Table 7.

Table 7 The enthalpy of formation of chemical species in the flow sections “2”.
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Because the Eqs. (7 to 15) includes large quantities, i.e. a, b, c, d, and small, very small or negligible quantities, i.e. e, f, g, h, i, the computational procedure starts with Eqs. (11 to 16) in which the temperature Tfg was imposed and supposed that a ≃ rN2, b ≃ rO2, c ≃ yCO2 and d ≃ zH2O. The preliminary evaluation of e, f, g, h and i allows the first evaluation of a, b, c and d through Eqs. (7 to 10) followed by a preliminary evaluation of excess air through Eq. (16). Knowing the first values of all chemical species and x, the next step re-uses the Eqs. (7 to 15) and after those Eqs. (11 to 15, 16). The iterative procedure is finished when the imposed error between two successive iterations is reached. The numerical results are included in the Tables 8 and 9.

Table 8 The mole composition of flue gases function of temperature Tfg, flow section “2”.
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Table 9 The mole composition of flue gases function of temperature Tfg, flow section “2”.
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Tables 8 and 9 present the main combustion noxious emissions, i.e. CO, NO and NO2 calculated using the chemical modeling algorithm at different temperatures, pressure and excess of air/oxygen. The emission levels calculated using the model had in most cases the same variation trend. For example, at high excess of air/oxygen and low temperature the emission of CO, NO and NO2 calculated have the minimum values. This result was expected, as the combustion process was performed under conditions of high oxygen excess that maximize the formation of complete combustion products. Opposite, at low air/oxygen excess and high temperature, the combustion noxious emission presents maximum values, but in most cases, above the legal limit. However, emissions under hydrotreating conditions should not be compared with proposals within legal limits, since these limits refer mainly to combustion conditions with excess air 44. The gaseous fraction resulted from hydrotreating experiments contains a high amount of unreacted hydrogen. In conclusion, the developed model allows the evaluation of harmful emissions depending on the temperature and pressure set on the flue gases. This calculation algorithm can be applied to any pressurized or atmospheric combustion engine fueled with different mixture of combustible gases such as: natural gas, hydrogen-enriched natural gas or oxy-fuel.

Conclusions

CoMo/γ-Al2O3-HMS catalyst was tested in the hydrotreating process of pyrolysis bio-oil. Based on the presented results at mild conditions of 320 °C and 40 bar, the catalyst is very active in the hydrotreating process leading to a decrease of total acid number of hydrotreated bio-oil with almost 89% and bio-oil conversion of 87.23%. In addition, in order to evaluate the harmful emissions resulted from combustion of gaseous phase obtained in the hydrotreating process a chemical modeling algorithm was developed.

Methods

Catalyst characterization

The surface area, pore volume and mean pore diameter of the catalyst width was calculated by the Brunauer-Emmet-Teller (BET) method. The nitrogen sorption measurements were carried out at 77 K using a Nova 1000 Quantachrome instrument. Prior to analysis, the samples were heated to 200 °C for 1 h. The X-ray diffraction (XRD) analysis was carried out using a fully automated, modular Rigaku Smart Labdiffractometer, operated at 45 kV and 200 mA, with Cu Kα radiation (1.54059 Ǻ), parallel beam configuration (2θ/θ scan mode) and scanning range between 2–90° (2θ), with a 0.02° step. Fourier transform infrared (FT-IR) analysis was performed with a FT-IR Tensor 27—Bruker spectrometer, using KBr pellet technique 45. Scanning Electron Microscopy (SEM) analysis was conducted using a FEI Inspect, S model microscope. The X-Ray Photoelectron Spectroscopy (XPS) spectra were obtained on a SPECS spectrometer by using the Al anode (1486.6 eV) radiation. The pyridine-adsorbed FT-IR spectrum was recorded using Jasco 610 spectrometer, with a scanning range from 4000 to 400 cm−1, a scan rate of 4 cm-1·s-1 and an average of 64 measurements in the final spectrum. The total number of acidic sites (sites/m2) was measured by temperature programmed desorption of pyridine (TPD-pyridine) as described by using a TGA Q5000 v3.13 from TA Instruments, USA For the TPD-pyridine profile as described by using TGA Q5000 v3.13 from TA Instruments, USA.

Experimental part

Catalyst preparation

The HMS material was synthesized via templating pathway using tetraethylorthosilicate (TEOS) as silica source and dodecylamine (DDA, Merck) as template and ethanol (EtOH) and water as solvents. Tetraethylorthosilicate (TEOS) was added to a mixture of dodecylamine (DDA), water and ethanol at room temperature. The mixture was stirred for 1 h at 40 °C and then let for 24 h at 25 °C. The molar composition of the material was 1.0 SiO2: 0.27 DDA: 8.5 EtOH: 29 H2O. The solid was filtered, washed, dried at room temperature and calcined in air at 550 °C for 8 h (1 °C/min), to remove the template. The γ-Al2O3-HMS support was obtained as cylindrical extrudates with the average size of 2 mm. The powdered materials (γ-Al2O3/HMS: 40/60) were mixed with a 10% (wt%) HNO3 solution gradually dosed for 1 h. After dosing the nitric acid, stirring is continued for 3 h at ambient temperature. The resulted homogeneous paste is inserted into a manually operated extruder with interchangeable diameters, the diameter of a die having the size of 1 mm. The resulted catalyst were dried for 6 h at 160 °C and annealed at 450 °C (10 °C/min). The molybdenum (8%) and cobalt (4%) catalyst was prepared by incipient wetness impregnation method using appropriate concentrations of aqueous solutions of ammonium heptamolybdate ((NH4)6Mo7O24, Aldrich 99%) and cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O), Aldrich 98%). After impregnation on γ-Al2O3-HMS support the wet cylinders were dried at 120 °C for 4 h and calcined at 450 °C for 2 h.

Catalytic tests

The activity tests of 4%Co8%Mo/ɤ-Al2O3-HMS catalyst were carried out in a fixed bed flow reactor (length of 0.5 m, volume of 200 cm3) heated by an electrical furnace. An amount of 50 cm3 catalyst was loaded in the middle zone of the reactor. The catalyst was activated in situ with a flow rate of 15 L/h hydrogen at 450 °C for 6 h. The bio-oil used in this report was obtained by pyrolysis of biomass derivate from biogas process and conditioned with waste vegetable oil at 425 °C 46. Bio-oil was introduced into the reactor using a metering pump, mixed with H2, and preheated to a desired temperature. The liquid samples were analyzed by GC/MS 7000 Triple Quad MS (Agilent Technologies) system equipped with HP-FFAP (30 m, 250 μm, 0.25 μm) column and He as carrier gas with volumetric flow of 1 ml/min. The oven program started from 30 °C with 2 °C/min rate and until reached 100 °C. The injector temperature was set at 250 °C. The compounds were identified using NIST MS Search 2.0 Library. The water content in the samples was determined by Karl Fischer method (ASTM D6869). Density, total acid number and saponification number were determined using standardized methods 47.

The gaseous fraction generated during the experiments was analyzed using a Agilent Technologies 6890 N Gas-Chromatograph with a Thermal Conductivity Detector (TCD), connected on-line, equipped with a SHINCARBON ST 80/100 2 M, 2 M ID,1/8"OD, SILCO, HP column and He as carrier gas with volumetric flow of 50 mL/min. The column temperature was set up to 40 °C (hold time) 300 °C, program rate: 20 °C/min, injection temperature: 300 °C, detector temperature 300 °C, sample volume: 0.75 mL.

The product yields in were calculated using the following equations:

$${\eta }_{i}(\%)=\frac{\frac{{x}_{i}\cdot {\overline{M}}_{oil}}{{M}_{i}}}{(\sum_{i=1}^{n}\frac{{x}_{i}\cdot {\overline{M}}_{oil}}{{M}_{i}})+{x}_{unreacted}}\cdot 100$$

where, Moil -average molecular weight of the bio-oil, Xi-mass fraction of the component I, Mi-molecular weight of component i

$$Conversion(\%)=\sum_{i=1}^{n}{\eta }_{i}(\%)$$