Question

Biodiesel is most frequently produced from vegetable oils however, lignocellulosic feedstocks can be converted to biodiesel using oleaginous yeasts. Lignocellulosic feedstocks are also highly valued for bioethanol production. For both processes wheat straw (LHV 19 MJ/kg) is processed through a steam explosion pre-treatment followed by enzymatic hydrolysis to recover 92% of available hexose and 98% of available pentoses. The organic residue material that remains after sugar extraction has an LHV of 22 MJ/kg-od and can be combusted to generate energy. Wheat composition: glucan 33%, xylan 23% galactomannan 4%, lignin 25%. extractives 3%, protein 6% and ash 6% For a biorefinery that has a 120.000 tonne-dry/year capacity, what is the total energy yield from the Biodiesel biorefinery per year? From the Bioethanol biorefinery? (Note: organic residue can be used for direct combustion energy). Which system would you recommend and why? Assumptions: FOR the Bioethanol process Saccharomyces cerevisiae yeast is used and this yeast can only ferment hexoses. - Fermentation efficiency is 82% - LHV of bioethanol = 29.7 MJ/kg FOR the Biodiesel process the oleaginous yeasts will utilize both hexoses and pentoses for production of lipids. Carbohydrate consumption is 1.89g/g yeast cell mass, with a 55% lipid content. Extraction efficiency of the lipids from the yeast cells is 72% • Tranesterification efficiency of lipids to biodiesel is 98% - LHV of biodiesel = 42 MJ/kg

Answer 1

120,000 tonnes or 1.2 x 108 Kg of organic residue used for direct combustion would yeild: 1.2 * 108 * 19 = 22.8 * 108 MJ of energy. [Note: 1 tonne = 1000kg]

• Glucan and Galactomannan are made of hexoses.
• Xylan is made of pentose.

The dry residue contains (33 + 4)% or 37% hexose and 23% pentose

After steam exploson pre treatment and enzymatic hydrolysis, we'll have-

Hexose: (37/100 * 120,000) * 92/100 = 40,848 tonne = 4.0848 * 107Kg

Pentose: (23/100 * 120,000) * 98/100 = 27,048 tonne = 2.7048 * 107Kg

Organic residue: 40/100 * 120,000 = 48,000 tonne = 4.8 * 107Kg

For bioethanol:

Energy yeild from hexose: (82/100 * 4.0848 * 107 ) * 29.7 = 9.948 * 108 KJ

Energy yeild from pentose: NIL (as it is not converted into bioethanol)

Energy yeild from organic residual matter: 4.8 * 107 * 22 = 10.56 * 108 KJ

Hence, total energy yeild: 20.508 * 108 KJ

For biodiesel:

Energy yeild from hexose: (1/1.89 * 55/100 * 72/100 * 98/100) * (4.0848 * 107) * 42 = 3.522 * 108

Energy yeild from pentose: (1/1.89 * 55/100 * 72/100 * 98/100) * (2.7048 * 107) * 42 = 2.333 * 108

Energy yeild from organic residual matter: 4.8 * 107 * 22 = 10.56 * 108 KJ

Hence, total energy yeild: 16.415 * 108 KJ

So, from all this we can conclude that direct combustion yeids the most amount of energy of the 3 processes, followed by bioethanol and finally biodiesel.