Mixed biomass pellets for thermal energy production: A review of combustion models

doi:10.1016/j.apenergy.2014.04.042

Applied Energy

Volume 127, 15 August 2014, Pages 135-140

https://doi.org/10.1016/j.apenergy.2014.04.042 Get rights and content

Highlights

•
The use of biomass is increasingly important for clean energy generation from renewable sources.
•
An analysis of the current situation of the production of pellets (mixed biomass types) is provided.
•
The main emphasis is on the review of different combustion processes.
•
The evaluation of each individual stage of the combustion process is very important.

Abstract

The need to generate thermal and electrical energy, global warming caused by increased emissions of greenhouse gases, rising fossil fuel prices and demand for energy independence, have created a new industry focused on energy production through the use of renewable sources. Among the different options, biomass is the third most important source for obtaining electricity, and is the main source for the production of thermal energy. However, problems related to the low density of the different types of biomass, and the difficulty of transportation and storage, have led to the need to find solid fuels with higher density and greater hardness, known as pellets and briquettes. This paper seeks to develop an analysis of the current situation of the production of pellets, mainly with mixed biomass types, and the possible uses they have, with the main emphasis on the review of different combustion processes.

Introduction

In recent times, humanity’s development has been directly related to energy production, both for use as electricity and for thermal applications [1]. However, the increase in energy production has caused a considerable increment in the emission of greenhouse gases generated by fossil fuels such as coal, oil and natural gas. In fact, the production of CO₂ has grown from 4 million tons/year to more than 28 million tons/year over the past 60 years [2].

Due to high levels of CO₂, global warming and the rising cost of fossil fuels, the need to find clean and renewable new sources of energy has become imperative. This is reflected in the increasing investment in renewable energy projects all over the world [3]. This has allowed the creation and development of new technologies and new industries devoted to energy generation from renewable sources, representing presently more than 3% of the global energy produced from all sources [4].

Biomass comprises compounds resulting from photosynthesis processes and due to its carbon content it may produce energy by heat or chemical processes [5].

Among the main advantages of using this type of energy source are: the permanent availability with large amounts of biomass growing, low levels of greenhouse gas emissions produced by the processes of transformation and the low cost of recollection. It is possible to produce various types of solids, liquids and gaseous biofuels from biomass, such as briquettes, pellets, charcoal, alcohols, pyrolysis oils, biogas and biohydrogen, among others [6].

The main objective of this paper is to develop an analysis of the current situation of production of mixed biomass pellets (MBP), and their possible uses in thermal and electric energy production, with the main emphasis on the review of different combustion processes.

Section snippets

Pellets production

To evaluate biomass potential, it is necessary to consider several aspects. Among these are some physicochemical properties such as moisture content, carbon content, heating value and density, which are very important since they determine the use and the actual application that can be given to certain types of biomass [7].

By analysing the high heating value (HHV), it is possible to see how biomass can release large amounts of energy generated per kilogram of substance during the combustion

Biomass combustion

Combustion is a complex phenomenon that involves successive homogeneous and heterogeneous reactions. Over the years, many studies have been conducted in order to provide mathematical models that express the combustion of biomass [29].

Since biomass fuels are primarily composed of carbon, hydrogen and oxygen, the main products from burning biomass are carbon dioxide and water. Flame temperatures can exceed 2000 °C, depending on the heating value and moisture content of the fuel, the amount of air

Process analytical models

Biomass combustion is carried out in equipment that varies according to the particle size, and the residence time of the oxidizing agent, among other variables. The main types of combustion equipment are fixed bed (grates) in which the solid material remains firm and the oxidizing agent flows through the spaces generated by the solid particles to carry out the reactions, and fluidized bed equipment in which the bed is moving freely within the oxidant due to the pressure of the latter.

Conclusions

The use of biomass is increasingly important for clean energy generation from renewable sources. An important aspect is the need to increase the density of several wood waste forms, mainly from agriculture and forestry, which have generated a growing industry in Europe, Canada and the United States that is capable of producing about 10 million tons of biomass pellets per year. One of the main problems facing the pellets industry is the fact that the vast majority of these are being produced

Acknowledgements

This work was supported by FEDER funds (European Union) through COMPETE, and by Portuguese funds through FCT, under Projects FCOMP-01-0124-FEDER-020282 (Ref. PTDC/EEA-EEL/118519/2010) and PEst-OE/EEI/LA0021/2013. Also, the research leading to these results has received funding from the EU Seventh Framework Programme FP7/2007-2013 under Grant agreement No. 309048.

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Mixed biomass pellets for thermal energy production: A review of combustion models

Highlights

Abstract

Introduction

Section snippets

Pellets production

Biomass combustion

Process analytical models

Conclusions

Acknowledgements

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