Techno-economic evaluation of stillage treatment with anaerobic digestion in a softwood-to-ethanol process

Table 2 Process details of the various scenarios

		Reference	A1	A2	A3	A4	B
Organic matter to anaerobic digestion	t COD/h	1.9	9.7	9.7	9.5	9.5	30.9
Polysaccharides	t COD/h	-	0.1	0.1	0.1	0.1	2.4
Soluble sugars	t COD/h	-	2.2	2.2	2.2	2.2	2.4
WIL	t COD/h	-	0.9	0.9	0.9	0.9	17.8
Others*	t COD/h	1.9	6.5	6.5	6.3	6.3	8.2
Organic matter removed	t COD/h	1.1	5.8	5.8	5.7	5.7	8.5
COD removed/COD fed incl. WIL	%	58	60	60	60	60	28
COD removed/COD fed excl. WIL	%	58	66	66	66	66	65
Energy flow† of raw biogas	MW	3.8	19.9	19.9	19.5	19.5	29.4
Energy flow† of upgraded biogas	MW	-	18.9	-	18.5	-	27.9
Overall heat duty of the process	MW	27.8	17.6	17.6	17.9	17.9	17.1
Total power demand of the process	MW	4.1	4.4	4.5	3.8	4.0	4.5
Electricity produced in CHP	MW	5.9	3.0	4.0	11.7	17.1	9.8
District heat	MW	-	-	4.7	34.1	48.9	31.1
Energy flow† of pellets‡	MW	48.5	39.3	56.2	-	-	-

The following input energy flows (in MW, based on lower heating values) were the same in all the scenarios: raw material 105.1; molasses 1.7; and enzymes 0.7. The positive and negative differences between power demand and electricity produced indicate purchasing or selling of electricity, respectively. A summary of the scenarios is given in Table 1.
* Includes organic acids, ethanol, glycerol, enzyme, yeast, extractives, degradation products and water-soluble lignin, depending on the scenario.
^† Based on lower heating values.
^‡Only the fraction of the solid fuel sold is given here, i.e. the fraction incinerated on-site is excluded.
CHP: combined heat and power production; COD: chemical oxygen demand; WIL: water-insoluble lignin.