Combustion properties of biomass

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Abstract

Properties of biomass relevant to combustion are briefly reviewed. The compositions of biomass among fuel types are variable, especially with respect to inorganic constituents important to the critical problems of fouling and slagging. Alkali and alkaline earth metals, in combination with other fuel elements such as silica and sulfur, and facilitated by the presence of chlorine, are responsible for many undesirable reactions in combustion furnaces and power boilers. Reductions in the concentrations of alkali metals and chlorine, created by leaching the elements from the fuel with water, yield remarkable improvements in ash fusion temperatures and confirm much of what is suggested regarding the nature of fouling by biomass fuels. Other influences of biomass composition are observed for the rates of combustion and pollutant emissions. Standardized engineering practices setting out protocols of analysis and interpretation may prove useful in reducing unfavorable impacts and industry costs, and further development is encouraged.

Introduction

The conversion of solar energy by plant photosynthesis yields the food energy supporting animal life on earth. Plant matter, or biomass, has also long served as one of the primary energy forms utilized by humans for essential activities aside from nutrition. It will continue to do so, in increasingly economic important ways, into the future. The energy invested by nature in the production of photosynthates is enormous; incident on the top of the atmosphere is a continuous radiant power of over 1017 W. Of this, plants collect and utilize 0.02%, producing a total annual energy storage of 1021 J. Plant photosynthesis also serves as the principal generator of atmospheric oxygen, critical to the respiration of plants and animals, as well as for the all important combustion reactions which drive modern human society. Prior to human industrialization, total energy in biomass was well in excess of human needs. Currently, the total is only a little better than three times the total human consumption of non-food commercial energy including all forms—fossil, nuclear, geothermal, gravitational, as well as solar. Biomass now contributes 6% of global non-food energy consumption, much of this through primitive low efficiency and highly polluting combustion in poorly controlled heating and cooking fires which support a major share of the world's population. Technical enhancements in the contribution of biomass to commercial energy needs are focused on improving both the efficiency and environmental impacts of biomass conversion. To this end, we seek a better understanding of the properties of biomass and their role in conversion. In this regard, we also seek an understanding of not only how the conversion technology can be adapted to fit the properties of the biomass fuel, but how properties of the fuel might be varied to suit the conversion technology of choice.

Plants rely on certain fundamental processes for growth and reproduction, yet have evolved to accommodate a great diversity of ecosystems and environmental conditions. Accordingly, they exhibit certain gross similarities in properties, yet with substantial specific variation. Many of these properties are critical for proper design and operation of conversion facilities, although not all properties are equally important for all conversion techniques. This paper attempts to briefly summarize some of the important properties for combustion, especially with respect to the use of biomass for electricity generation. As the biomass-fueled power generation industry has expanded in recent decades, the diversity of fuel types utilized has expanded as well, often with unanticipated and undesirable impacts on facility operation. Although the fundamental combustion behavior of biomass fuels has received increasing attention of late, there remains no comprehensive compilation of combustion properties, nor even a general recognition of the importance of certain properties for facility implementation and design. An objective of this paper is to stimulate the larger development of industry standards in the analysis, reporting, and interpretation of biomass properties.

Section snippets

Analytical methods

Combustion is a complex phenomenon involving simultaneous coupled heat and mass transfer with chemical reaction and fluid flow. Its prediction for the purposes of design and control requires knowledge of fuel properties and the manner in which these properties influence the outcome of the combustion process. A global reaction for the combustion of a biomass fuel in air might take the following form, where the first reactant compound is a biomass fuel:Cx1Hx2Ox3Nx4Sx5Clx6Six7Kx8Cax9Mgx10Nax11Px12

Composition of biomass

Photosynthesis results in the production of structural and non-structural carbohydrates comprising the plant tissues. The components of biomass include cellulose, hemicelluloses, lignin, lipids, proteins, simple sugars, starches, water, HC, ash, and other compounds. The concentrations of each class of compound varies depending on species, type of plant tissue, stage of growth, and growing conditions. Cellulose is a linear polysaccharide of β-D glucopyranose units linked with (1–4) glycosidic

The energy value of biomass

The standard measure of the energy content of a fuel is its heating value, sometimes called the calorific value or heat of combustion. In fact, there are multiple values for the heating value, depending on whether it measures the enthalpy of combustion, the internal energy of combustion, and whether, for a fuel containing hydrogen, product water is accounted for in the vapor phase or the condensed (liquid) phase. The enthalpy of combustion is determined at constant pressure, and so includes

Rates of combustion

In addition to the energy released by combustion, the rate of combustion is also important in the design of combustion systems. Occasionally, biomass combustion power plants have been observed to be underdesigned in terms of boiler volume and grate area for the rated capacity. Typical design heat release rates (expressed per unit grate area) for a stoker fired travelling grate combustor are in the range of 2 to 4 MW (thermal) m−2. A whole tree combustion concept (not yet built) utilizes a

Pollutant emissions

Critically related to the properties of biomass are pollutant emissions generated by combustion. Primary pollutants formed are particulate matter (PM), CO, HC, oxides of nitrogen (NOx, principally NO and NO2), and oxides of sulfur (SOx, principally as SO2). Acid gases, such as HCl, may also be emitted, as may lead and other heavy metals. CO and HC, including volatile organic compounds (VOC) and polycyclic aromatic hydrocarbons (PAH), are products of incomplete combustion. These species are

Engineering practice

This paper has briefly discussed only a few of the properties of biomass important to the design and development of combustion and other types of energy conversion systems. The literature is not entirely consistent in the reporting of properties, and there are not standard engineering practices yet available to which the industry can refer. The issue of standardizing biomass analysis methods has been addressed previously (e.g., Refs. 33, 34, 35). Some protocols have been defined to provide a

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