**2.14 Supercritical conversion**

Supercritical conversion is a new technique that uses high temperature and highpressure fluids, above their critical point, to achieve the transformation of waste. Compared with conventional WtE and WtL technologies, this method may ignore drying or dehydration pretreatments, reduce reaction temperature, shorten reaction times, and increase product yield. In recent decades, supercritical conversion has gained interest not only for chemical extraction, but also in chemical conversion by replicating processes such as transesterification, gasification, hydrolysis, and others (**Figure 8**). Studies with real biomasses and at larger scales are lacking, but

**45**

H2 kg<sup>−</sup><sup>1</sup>

**Figure 8.**

**3. Economics of WtE**

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

*Example scheme of supercritical conversion technology.*

the reviewed research generally suggests that, for example, supercritical gasification in both biorefineries and cogeneration has enormous potential. Supercritical water gasification of olive oil mill wastewater, for example, has been investigated recently with different alkali catalyst types [88]. The tests proved that an increment in catalyst concentration would improve hydrogen yield to a maximum of 76.73 mol

 in specific conditions. Extraction with supercritical carbon dioxide for biodiesel production is another process investigated [89]. In the example study

The most apparent barrier for the implementation of WtE technologies is capital cost, specifically the upfront expense of building and installing the energy generation system. While not enabling a detailed view of project economics, an assessment of capital costs offers simple and clear information which can be used to evaluate the status of different commercial technologies. **Figure 9** shows estimates of capital cost for a range of WtE power generation technologies. Capital costs are low for mature technologies such as cocombustion and anaerobic digestion integrated with ICE or gas turbine (GT). For early-stage technologies, the capital costs are extremely uncertain, and as such many were not included in the analysis. Pyrolysis, plasma arc gasification, and refused-derived fuel (RDF) direct combustion, for

of 500 bar and 40°C. Fatty acid content was observed to decrease with increasing pressure. In fact, the extraction of fatty acids and transesterification in a single step

at a pressure

reviewed, the best productivity was 0.312 kg of oil per kg of seed<sup>−</sup><sup>1</sup>

are considered one of the greatest potentials for this technology [90].

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

*Elements of Bioeconomy*

chemicals from bio-oils [78, 79].

**2.13 Transesterification**

than mono-solvent extractions, with a 50/50 mixture of chloroform and ethanol leading to 11.76% lipid extracts. As a mono-solvent, chloroform resulted in the highest quantity of lipids extracted at 10.78% with 3 h showing the best extraction efficiency [77]. Solvent extraction also has the potential to be integrated with other processes like supercritical extraction or pyrolysis in order to produce higher value

Transesterification is the main process used in the production of biodiesel in which vegetable oils are broken into methyl or ethyl esters by reacting with an alcohol and catalysts (acids, alkalis, and enzymes) with glycerol as the only byproduct. Biodiesel production has increasingly been seen as a carbon mitigation tool, assuming increasing importance in promoting sustainability in European countries. Since January 1, 2010, for example, all commercial diesel fuel sold in Portugal has a 7% incorporation of biodiesel. Biodiesel production is a controversial issue due to the use, availability, and cost of raw materials, as well as greenhouse gases emission and food competition. In this context, the use of waste oils and nonfood crops seems to compose the best option for the widespread production of biodiesel in the future [80]. In Europe, it is estimated that about 4 million Mg of waste cooking oil are to be collected annually, seven times more than the current collected amount [81]. This underdeveloped collection chain led to record level imports in the first 8 months of 2018 with more than 235,000 Mg of waste cooking oil entering the EU from China. Biodiesel market thus does not show signs of slowing down [82]. Although already mature and well established commercially, biodiesel still needs a lot of research and development to achieve significant improvements in its production [83, 84]. In this regard, continued interest in the use of biodiesel as an alternative fuel has led to increased efforts to develop a new generation of biofuels. Heterogeneous catalysts have been increasingly tested since they offer process improvements over homogeneous catalyzed commercial production employing liquid bases. In more detail, the use of solid catalyst facilitates post-process separation and fuel purification, along with the continuous synthesis of biodiesel. The increasing use of low-grade waste cooking oil remains a challenge for existing heterogeneous catalysts since the high concentration of impurities (acid, moisture, and heavy metals) induces rapid deactivation in flow and requires purification. The development of more robust catalyst formulations tolerant to such components is, therefore, a necessity [85]. Cement was recently tested in the transesterification of *Pongamia pinnata* and sunflower oil with somewhat low conversion rates (76%), but research should continue in upcoming years [86]. In terms of process coupling, the blend of biodiesel with pyrolysis oil derived from lignocellulosic wastes is an attractive route as an alternative to diesel fuel [87]. Microalgae are also considered an attractive feedstock alternative to reduce

costs in the extraction and conversion of this renewable fuel.

Supercritical conversion is a new technique that uses high temperature and highpressure fluids, above their critical point, to achieve the transformation of waste. Compared with conventional WtE and WtL technologies, this method may ignore drying or dehydration pretreatments, reduce reaction temperature, shorten reaction times, and increase product yield. In recent decades, supercritical conversion has gained interest not only for chemical extraction, but also in chemical conversion by replicating processes such as transesterification, gasification, hydrolysis, and others (**Figure 8**). Studies with real biomasses and at larger scales are lacking, but

**2.14 Supercritical conversion**

**44**

**Figure 8.** *Example scheme of supercritical conversion technology.*

the reviewed research generally suggests that, for example, supercritical gasification in both biorefineries and cogeneration has enormous potential. Supercritical water gasification of olive oil mill wastewater, for example, has been investigated recently with different alkali catalyst types [88]. The tests proved that an increment in catalyst concentration would improve hydrogen yield to a maximum of 76.73 mol H2 kg<sup>−</sup><sup>1</sup> in specific conditions. Extraction with supercritical carbon dioxide for biodiesel production is another process investigated [89]. In the example study reviewed, the best productivity was 0.312 kg of oil per kg of seed<sup>−</sup><sup>1</sup> at a pressure of 500 bar and 40°C. Fatty acid content was observed to decrease with increasing pressure. In fact, the extraction of fatty acids and transesterification in a single step are considered one of the greatest potentials for this technology [90].
