Effects of steam pretreatment and co-production with ethanol on the energy efficiency and process economics of combined biogas, heat and electricity production from industrial hemp

Table 1 Details of anaerobic digestion in the various scenarios

Scenario		AD	AD-R	SP-AD	SP-AD-R	Et-AD	Et-AD+
AD system		CSTR	CSTR	CSTR	CSTR	CSTR	CSTR	UASB
Degradable components fed¹	t/h	22.6	27.2	22.6	24.3	18.0	10.1	7.9
In main stream²	t/h	18.7	23.3	17.5	19.2	9.3	6.2	3.1
In leaves	t/h	3.9	3.9	3.9	3.9	3.9	3.9	-
In flash stream	t/h	-	-	1.2	1.2	1.2	-	1.2
In liquid fraction after SP	t/h	-	-	-	-	3.6	-	3.6
Non-degradable components fed⁴	t/h	6.8	7.7	6.8	7.7	6.8	5.6	1.2
In main stream²	t/h	6.3	7.2	6.3	7.2	5.4	5.1	0.3
In leaves	t/h	0.5	0.5	0.5	0.5	0.5	0.5	-
In flash stream	t/h	-	-	0	0	0	-	0
In liquid fraction after SP	t/h	-	-	-	-	0.9	-	0.9
C flow fed⁵	t/h	13.2	16.8	13.2	14.8	10.1	4.4	5.6
N added	kg/h	370	185	370	274	67	0	227
P added	kg/h	45.1	61.0	45.1	50.9	0	14.1	0
Fe added	kg/h	17.3	21.2	14.3	16.2	30.1	30.1 ⁶
Ni added	g/h	4.5	8.8	0	0.8	29.1	29.1 ⁶
Co added	g/h	97	114	82	90	162	162 ⁶
Degradation ratio³	-	0.53	0.48	0.68	0.65	0.66	0.65	0.49
Sludge DM in the effluent	t/h	1.29	1.42	1.55	1.60	1.23	0.67	0
Raw biogas produced	Nm³/h	9412	11275	10803	11456	7755	4425	3254

A summary of the scenarios is given in Figure 3.
AD: anaerobic digestion, CSTR: continuous stirred tank reactor, UASB: upflow anaerobic sludge blanket, SP: steam pretreatment, DM: dry matter
¹ This value refers to carbohydrates, proteins, lipids, extractives, organic acids, ethanol, glycerol, enzymes, yeast, and sugar degradation products.
² This figure refers to hemp stems, or steam pretreated hemp stems, or whole stillage, or thin and thick stillages, depending on the scenario. It includes also the recycled liquid fraction of the AD effluent.
³ The degradation ratio is defined as the difference between the mass flow of degradable components in effluent and input, divided by the mass flow of degradable components input.
⁴ Water-insoluble and water-soluble lignin, ashes and other unknown components are considered to be non-degradable.
⁵ This value includes the carbon flow of the recycled liquid fraction of the AD effluent.
⁶ Experimental data for trace metal contents are available only for the feedstock (Table 2), and thus the distribution of trace metals between CSTR and UASB is unknown. The total requirements of the two systems have therefore been assumed to be the same as those of Scenario Et-AD.

ISSN: 2731-3654