coal-fired power stations. Report ECN-C--05-013,. 2005. [5] Bridgeman, T.G., Jones, J.M., Shield, I., Williams,. P.T. Torrefaction of reed canary grass, wheat straw.
24th European Biomass Conference and Exhibition, 6-9 June 2016, Amsterdam, The Netherlands
HYDROTHERMAL CARBONIZATION OF MIXED BIOMASS: EXPERIMENTAL INVESTIGATION FOR AN OPTIMAL VALORISATION OF AGROFOOD WASTES Gabriele Di Giacomo, Alberto Gallifuoco, Luca Taglieri University of L’Aquila - Department of Industrial and Information Engineering & Economics Via G.Gronchi, 18 – 67100 – L’Aquila – ITALY
ABSTRACT: Hydrothermal carbonization (HTC) of biomass is a well-known thermochemical process for increasing the energetic density of organic feedstock, but literature lacks sufficient data for designing flexible processes, possibly operating continuously. This work starts filling this gap, focusing on products, on mass and energy balances, and on the life cycle assessment for the energetic and environmental effectiveness. Attention is paid to energetic performances and end-products physic-chemical properties. European silver fir is prevalently used as model biomass in batch tests (T=200 °C and T=250 °C; τ=15÷300 min). Solids (hydrochar) and liquids (biocrude) are analysed for the key parameters (solid/liquid yields, elemental analysis, calorific value, energetic density). The possible effect of process pressure is investigated up to P=50 MPa. The obtained hydrochar pellets demonstrate good hydrophobicity, high mass and energy density and a considerable carbon densification, thus confirming potential heat and power applications of this matrix. Furthermore preliminary tests show that similar results can be obtained a variety of matrices such as wastes of the agrofood industry. LCA analysis of preliminary process layouts gives optimal energy recovery and environmental impact. Keywords: Torrefaction, biomass, life cycle assessment (LCA), agroindustrial residues.
1
INTRODUCTION
to face safety issues. Moreover, if compared to common organic waste treatments (e.g. composting or anaerobic digestion), HTC involves much lower residence times, which would result into reduced volumes of the equipment. The HTC process is also sufficiently flexible for managing variable biomasses, i.e. coming from different sources, as well as input feedstocks which can undergo broad variations of the chemical and physical characteristics. Therefore, to date, many authors have studied the possibility to exploit hydrothermal carbonization for organic waste management and treatment [15-20]. In this scenario, the correct developing of a process highly adaptable to variable waste materials is of the utmost importance, considering the fact that additional costs due to preliminary separations are not affordable. This latter topic deserves additional attention, and requires adequate research in the area of process engineering in order to optimize the production of an energy densified, uniform biomass starting from waste materials with a high grade of heterogeneity. The scientific literature suffers from a certain lack of data useful for designing adaptable processes. Moreover, no significant progress are reported on the way to a practical development of a continuously operating HTC process, which would be more suitable for recover economically energy from wastes. The present aims to direct our investigations toward this goals, and reports preliminary results on the wet torrefaction of a model organic substrate, silver fir. In this paper, the focus is on products, on mass and energy balances, and on the life cycle assessment for the energetic and environmental effectiveness. Special attention is paid to energetic performances and endproducts physic-chemical properties.
The negative impact of the increasing use of fossil fuels on environment security urges extremely the development of green renewable energy sources [1,2]. Biomass has been assigned many roles to play in strategies for sustainable and environmental friendly consumption. A major disadvantage for almost all applications is the high degree of heterogeneity in the form, composition and water content of biomass. Therefore, drying and/or conversion processes are usually required to improve material properties for easier handling, transport and storage of such materials [3,4]. A variety of thermochemical or biological processes can be used, in the absence of oxygen, to convert biomass into products with higher degrees of carbon content. Currently, researchers in many disciplines are involved in the search for environmentally sound processes and applications for biomass conversion. In hydrothermal processes, during the treatment the solid material is surrounded by water, kept in a liquid state by allowing the pressure to rise with the steam pressure in (high)-pressure reactors. As an example, during hydrothermal carbonization/ wet torrefaction, which is carried out in hot compressed water, the chemical structure of biomass is stimulated and the subsequent conversion of the resulting solid product is enhanced. In wet torrefaction processes, hemicellulose in biomass can be completely solubilized into aqueous compounds, the lignin seal is broken, while cellulose is almost entirely preserved in the solid product. Cellulose in the wet torrefied biomass is more readily accessible to enzyme, and the enzymatic digestibility of cellulose is enhanced. Thus hydrothermal carbonization is also considered as an effective pretreatment for biomass bioconversions [5-14]. The typical range of process parameters, such as temperature and the pressure, and the type of products obtained make HTC interesting for several industrial applications. With respect to other thermochemical processes, a HTC plant would require much reduced investment and operating costs, as well as easier solutions
2
MATERIALS & METHODS
2.1 Apparatus Fig. 1 shows the experimental apparatus, designed and constructed at the Department of Industrial and Information Engineering and Economics–University of
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L’Aquila, consisting of a stainless steel (AISI 316) batch reactor (V = 200 mL) and a heating system. Pipes of 1.5 mm internal diameter are connected to the reactor flanged cover (Fig. 2). V1 valve allows creating vacuum inside the reactor with P1 pump; V2 valve evacuates the HTC process gaseous end products, quantified by FRC. An internal embedded thermocouple passes through the reactor flanged cover, two other are inside the oven. Temperature and pressure signals are sent to the automatic controller.
were accurately weighted; after loading, the reactor was vacuum-sealed, and heated up to the desired set point, after which τ was computed. At the end of the experiment the reactor was quenched to 30 °C with air, valve V2 was opened to recover the excess gas phase, and finally was opened. Liquid and solid products were filtered and collected. The wet spent filter and the solid phase were weighed before and after drying (70 °C, 24 h). The hydrochar yield was computed. The hydrochar was stored in vials, before CHNS characterization. LHV were calculated according to the Du-Long equation. 2.3 LCA Procedure An unavoidable verification in the production of solid biofuels is to quantify the sustainability and the main environmental implications. This was performed with the LCA methodology. In particular, the commercial LCA software SimaPro7.0 was used along with CML-2 Baseline 2000 methodology applied to a plant with a potentiality of 1 ton/h of hydrochar.
3
RESULTS AND DISCUSSION
Fig. 3 shows the hydrochar yields obtained at 200 and 250 °C and the energetic density. As expected, the higher the T and τ, the lower the amount of hydrochar. The hydrochar yield was more sensitive to T than to τ. Table I shows the results obtained by CHNS analysis, while Fig. 4 reports the energetic yields calculated at 200 and 250 °C and all τ investigated. The results allow stating that:
Figure 1: 3D design drawing of the HTC reactor.
the hydrochar was much more carbon enriched at 250 °C than at 200; H and S decrease with T (200 °C: -8%, 20%; 250 °C: -22%, -35%); the energetic density was more sensitive to T than to τ; the hydrochar obtained at 200 °C and 30 min of τ maintained 99% of the energetic content of substrate.
LCA analisys results of the preliminary industrial process, presented in fig. 5, are reported in fig. 6. Fig. 7 shows the results of energetic balances:
Figure 2: P&ID of the laboratory apparatus. 2.2 Experimental Procedure The experiments aimed to get information on both the yield and the carbon, hydrogen, nitrogen and sulfur content of the hydrochar produced. Two different temperature were selected, (200 and 250 °C) and five process time, τ (30, 60, 120, 240, and 300 min). Water to dry biomass mass ratio was kept constant at 5. The model biomass tested was silver fir, which is a good representative of a typical organic waste. This material possesses sufficiently reproducible characteristics for correctly assess the effect of process parameters, and at the same time is not too simple to give results void of practical usefulness. Biomass and demineralized water
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Energy required is about 82% of hydrochar energy content; After proposed energy recovery, energy required decrease to 30%; The Non-renewable energy supplied represent about 4% of therenewable energy obtained.
24th European Biomass Conference and Exhibition, 6-9 June 2016, Amsterdam, The Netherlands
GAS
Electric Energy
Thermal Energy
Electric Energy
BIOCRUDE
BIOCRUDE BIOMASS
GRINDER
HTC REACTOR
DEHYDRATHOR HYDROCHAR
WATER HYDROCHAR
Figure 3: Solid Yield (kgHydrochar/kgSubstrate∙100) and Energetic Density (LHVHydrochar /LHVSubstrate) vs Time and T.
PELLET OF HYDROCHAR
T
200 30
60
120
30
60
120
C
53.61
53.12
54.4
69.18
70.25
70.69
5.84
5.79
5.79
4.84
4.92
5.00
0.81
0.87
0.81
0.68
0.69
0.68
10.4
9.4
12.1
42.5
44.7
45.6
-7.5
-8.2
-8.2
-23.3
-22.0
-20.7
-23.6
-17.9
-23.6
-36.3
-35.4
-35.8
%w/w
H %w/w
S %w/w
C %
H %
S %
WATER
Thermal Energy
250
min
DRYER HYDROCHAR
Electric Energy
Table I: Composition and Yield in C, H, S of Hydrochar vs Time and T. (µi=([ihydrochar]-[iSubstrate])/[iSubstrate]∙100)
°C
PELLETIZER
Figure 5: Block diagram of the proposed preliminary industrial process.
Figure 6: Results of LCA analisys (CML-2 Baseline 2000).
Figure 4: Energetic Yield of Hydrochar (Solid Yield ∙ Energetic Density) vs Time and T.
Figure 7: Results of Energetic Balances.
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24th European Biomass Conference and Exhibition, 6-9 June 2016, Amsterdam, The Netherlands
4
CONCLUSIONS
In this paper the technical characteristics of a benchscale batch reactor for HTC of wet biomass were described. The HTC reactor was utilized to perform the carbonization of two substrates in order to study the influence of the process parameters (T, P and τ) on the hydrochar produced. The results, both in terms of hydrochar yield and hydrochar chemical and thermal properties, showed that HTC represents an effective way to obtain a solid product with quite good characteristics as energy vector. In this perspective, the influence of P is negligible while high T and long τ increased the LHV of the hydrochar, although the yields resulted lowered. The grounds of these preliminary information are sufficiently robust for allowing the correct prosecution of the on-going investigation. The apparatus, specifically designed, proved to be a reliable and perfectly complies with the use of mixed wastes and for high pressure tests. Research is in progress aiming to test all of the process variable which contribute to define the correct design of a full scale plant for performing the flexible HTC of mixed waste biomass. The accuracy obtained during these preliminary experiments performed with the model biomass encourages to proceed with tests specifically aimed to assess the optimal values of the process parameters.
[9]
[10]
[11]
[12]
[13]
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[15] 5
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