Biomass is plant-based or animal-based material used as fuel to produce heat or electricity.
The most common biomass materials used for energy production are plants., wood and agricultural products, solid waste, landfill gas and alcohols.
The energy from these materials can be converted into usable energy in two ways: direct (heat, electricity) or indirect (biofuel production). Biomass can also be simply burned by thermal convection in process of firing or co-firing (with fossil fuel).
The methods to produce biofuels or coal as a energy source are pyrolysis (feedstock is heated up to 200o-320 oC in oxygen-free conditions), gasification (feedstock is heated up to 700 oC, controlled atmosphere) or anaerobic decomposition (in presence of bacteria, result is methane). Biomass can be also converted into Ethanol or Biodiesel directly. Products of pyrolysis are pyrolysis oil (liquid, can be burned), synthetic gas (can be converted into methane) and biochar (can be used in agriculture). Products of gasification is syngas, mixture of hydrogen and carbon monoxide. Syngas can be used for energy or heat generation. Liquid products of biomass conversion – ethanol and biodiesel can be used for powering vehicles or burn for heat and/or energy generation. However, one can use biomass in gasification process to produce hydrogen for cells and use clean hydrogen to generate power and fuel vehicles.
The proper characterization of biomass is an important aspect for producing energy out of it.
All processes in elevated temperature require knowledge about thermophysical properties of biomass samples. Thermal convection procedures consist of many steps incl. drying, evaporation, fusion, sorption-desorption, decomposition, torrefaction and combustion. There is also a strong influence of oxygen on the pyrolysis or gasification process we can reproduce in laboratory conditions.
It is possible to analyse biomass behaviour using the methods of thermal analysis, mainly TGA and DSC. Pressure- and temperature-controlled experiments provide capacities to model and scale-up industrial processes. Heat and energy generation can be optimized since biomass properties affect the conversion pathway, yield and quality of products and processing costs.
Thermal analysis can be also very useful for biomass characterisation. TGA (thermogravimetry) records mass change, DSC (Differential scanning Calorimetry) measures heat flow and enthalpy of changes, TMA (Thermomechanical analysis) measures change of dimension, deformations, penetration, bending, Dilatometry records change in dimensions/volume, EGA (evolved gas analysis) gives valuable information about gas products released in heating process. All measurements are recorded as function of temperature in controlled conditions (temperature, pressure, gas flow, presence of oxygen). Methods above can be combined (TGA-DSC-EGA).
The standard testing procedures were described in ASTM:
ASTM D2584 – Ignition Loss of cured Reinforced Resins
ASTM E1131 -Compositional analysis by Thermogravimetry
ASTM E1641 -Decomposition Kinetics by Thermogravimetry using Ozawa/Flynn/Wall Method
ASTM E2008 – Volatility Rate by Thermogravimetry
Thermogravimetry allows to classify chemical changes in sample, based on TG signal (desorption, single or multistage cracking, single or multiple reactions). There are several factors affecting on TG curves: heating rate (decomposition kinetics, pyrolysis products ratio), sample size (reaction rate, diffusion rate, temperature distribution), atmosphere (decomposition products ratio), pressure or gas flow (decomposition products ration, reaction rate).
Thermogravimetry can be a tool for investigation of the thermal stability (decomposition studies), oxidative stability (storage life) and composition analysis.
One special application of a thermo-balance is a coal gasification or biomass gasification experiment where carbon containing samples undergo a reaction (with or without pressure ramp) in hot water vapor to form CO and CO2 out of the bound carbon. This setup is a small scale experiment of big reactor processes that are commonly used for biomass conversion and can therefore easily be optimized.
DSC can be considered as useful method for biomass analysis to characterize heat flow and heat capacity in process of pyrolysis and combustion. DSC curve indicates the reaction pathway of the biomass. DSC allows to evaluate the effect of evaporation on the deviation of the sample temperature from the heating gas temperature and adsorption energy on the surface of the sample. It gives information on the endothermic/exothermic nature of the processes, thus contributing significantly to interpret the weight changes events detected by the TG curves. Also, DSC curves can be used as discriminant characteristics (i.e. for lignin/hemicellulose).