Abstract
The growing need for food, energy and materials demands a resource efficient approach as the world’s population keeps increasing. Biochar is a valuable product that can be produced in combination with bio-energy in a cascading approach to make best use of available resources. In addition, there are resources that have not been used up to now, such as, e.g., many agro-residues that can become available. Most agro-residues are not suitable for high temperature energy conversion processes due to high alkali-content, which results in slagging and fouling in conventional energy generation systems. Using agro-residues in thermal processes, therefore, logically moves to lower temperatures in order to avoid operational problems. This provides an ideal situation for the combined energy and biochar production. In this work a slow pyrolysis process (an auger reactor) at 400 °C and 600 °C is used as well as two fluidized bed systems for low-temperature (600 °C–750 °C) gasification for the combined energy and biochar generation. Comparison of the two different processes focuses here on the biochar quality parameters (physical, chemical and surface properties), although energy generation and biochar quality are not independent parameters. A large number of feedstock were investigated on general char characteristics and in more detail the paper focuses on two main input streams (woody residues, greenhouse waste) in order to deduct relationships between char parameters for the same feedstock. It is clear that the process technology influences the main biochar properties such as elemental- and ash composition, specific surface area, pH, in addition to mass yield quality of the gas produced. Slow pyrolysis biochars have smaller specific surface areas (SA) and higher PAH than the gasification samples (although below international norms) but higher yields. Higher process temperatures and different gaseous conditions in gasification resulted in lower biochar yields but larger TSA, higher pH and ash contents and very low tar content (16-PAH). From the feedstock data looked at in more detail, a few trends could be deducted in the attempt to learn how to steer the biochar characteristics for specific uses.
Highlights
IntroductionCo-production of bioenergy and products in a cascading approach will achieve a more optimized use of resources while at the same time limiting the overall greenhouse gas (GHG) emissions
As the data from ultimate analysis shows, pyrolysis and gasification leads to release of H and
Special attention is given to the extraction process of the biochar in order to avoid any adsorption of hydrocarbons on the biochar surface
Summary
Co-production of bioenergy and products in a cascading approach will achieve a more optimized use of resources while at the same time limiting the overall GHG emissions. Resources not used up to now, such as, e.g., many agro-residues need to be taken into account and new strategies must find a balance in agricultural applications, energy plus materials production and environmental effects. The use of biochar as a soil amendment is a promising agricultural practice that reduces N losses achieving a more effective N fertilizer use, and at the same time promotes soil organic carbon (SOC) accumulation in soils [3]. The application of biochar as a soil amendment could even bring areas lost for agriculture back into production by compensating for acidity, too low organic carbon and water retention [4]
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