biomass
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Global energy consumption and energy-related CO2 emissions hit a record high in 2018, with the driving factors being stronger heating and cooling needs in some regions across the world. This calls for greater urgency in deploying cleaner resources and closing the carbon cycle, which brings to mind biomass technology.

This article first appeared in ESI Africa Edition 2, 2019. You can read the magazine's articles here or subscribe here to receive a print copy.

The increase in energy-related carbon emissions is outlined in the findings contained in the International Energy Agency’s Global Energy & CO2 Status Report. One overarching factor from the IEA report is the continual rise in global carbon emissions – once again demonstrating the urgency needed to develop cleaner energy solutions.

Currently, fossil fuels are the dominating source of energy; however, reserves are slowly depleting. Among the alternative energy sources, biomass is a promising sustainable energy source, due to its high diversity and availability. Different types of energy can be generated through the thermal conversion of biomass; such as combustion, pyrolysis, gasification, fermentation, and anaerobic decomposition. Out of all the various techniques of biomass conversion, the pyrolysis process offers a good number of benefits, including fewer emissions and the fact that all the by-products can be reused.

During the process, pyrolysis produces solid or carbonised products, liquid products (bio-oils, tars, and water) and a gas mixture composed mainly of CO2, CO, H2 and CH4. The oil resulting from the pyrolysis of biomass, usually referred to as bio-oil, is a renewable liquid fuel, which is the main advantage over petroleum products. It can be used for the production of various chemical substances.

The factors that influence the distribution of the products are the heating rate, final temperature, composition of the raw material, and pressure. The pyrolysis process has great market potential; in this process, biomass is used as raw material in order to produce energy. Therefore, intense research is taking place around the world to improve this method of energy production.

Advantages of pyrolysis technology

From a variety of technologies in the generation of energy from biomass such as digestion, fermentation and mechanical conversion, thermos-conversion for producing energy – the pyrolysis process – is relatively new from a commercial perspective, and is gaining more attention because of its technical and strategical advantages. Pyrolysis technology is the decomposition of heated organic matter in the absence of atmospheric oxygen, where heating is controlled by temperature ranges and provides the energy needed to break down the structures of the macromolecules present in biomass.

Though the research into pyrolysis technology indicates that pyrolysis is a more promising option for sustainable development, the technology still needs further improvement, and several challenges need to be tackled to gain its full potential benefits. Currently, there are three pyrolysis processes in the world: slow pyrolysis, fast pyrolysis, and ultrafast pyrolysis.

Slow pyrolysis

Slow or conventional pyrolysis consists of systems known as ‘charcoal’ or continuous systems, with slow biomass heating above 400°C in the absence of oxygen. In this process, the biomass is pyrolysed with low heating rates, around 5 to 7°C/minimum, where the liquid and gaseous products are minimal, and the coal production is maximised. Slow pyrolysis of wood, with a 24-hour endurance, was a very common technology in industries until the early 1900s, where coal, acetic acid, methanol, and ethanol were obtained from wood. Slow pyrolysis is characterised by small heating rates and a maximum temperature range of around 600°C, and the biomass time in the reactor is between 5 and 30 mininutes. The main products are bio-oil, coal, and gases.

Rapid pyrolysis

This type of process is a promising method for conversion of biomass into a liquid product. The produced pyrolysis oil (bio-oil) is an intermediate dense energy fuel, which is possible to upgrade to hydrocarbons in diesel and gasoline. In rapid pyrolysis, the biomass decomposes very quickly, generating mainly vapours and aerosols, and a small amount of coal and gas.

After cooling and condensation, a homogeneous mobile dark brown liquid is formed, which has a calorific value corresponding to half of the conventional fuel oil. Rapid pyrolysis technology is used globally, in large scale, for the production of liquids (bio-oils), and there is a lot of interest regarding this technology among biofuel researchers. Several reactors are used in the rapid pyrolysis process. Among them are the dragged-flow reactor, vacuum furnace reactor, vortex reactor, rotary reactor, and bubbling fluidised bed reactor to mention a few.

Ultrafast pyrolysis

The ultrafast pyrolysis has very high heating rates and very low residence time of the biomass in the reactor. These characteristics favour the production of vapours, and make the process very similar to gasification. Due to the high heating rate, where biomass residence times are only a few seconds, reactors are needed to meet these heating needs. These reactors have a fluidised bed and are flow-dragged. The fluidised bed reactor is used in the execution of multiphase chemical reactions, where a catalyst, usually sand, is used, working the same as with a fluid inside.

In conclusion, the pyrolysis of biomass produces bio-oil, also known as pyrolysis oil, which has a number of applications. These applications can be improved to be used as a transport fuel and it can also be used in turbines and electric power generation engines, or in boilers to generate heat. ESI

This article first appeared in ESI Africa Edition 2, 2019. You can read the magazine's articles here or subscribe here to receive a print copy.

Acknowledgment

This article is based on the article Overview of Recent Developments in Biomass Pyrolysis Technologies, by N. Uddin, M & Techato, Kuaanan & Taweekun, Juntakan & Rahman, Md. Mofijur & Rasul, Mohammad & Mahlia, T M Indra & Rahman, S M Ashrafur. (2018). It is republished with edits under CC BY 4.0, under license Energies.