**5.2 In situ upgrading**

Other strategy to improve the properties of the bio-oil obtained by pyrolysis process is the modification in situ, i.e., inside the own reactor by change of the variables and the parameters. Two strategies that have gained prominence and show good results in obtaining a bio-oil with good qualities are hydrogenation and catalytic pyrolysis, and the hybrid process with both.

The addition of H2 as a pyrolysis gas agent in the absence of catalysts promotes hydrogenation reactions mainly with the volatiles released and may have various effects such as increased formation of less oxygenated species, greater selectivity in phenolic compounds among others. For example, the effect of adding hydrogen in the carrier gas (up to 15% vol) in the pyrolysis of food waste in a dawndraft reactor promoted the formation of a bio-oil more selective in aromatic, phenolic compounds while reducing the amount of heterocycline nitrogenous species such as (quinoline) [55]. Other biomasses, such as poplar wood showed the same behavior upon pyrolysis in H2 atmosphere (6% vol) compared to pure N2 atmosphere obtaining higher yield and selectivity in phenolic species [56]. Another important effect of the addition of H2 in the pyrolysis process is that, regardless of the presence of catalysts, it can promote the production of bio-oil with higher H/C ratio which is highly beneficial for the use of bio-oil as liquid fuel [57].

Recycling the pyrolysis gas to the reactor as a reactive atmosphere is a potential alternative, although its effect is not yet clear. When fast pyrolysis of biomass takes place under a N2 atmosphere, the main gaseous products of pyrolysis are CO, CO2, H2, CH4 and low levels of light hydrocarbons, therefore, when this gas is recycled to the reactor, it can promote a reactive atmosphere and not more inert. Studies in fluidized bed reactors, in the temperature range of 430–500°C, show that the increase in gas recycling rates can lead to a decrease in the production of organic liquids, but some biomasses, such as oak and switch grass, show good results. Deoxygenation effect and HHV increase in your bio-oil with gas recycling rates of up to 80%. However, other biomasses, such as *Pennycress Presscake* and pine wood, had no effect on the yield and composition of bio-oil with the pyrolysis gas recycled in the reactor [58, 59].

Among all the in situ alternatives to improve the obtainment of higher quality bio-oil, catalytic pyrolysis stands out as the most promising. The process basically consists of the insertion of a solid catalyst in the pyrolysis reactor capable of acting in the set of pyrolysis reactions, promoting the formation of bio-oil with better properties and selectivity in the components of interest of interest. Several authors have already published works testing different catalysts, in different process configurations, attesting to diverse improvements in obtaining bio-oil. Among these improvements in the use of catalysts in the fast pyrolysis process, it is worth mentioning: [60].


Despite promising experimental results, the fast pyrolysis process using catalysts still needs to overcome some obstacles to reach the industrial level. The basis for these challenges is the development of catalytic reactors capable of supporting the pyrolysis atmosphere without expressive rates of catalyst deactivation, expanding the selection of desired products in bio-oil, in addition to efficient developments in the recovery and regeneration of catalysts [41, 60]. **Table 4** below shows a set of


### **Table 4.**

*Some research about catalytic pyrolysis.*

works on catalytic pyrolysis of biomass at different scales, the characteristics of the catalysts used as well as the yield in bio-oil.
