**2.2 Gasification**

Gasification is a thermochemical decomposition process which occurs without the presence of sufficient oxygen for a complete combustion and allows the transformation of waste feedstocks into a combustible gas known as syngas, a fuel with many potential applications (**Figure 2**). As a technology, gasification has a several-centuriesold history with progress made by advances and stalls. Widely used during industrial revolution in the 1850s to illuminate factories, streets, and houses, this technology fell into disuse during the twentieth century and only recently gained a continuous support for its development due to energy security threats and climate change.

Among WtE processes, gasification is one of the most promising with some specific barriers explaining its lack of penetration in the domestic and commercial sectors [11]. An extensive review on technology progress identified 50 companies offering "commercial" gasification plants in Europe, the USA, and Canada, mostly downdraft and fluidized bed systems (75 and 20%, respectively) [12]. Moreover, in 2013 there were more than 272 operating gasification plants worldwide with more under construction and planned until 2019 [13].

Supply chain development, waste pretreatment (drying/grinding/pelletization), and the potential need for treatment of syngas are usually pointed out as the main barriers to be overcome. Conventional drying systems are known to be expensive and energy intensive. In addition, complete drying of the biomass represents a decrease in

**37**

**Figure 3.**

*Review of Biofuel Technologies in WtL and WtE DOI: http://dx.doi.org/10.5772/intechopen.84833*

the amount of hydrogen that is potentially producible during gasification. Solar drying, though inefficient, is cheap and should be studied and viewed as an alternative. The potential presence of tars, particulate emissions, SOx, NOx, and NH3 in the syngas also limits its range of use. Filtration of the syngas is important to obtain a syngas free of contaminants but requires constant cleaning of the filters as a way to prevent blockage and pressure drops. Tars are seen as the most complicated contaminant. In addition to filtration, it is also possible to resort to thermal decomposition and catalytic cracking as a form of treatment [14]. Thermal decomposition leads to melting of the ashes, which can also result in mechanical problems. Catalytic treatment is seen as the most effective for dealing with tars but ineffective against particles and other toxic

gases. The combination of various forms of treatment is the best solution [15].

option and is already considered economically viable [22–24].

**2.3 Explosive steam decompression**

*Example scheme of explosive steam decompression technology.*

Pretreatment of the waste and biomass to be gasified, as well as reactor design and optimization of operational conditions, has been proven to be of great importance to maximize conversion efficiency, viability, and profitability [16]. In this regard, procedures such as sorting, grinding, and sifting are simple but essential. Fluidized bed reactors are considered the most suitable for a good and efficient process. Fluid bed material consisting of natural rocks such as dolomite and olivine is usually the best option due to reasonable prices. As for optimized conditions, mathematical models using 2D computational fluid dynamics (CFD) confirmed that gasification temperature has a key influence on the calorific value of the syngas produced [17]. Co-gasification of several wastes has been reported with promising results [18–20]. Inorganic additives such as calcium oxide (CaO) have been observed to decrease CO2 and increase the quality of the syngas [21]. Integrating gasification and co-gasification into solid oxide fuel cells (SOFC) or internal combustion engine (ICE) cogeneration systems is a very promising

Explosive decompression is a thermochemical pretreatment process which disrupts the rigid structure of lignocellulosic materials using steam and high pressures. Patented in 1931 by Mason [25], this process consists in heating the waste in hot steam at 285°C and at a pressure of 3.5 MPa for 2 min, before increasing the pressure once again, this time to 7 MPa for 5 s [26]. Naturally, time and temperature are a major influence in the disruption of the fibers composing the biomass, with the pretreatment process potentially resulting in just some grooves in the wood or in the total conversion into pulp. The main application of this technology is as pretreatment of lignocellulosic materials (**Figure 3**) which is essential for making the

**Figure 2.** *Example scheme of gasification technology.*

*Review of Biofuel Technologies in WtL and WtE DOI: http://dx.doi.org/10.5772/intechopen.84833*

*Elements of Bioeconomy*

**2.2 Gasification**

**Figure 1.**

Gasification is a thermochemical decomposition process which occurs without the presence of sufficient oxygen for a complete combustion and allows the transformation of waste feedstocks into a combustible gas known as syngas, a fuel with many potential applications (**Figure 2**). As a technology, gasification has a several-centuriesold history with progress made by advances and stalls. Widely used during industrial revolution in the 1850s to illuminate factories, streets, and houses, this technology fell into disuse during the twentieth century and only recently gained a continuous sup-

port for its development due to energy security threats and climate change.

under construction and planned until 2019 [13].

*Example scheme of incineration/combustion technology.*

Among WtE processes, gasification is one of the most promising with some specific barriers explaining its lack of penetration in the domestic and commercial sectors [11]. An extensive review on technology progress identified 50 companies offering "commercial" gasification plants in Europe, the USA, and Canada, mostly downdraft and fluidized bed systems (75 and 20%, respectively) [12]. Moreover, in 2013 there were more than 272 operating gasification plants worldwide with more

Supply chain development, waste pretreatment (drying/grinding/pelletization), and the potential need for treatment of syngas are usually pointed out as the main barriers to be overcome. Conventional drying systems are known to be expensive and energy intensive. In addition, complete drying of the biomass represents a decrease in

**36**

**Figure 2.**

*Example scheme of gasification technology.*

the amount of hydrogen that is potentially producible during gasification. Solar drying, though inefficient, is cheap and should be studied and viewed as an alternative. The potential presence of tars, particulate emissions, SOx, NOx, and NH3 in the syngas also limits its range of use. Filtration of the syngas is important to obtain a syngas free of contaminants but requires constant cleaning of the filters as a way to prevent blockage and pressure drops. Tars are seen as the most complicated contaminant. In addition to filtration, it is also possible to resort to thermal decomposition and catalytic cracking as a form of treatment [14]. Thermal decomposition leads to melting of the ashes, which can also result in mechanical problems. Catalytic treatment is seen as the most effective for dealing with tars but ineffective against particles and other toxic gases. The combination of various forms of treatment is the best solution [15].

Pretreatment of the waste and biomass to be gasified, as well as reactor design and optimization of operational conditions, has been proven to be of great importance to maximize conversion efficiency, viability, and profitability [16]. In this regard, procedures such as sorting, grinding, and sifting are simple but essential. Fluidized bed reactors are considered the most suitable for a good and efficient process. Fluid bed material consisting of natural rocks such as dolomite and olivine is usually the best option due to reasonable prices. As for optimized conditions, mathematical models using 2D computational fluid dynamics (CFD) confirmed that gasification temperature has a key influence on the calorific value of the syngas produced [17]. Co-gasification of several wastes has been reported with promising results [18–20]. Inorganic additives such as calcium oxide (CaO) have been observed to decrease CO2 and increase the quality of the syngas [21]. Integrating gasification and co-gasification into solid oxide fuel cells (SOFC) or internal combustion engine (ICE) cogeneration systems is a very promising option and is already considered economically viable [22–24].

### **2.3 Explosive steam decompression**

Explosive decompression is a thermochemical pretreatment process which disrupts the rigid structure of lignocellulosic materials using steam and high pressures. Patented in 1931 by Mason [25], this process consists in heating the waste in hot steam at 285°C and at a pressure of 3.5 MPa for 2 min, before increasing the pressure once again, this time to 7 MPa for 5 s [26]. Naturally, time and temperature are a major influence in the disruption of the fibers composing the biomass, with the pretreatment process potentially resulting in just some grooves in the wood or in the total conversion into pulp. The main application of this technology is as pretreatment of lignocellulosic materials (**Figure 3**) which is essential for making the

**Figure 3.** *Example scheme of explosive steam decompression technology.*

biopolymers accessible for further treatment via other processes such as fermentation, hydrolysis, anaerobic digestion, and densification. The production of biogas by anaerobic digestion using lignocellulosic wastes, for example, is considered a huge challenge due to its recalcitrant nature (non-biodegradability) [27]. In this regard, the use of explosive steam decompression as a form of pretreatment has been proven to enhance the production of biogas. Moreover, ethanol production and syngas production using lignocellulosic feedstocks have also been reported to proceed with higher calorific value and lower temperatures, respectively, when precluded with steam explosion [28, 29]. A promising solution for continuous steam explosion has been presented by a research team from South China University of Technology [30] allowing for process scale-up and its potential integration in second-generation biorefineries.
