Theoretical Considerations about the Steady State Combustion of Wood Char in a Bubbling Fluidized Bed Reactor

Carlos Pinho

Transport Phenomena Research Center, Department of Mechanical Engineering, Faculty of Engineering, University of Porto, Porto, Portugal.

DOI: 10.4236/epe.2013.53021                

Abstract

A theoretical study on the performance of steady state bubbling fluidized bed burners is presented using a simple mathematical model. The proposed model has pedagogical and practical advantages due to its simplicity. The calculations, whose results are plotted in several graphics, were based on data obtained in laboratory scale experiments. The experiments were carried out with wood chars and the model allows a proper evaluation of physical and chemical phenomena taking place inside the reactor, as well as a fast approach to the pre-design phase, before going towards more complex and time consuming numerical modeling. In the first part of the paper the steady state modeling is compared with the combustion of successive batches of char particles. Afterwards, the performance of a 1 m diameter bed operating from 700℃ to 800℃ is shown.

Keywords

Biomass; Wood Char; Fluidized Bed

1. Introduction

The urgent need to use energy sources that could guarantee a reduction or the control of CO₂ emissions has led many researchers to look for the renewable sources of energy, like biomass [1]. Wood is the oldest and the most relevant solar energy conveyor known and used by mankind [2-7]. Presently more than 15% of the primary energy consumed by mankind comes from the combustion of wood or ligneous residues [8], but the efficiency of combustion under these circumstances is low. The interest on this energy resource is raising and European Union targets as far as bioenergy is concerned are rather ambitious [9].

The use of biomass as an energy source in a country like Portugal, through the combustion of wood, can be considered under two ways, fast growing plant species exclusively dedicated to combustion leading to an exploitation rate of about 80 t/ha/year, or the use of the forest for other ends and using the corresponding ligneous forest cleaning waste of about 2 to 3 t/ha/year. These numbers are typical values for the Atlantic coast of the Iberian Peninsula [10-13]. In the use of forest cleaning residues it is necessary to avoid soil depletion of minerals and nutrients [14]. The forest cleaning process can then be considered, under very limited restrictions, a source of biofuel easing the economic costs of the cleaning operation [15]. In either of these situations the main attracttiveness is on the achievement of a closed carbon cycle [16-19].

In developing countries biomass is frequently the most important source of primary energy corresponding to 35 % of their energy needs [8,9,15]. In the developed countries the increase of the efficiency of biomass utilization is one important target. In the USA the efficiency of a power plant stays around 20% - 25% based on the higher calorific value of the biomass, while forecasts concerning the maximum efficiency through biomass gasification plants can go up to 43% [17,20].

Fluidized bed burners are flexible in terms of fuel quality and have a relatively fast response to load variations [2-6]. In fluidized bed combustion, fuel particles below 25 mm burn inside a bed of inert particles commonly of 0.5 to 1 mm diameter [21]. The amount of fuel particles is a small mass fraction of the bed, around 1%. The solid fuel is sent to the bed and is rapidly heated up to the bed operating temperatures, this provokes a strong devolatilization of the fuel particles and the majority of the volatiles burn above the free surface of the bubbling bed, while the charcoal solid core of the particle stays inside the bed while burning [22-32]. With this type of low temperature combustion (800˚C to 900˚C), low levels of NOx emissions are obtained. Other advantages are the high heat transfer coefficients that can be obtained if the heat transfer surfaces are placed inside the bubbling bed [33], a relatively wide operating ratio and its ability to burn different fuels with high combustion efficiency, namely low calorific value fuels as biomass [34]. The rice husk is a good example, it cannot be efficiently burned in conventional furnaces whereas it has been burned with combustion efficiencies of 95% to 99% in fluidized bed burners [35].

When the sulfur content of the biomass is low the co-combustion of wood and coal is very attractive by keeping the sulfur oxides concentration in the flue gases under acceptable values [36]. Also due to the low nitrogen concentration of the biomass, the co-combustion can lead to a reduction of the formation of NOx [36-38].

Due to the high flexibility of fuel utilization the fluidized bed combustion is becoming a reference in the combustion of coal and biomass [21].

2. Steady State Combustion of Coke or Char Particles in a Fluidized Bed

In continuously working furnaces, the fuel supply can be considered in steady state regime although changes arrive according to the energy demands from the boiler user. As the fuel concentration inside the bubbling bed is very small, many authors consider that the individual behavior of a burning particle is not affected through interaction with other fuel particles and that the steady state combustion can be considered as combustion of a sequence of batches of solid particles [21,39]. There are however more or less elaborated models for this steady state combustion process [39], but such models are very complex.

Here a simple mathematical model for the steady state combustion is proposed. It is an evolution of a mathematical model for the combustion of batches of coke or char particles in bubbling fluidized bed reactors [22-30]. This model can be based upon experimental data obtained in laboratory studies of the combustion of batches of coke or char particles and is simple enough to allow the students to rapidly grasp the relative importance of the different phenomena taking place during the combustion process.

The fluidized bed reactor is considered an isothermal reactor and the burning particles are at bed temperature. This supposition can lead to some errors, for coke particles burning in a bubbling fluidized bed at 930˚C Roscoe et al. [40] verified experimentally that particles could be burning at temperatures around 130˚C to 160˚C above the bed temperature. This was obtained from visual analysis of the particles floating at the bed surface; it may be possible that they are not representative of the overall behavior of the particles that compose the majority of the batch under combustion. There is some contradiction among several authors that more recently looked at this subject [21,41]. For example, Khraisha [42] considers that particles burn at bed temperature, while Komatina et al. [43] consider that the type of coal, the batch size and the O₂ concentration, all are important to define the evolution of the temperature of the particles during the combustion process. Here it is considered that the combustion takes place in isothermal conditions.

A solid particle undergoing a combustion process, Figure 1, takes an elemental time dt to suffer an elemental reduction of its diameter [22-30],

(1)

and taking into account the overall resistance to combustion

(2)

But this particle belongs to a flow of particles that is continuously introduced into the bed and they compete among them for the available oxygen. To account for such competition a multiplying factor is used for the O₂ concentration [44],

(3)

A value of close to one means that the inter particles competition for the available oxygen is small, whereas low values of mean low oxygen availability. There is some subjectivity on the choice of values, but this it is nonetheless a simple and attractive way to quantify the importance of the easiness of access to the available O₂.

Figure 1. Schematic representation of the combustion of a carbon particle.

Figure 2 shows the O₂ balance inside the fluidized bed. Oxygen enters the bed through the fluidizing air and is distributed into the dense and the bubble phase [45]. In the dense phase the O₂ concentration is uniform and equal to, for the bubble phase the oxygen concentration is given by

Conflicts of Interest

The authors declare no conflicts of interest.

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