**5.3** *Ex situ* **upgrading**

*Ex situ* processes are carried out outside the pyrolysis reactor, ie outside the main reaction zone. Esses processos podem ser de natureza física ou físico-química, promovendo alguma melhoria na qualidade do bio-óleo bruto gerado na pirólise. Dentre esses, pode se citar: [76–78].

	- Solvend adition
	- Emulsification
	- Hot vapor filtration
	- Catalytic hydrodeoxygenation (HDO)
	- Steam reforming
	- Esterification
	- Supercritical fluids

Although there are several strategies to improve the quality of bio-oil, the most studied processes, aiming at the industrial potential, are hot vapor filtration (HVF), catalytic hydrodeoxygenation (HDO) and steam reforming.

HVF is one of the most common and simple able to improve some properties of bio-oil. It consists of passing the pyrolysis vapors through a filtering medium, even at higher temperatures. This method, in addition to being more efficient than traditional cyclones for removing small particles of coal and ash, can add better properties to the condensed bio-oil later on [48, 79].

Some types of filter (fixed bed glass wool, ceramic candles) and biomass raw material (sugarcane, rice and cassava waste) for pyrolysis were tested in the configuration: reactor + hot filter. In short, the addition of the filter introduces a longer residence time of the bio-oil organic vapor at high temperatures, promotes cracking reactions of organic molecules and causing a loss in bio-oil yield (around to 5%), increase in yield of the gas in addition to the higher content of H2O in the bio-oil. But the condensed bio-oil tends to present a series of improvements such as: practically free of char and ash, less viscous. There may also be a certain deoxygenation of the bio-oil (decrease in the O/C molar ratio) but this effect is not fully understood, depending on the type of hot filter, biomass and operating conditions of the pyrolysis [79–81].

Another upgrading process, of the great highlighted is the hydrodeoxygenation (HDO) of the bio-oil. The HDO it's a particular case of hydrotreatment process, where the bio-oil is its reacted with H2 under specific conditions of temperature, pressure, catalystic, and fluidynamis (**Figure 7**). In this way, the organic compounds of the bio-oil are submitted to a set of reactions, mainly hydrogenation and hydrodeoxygenation, providing an improvement in properties, forming less oxygenated and more stable compounds.

### *Recent Perspectives in Pyrolysis Research*

**Figure 7.**

*Schematic of the catalytic hydrodeoxygenation process, its variables and products.*

Good control of the variables and parameters is essential to obtain a bio-oil with a high degree of deoxygenation and with lower H2 consumption. The process occurs in the range 200–450°C and higher temperatures, in general, increase the degree of deoxygenation but at the expense of a decrease in the yield of improved bio-oil due to higher gas production. The partial pressure of H2 is a variable of great relevance in the process and a minimum value of around 80 bar is required for good solubilization of the bio-oil, increased catalytic activity and minimization of adverse effects such as repolymerization of the bio-oil forming unwanted solid products [82, 83].

Finally the selection of the catalyst, jutanly with the support, is fundamental. The first catalysts tested in this process were CoMo/γ-Al2O3 and NiMo/γ-Al2O3 due to their use in oil refineries for nitrogen and sulfur removal processes. About these a number of sulfide, noble metal, and transition catalysts have been studied, each presenting different advantages with respect to higher catalytic activity, selectivity, and deactivation rates upon the presence of inorganic elements such as sulfur and ash present in the bio-oil [83]. The catalytic hydrodeoxygenation becomes a promising alternative for obtaining a bio-oil of higher quality and opening the possibility for the production of biofuels from it.

The steam reforming process emerges as another alternative of great potential for the valorization of lignocellulosic currents and hydrogen production. H2 is a gaseous fuel with high added value due to its energy density and its combustion is free of carbon emissions. In this process, bio-oil, as well as fossil fuels, react with H2O vapor at temperatures in the range of 700–1000°C, in the presence of a catalyst (usually nickel-based) offering as main product the H2-rich syngas, along with CO2 [76–78]. The main advantage of this process is the simultaneous production of high value-added fuel (H2), and it allows the assimilation of CCSU (Carbon Capture Storage and Utilization) technologies. But one of the major disadvantages is the high energy demand to carry out the process.

The overall balance of this process can be given by Eq (1): [77, 78].

$$2\,\mathrm{C}\_{n}H\_{m}O\_{k} + \left(2n - k\right)H2O \to nCO2 + \left(2n + \frac{\mathrm{m}}{2\mathrm{k}}\right)H\_{2} \tag{1}$$

Where the coefficient expresses the maximum possible H2 yield per mole of carbon fed. The steam reforming of bio-oil can be performed mainly in fixed bed, fluidized bed or staged bed reactors. The process can be carried out using a wide range of catalysts (Dolomite, Ru, Ni, Co, Rh) and supports (Al2O3, ZrO2, MgAl2O4, etc.) but Ni based catalysts are the most active but also have the highest deactivation rates due to coke formation. However, changes in the processes, mainly concerning

the Vapor/Carbon ratio, besides the temperature and the catalytic support, can minimize the impact of the decay of the catalytic activity by coke formation in the steam reforming process [84, 85].
