The technologies for the conversion of biomass which includes biomass from landscape conservation and maintenance work into bioenergy including electricity, heat and fuel can be classified into two categories: Thermochemical conversion and Biochemical conversion.
The technologies for the conversion of biomass which includes biomass from landscape conservation and maintenance work into bioenergy including electricity, heat and fuel can be classified into two categories:
- Thermochemical conversion
- Biochemical conversion
The thermochemical technologies involve conversion of biomass to energy via chemical processes in addition to utilisation of heat in absence or presence of oxygen e.g. pyrolysis, combustion, torrefaction, gasification.
In the biochemical technologies enzymes, bacteria and other microorganisms break down biomass molecules and convert them to liquid fuels or biogas e.g. anaerobic digestion, fermentation and composting.
The selection of conversion technologies for a particular biomass is – among others – dependent upon its physical and chemical characteristics which determine the suitability of biomass as a feedstock for the system. A number of pre-treatments have been developed which can improve the biomass characteristics for efficient conversion processing as well as to make handling, transport or storage of biomass cost effective.
Biomass properties and their influence on conversion technologies
The performance of conversion pathways relies on the use of appropriate biomass feedstocks. Tanger et. al. (2013) described in their study the characteristics of lignocellulosic biomass which plays a major role on pairings of biomass feedstocks and advanced conversion technologies. Biopolymers (e.g., cellulose, lignin etc.), relative abundance of individual elements (e.g. C, H, O, N and S), relative proportions of fixed carbon (FC) and volatile matter (VM), moisture and elemental ash complete the mass balance of a unit of freshly-harvested biomass. Different combinations of these mass-based properties result in different bulk properties such as grindability (comminution), density and heating value.
The important feedstock properties that affect conversion effectiveness include:
- Heating or calorific value: It is the energy available in the feedstock and is a primary measure of quality of a feedstock. High moisture content and high mineral content leads to a decrease in heating value because minerals contribute little energy during biomass oxidation e.g. grasses and other herbaceous feedstock that can consist of up to 27 % ash have low heating values.
- Elements (C,H,O) ratio: The biochemical components in the cell wall influence the relative content of C, H, and O in biomass. Lignin has a lower H:C and O:C ratio than cellulose therefore, lignin is less oxygenated and has a higher heating value than cellulose or starch. High lignin containing biomass e.g. woody biomass is beneficial for thermal systems and advantageous for thermo-chemical conversion pathways targeting liquid fuels. Whereas, minimizing lignin in feedstock via pre-treatments improves hydrolysis and biogas yields. Feedstock with low lignin and cellulose and hemicellulose content, are more suitable for biochemical conversion technologies. Concentrations of non volatile fixed carbon and volatile matter are related to the relative yields and composition of solid, liquid, and gaseous products generated during thermochemical and biochemical conversion pathways.
- Mineral and elemental content: Mineral content of the biomass directly influence the operation of thermochemical conversion equipment besides lowering the heating value of biomass. During combustion, minerals present in plant biomass form a liquid slag or solid deposits as they cool down. High concentration of elements Na, K, Mg, Ca, Cl, S and Si in biomass are problematic for thermochemical processes. The ash content of grasses has a high proportion of Si that reacts with alkali metals like K and forms alkali silicates which increases slagging during thermochemical conversions. Similarly, high Cl containing biomass leads to elevated HCl and dioxin emissions and form corrosive deposits that degrade components of the boilers.
- Moisture Content: The amount of water in the biomass is usually expressed as percentage of total mass. The moisture content of biomass influence effectiveness of a conversion technology. Biomass with a moisture content of ~5 % is suitable for combustion or co-firing and for gasification biomass with a moisture content of around 20-30 % is acceptable. Wet biomass can be a suitable feedstock for hydrothermal combustion (HTC). Drying of biomass before transport maximizes its dry bulk density that allows more cost effective transportation. Reduction in moisture content of biomass may improve process efficiency for thermochemical pathways but it is not preferred for wet digestion processes. The grindability of biomass is also related to moisture content and composition of biomass.The degree of suitability of various conversion technologies with different types of LCMW feedstock is indicated briefly in the table below. An online tool to match the biomass with optimized conversion pathways has been developed within the framework of the EU funded project S2BIOM and is available at www.s2biom.eu.
|Conversion Technologies||Feed stock type|
|Woody Biomass||Herbaceous Biomass||Mixed Biomass|