Hydrothermal carbonization of mixed biomass: experimental investigation for an optimal valorisation of agrofood wastes

Hydrothermal carbonization (HTC) of biomass is a well-known thermochemical process for increasing the energetic density of organic feedstock. Data in the literature mainly concern batch reactor tests and some pilot plants, and do not allow designing flexible processes, nor to move to the continuous. 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. Different biomasses are compared in terms of energetic performances and end-products physic-chemical properties. European silver fir is prevalently used as model biomass in batch tests (200 and 250 °C; from 15 to 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 50 MPa. The pellets produced in situ demonstrate good hydrophobicity, high mass and energy density and a considerable carbon densification. LCA analyses of preliminary process layouts give optimal energy recovery and environmental impact: the non-renewable energy request and the emission of CO2eq are reduced of 95 and 93%, respectively. Hydrochar pellets turn out suitable for heat and power applications, thus allowing the exploitation of matrices such as wastes of the agro-food industry. PURPOSE OF THE WORK To develop a new mixed-biomass solid biofuel obtained by HTC and following densification, capable of competing with the traditional woody pellet with regard to calorific value, energetic density and to the environmental impact. Also, this research aims to get additional basic knowledge for designing satisfactory the industrial process, identifying a set of optimal operating parameters, assessing possible layouts and keeping a watch on a possible evolution toward the continuous process. APPROACH Batch experimental tests are focused to ascertain the relative influence and the applicable ranges of the process parameters on the characteristics of the final product. The basic information is obtained mainly using the model biomass, while tests for possible process improvements compare different waste biomasses in terms of energetic performances and physic-chemical properties of the end-products. The energetic and environmental effectiveness of the process layout is improved with the LCA methodology. SCIENTIFIC INNOVATION AND RELEVANCE The scientific innovation and relevance base on the idea to prove the feasibility of producing industrially an improved solid bio-fuel, comparable to traditional woody pellet, using largely or totally a variety of wastes from different sources: cultivation and maintenance of trees and other agro-industrial activities, non lignocellulosic plants, and possibly the organic fraction of municipal solid wastes. CONCLUSION AND RESULTS A solid bio-fuel, obtained from woody material mixed with other vegetables wastes is proposed as an alternative to traditional woody pellet. The technical feasibility was proved experimentally at laboratory scale using a device designed on purpose. A semi-continuous process layout for the industrial production of hydrochar pellet is described, assessed and optimizated by LCA analysis, implemented on a commercial software package, permits the optimization of a first-level process layout specially in terms of net renewable energy value and environmental impact, reducing of 95 and 93% the non-renewable energy request and the emission of CO2eq respectively of the production process.

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; t=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.