# Pretreatments Applied to Wheat Straw to Obtain Bioethanol
## 4 Pretreatment Limitations  
The lignocellulosic biomass pretreatment methods have certain limits:  
* Pretreatment with an organic solvent has a low biomass recovery rate. The optimization of the pretreatment conditions depends on the pretreatment time and temperature, the type of catalysts and the solvent concentration [93];
* During biological pretreatment, lignin derivatives can poison microorganisms.
* To shorten the pretreatment time and improve enzymatic digestibility, fungal pretreatment combined with other pretreatments is recommended [40; 94];
* Microwave irradiation implies a high energy demand, high cost of pretreatment, and a lack of large-scale equipment. The design and expansion of more energetically efficient microwave pretreatment reactors is recommended [95; 96];
* In pretreatment with ionic liquid, there are no economic solutions for recycling the ionic liquid [40];
* Chemical pretreatment with acid and alkaline implies a requirement of intensive energy, high costs, and harmful by-products [40].  
Through chemical, physical or biological methods, the total or partial solubilization of lignin and hemicellulose takes place [97; 98; 99].  
Pretreatment by chemical methods, in the presence of dilute acids, has the disadvantage of hemicellulose saccharides breaking down into furans and phenols, compounds that inhibit the fermentation process, thus reducing the yield of ethanol [98].  
Chemical methods (with diluted acids, alkaline media, ammonia, organic solvent, sulfur dioxide, and carbon dioxide) are the most used; however, for economic reasons and environmental protection, it is expected that they will be replaced by physical methods [100]. The choice of the type of pretreatment step influences the ethanol yield. Chemical pretreatment methods involve splitting the fibers in various conditions, from highly acidic to alkaline environments. In these conditions, different constituents of the biomass can be affected. For example, acid pretreatment causes the hydrolysis of hemicellulose while the cellulose and lignin fractions remain intact in the solid residue [101; 102]. In most cases we work with H2 SO4. Pretreatment in alkaline environment affects only lignin [98].  
For pretreatment in an alkaline medium, bases such as sodium hydroxide or calcium hydroxide are used. The entire amount of lignin and part of the hemicellulose is removed. An advantage is the increase in cellulose reactivity. The costs for raw materials are much lower compared to acid hydrolysis. However, the use of high concentrations of reagents requires the treatment of wastewater and an increase in costs for the neutralization of the waste. Alkaline pretreatment methods are, in general, more effective, as a large part of the lignin is solubilized but most of the hemicellulose remains in an insoluble state [98].  
The use of the process with solvents presents some advantages compared to traditional chemical methods, e.g., it can use small-sized lignocellulosic material, facilitating the removal of impurities; the possibility of isolating and recovering lignin in an unaltered form; and recovery with good yields of saccharides from hemicellulose.  
The biological method uses a low amount of energy. The disadvantages of the method include the very low speed of hydrolysis and lignin derivatives poisoning microorganisms. Biological methods of biomass pretreatment can be combined with chemical ones.  
Bioethanol contributes to the reduction in CO2 emissions, being a beneficial fuel for the environment [100]. Using a bioethanol blend as fuel for cars can significantly reduce oil use and greenhouse gas emissions. Cellulosic ethanol can generate a reduction in greenhouse gas emissions of approximately 90%, with carbon dioxide emissions becoming almost equal to zero.