**6. Conclusion**

**Fuel cell type Operating temperature**

**Table 5.** Characteristics of various fuel cell types (adapted from [54])

cryogenic process efficiency demands elevated costs [54].

**Fuel Way of storage Energy density by weight**

Hydrogen Gas (20 MPa) 33.3 0.53

Natural gas Gas (20 MPa) 13.9 2.58

Methanol Liquid 5.6 4.42 Gasoline Liquid 12.7 8.76 Diesel Liquid 11.6 9.7

**Table 6.** Energy density for some fuels (adapted from [58])

Gas (30 MPa) 33.3 0.75 Liquid (-253 ºC) 33.3 2.36 Metal hydrides 0.58 3.18

Gas (30 MPa) 13.9 3.38 Liquid (-162 ºC) 13.9 5.8

chemicals (metal hydride) [53].

490 Biofuels - Status and Perspective

**(ºC)**

Proton exchange membrane 60-110 0.01-250 40-55 Alkaline 70-130 0.1-50 50-70 Direct methanol 60-120 0.001-100 40 Phosphoric acid 175-210 50-1000 40-45 Molten carbonate 550-650 200-100,000 50-60 Solid oxide 500-1000 0.5-2000 40-72

Hydrogen is quite difficult to store or transport with current technology. There are many ways for storing hydrogen fuel; as a gas (hydrogen compressed), a liquid (liquid hydrogen) and

Hydrogen compressed in tanks (with similar technologies applied in natural gas compression) is the easiest and cheapest way to store it. These tanks can store hydrogen at a high pressure (about 25 MPa - 35 MPa), but even under these conditions the energy density by volume for hydrogen is lower than for gasoline or diesel as can be seen from Table 6. In liquid form (-253 ºC), the energy density has higher value than hydrogen in compressed form [58]. However, it is necessary to spend more energy to liquefy hydrogen than to compress it (up to 20% of the energy content of hydrogen is required to compress the gas and up to 40% to liquefy it), so the

**Eletrical power range**

**(kWh/kg) Energy density by volume (kWh/L)**

**Eletrical efficiency**

**(%)**

**(kW)**

The use of crude glycerol by biologic processes that generate PD, hydrogen and ethanol should be ensured for large scale production. Therefore, detailed economic studies and the optimi‐ zation of such processes are interesting subjects for future investigations. New strategies may involve developing a proper market for the bioconversion of crude glycerol, as this would determine the economic viability of obtains clean energy from the glycerol feedstock.

Crude glycerol from the biodiesel manufacturing processes is a potential feedstock for bacterial hydrogen, PD and ethanol production. It can be used as substrate for the production of these bio products instead of other more expensive, carbon sources such as sugars.

A high hydrogen yield is possible when acetic acid is produced as the end product of crude glycerol fermentation. Other similar strategies should be developed for a metabolic route of acetic acid generation during the fermentation of crude glycerol.

Most investigations on crude glycerol bioconversion have been performed in serum bottle scale batch reactors. Only a few studies have been performed in a continuous mode. The continual improvement of investigations into bacterial hydrogen production using the continuous mode is recommended.

Consortia of anaerobic bacteria from environmental sources or pure cultures may be used for bioconversion of crude glycerol to hydrogen PD and ethanol. However using co-cultures may reduce the accumulation of metabolites and improve hydrogen yield. Application of the biological processes to directly convert abundant crude glycerol into higher value products may represent a promising route to achieve economic viability in the biofuels industry.

The widespread utilization of H2 as an energy source requires solutions to several problems: the H2 must be able to be produced from a cheap and renewable source; refueling infrastruc‐ ture must be developed for fuel cells. The appropriate utilization of energy from hydrogen in large scale must be initially expected on-site. Further investigations for safe and economic storage of hydrogen are recommended.

### **Acknowledgements**

Authors would like to acknowledge ABES – Associação Brasileira de Engenharia Sanitária e Ambiental for their kind permission to reuse figures from previously published material Mr. Frank Garrik (University of St Andrews, Scotland) for their language review and FUNDU‐ NESP for finantial support.
