Research and Simulation on Biomass Energy Generation System

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Figure 23.3-D Velocity Profile of Updraft with Air Volume Flow Rate of 0.008m3/s

Figure 24. Velocity Profile of Updraft with Air Volume Flow Rate of 0.012m3/s

Figure 25. 3-D Velocity Profile of Updraft with Air Volume Flow Rate of 012m3/s

Figure 26. Velocity Profile of Downdraft with Air Volume Flow Rate of 0.004m3/s

Figure 27. 3-D Velocity Profile of Downdraft with Air Volume Flow Rate of 0.004m3/s

Figure 28. Velocity Profile of Downdraft with Air Volume Flow Rate of 0.008m3/s

Figure 29. 3-D Velocity Profile of Downdraft with Air Volume Flow Rate of 0.008m3/s

Figure 30. Velocity Profile of Downdraft with Air Volume Flow Rate of 0.012m3/s

Figure 31. 3-D Velocity Profile of Downdraft with Air Volume Flow Rate of 0.012m3/s

Figure 32. Velocity Profile of Updraft with Air Volume Flow Rate of 0.004m3/s at 2 inlets (Radial)

Figure 33. Velocity Profile of Updraft with Air Volume Flow Rate of 0.004m3/s at 2 inlets (Radial)

Figure 34. Velocity Profile of Updraft with Air Volume Flow Rate of 0.004m3/s at 2 inlets (Tangent)

Figure 35. Velocity Profile of Downdraft with Air Volume Flow Rate of 0.004m3/s at 2 inlets (Tangent)

Tables

Table 1. Comparison of Three Main Thermochemical Conversion Processes

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1. Introduction

Biomass is the oldest form of energy used by humans and is classified as a renewable energy source. Biomass is a biological living material or recently living organism such as, wood, agricultural crops and wastes for a few examples (Figure 1). It is considered as a renewable energy as it does not require a long period of time for it to form such as fossil fuels which requires millions of years. Biomass is also regarded as a greenhouse gas neutral as the combustion of biomass does not increase the total carbon dioxide (CO2) inventory of the earth (Basu 2010).

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Figure 1. Examples of Biomass

The conversion of biomass into energy is done by breaking down the biomass into useful fuel or gas which contains heating value by the means of biochemical or thermochemical conversion. Biochemical conversion involves the uses of bacteria or enzymes to break down the composition of the biomass and does not require external energy. However this method requires a longer period of time compared to thermochemical as it undergoes digestion, fermentation and composting and is therefore regarded as inefficient. On the other hand, thermochemical conversion uses external heat to convert the biomass into useful fuel or energy.

The thermochemical conversion of biomass through gasification is one of the most promising non-nuclear forms of future energy (Babu 2005). Therefore, this paper will be focusing on the thermochemical conversion only.

1.1 Thermochemical Conversion

Thermochemical conversion of biomass is categorized into mainly three processes which are (1) pyrolysis, (2) gasification, and (3) combustion. Pyrolysis is the process where the biomass is deformed and broken down in the absence of oxygen in an elevated temperature into solid, liquid and gaseous products.

Gasification is the process whereby the products from pyrolysis namely solid carbonaceous fuels are converted into synthesis gas (syngas) by reacting it with a controlled amount of air or steam in high temperature. Gasification packs energy into chemical bonds in an oxygen deficient environment and requires heat. The product, syngas which is a mixture of carbon monoxide (CO) and hydrogen (H2), contains heating value and can be used as a fuel source or as an intermediate in producing other chemicals such as synthetic petroleum.

The general equation for the gasification reaction is shown as below,

Boudouard reaction: C + CO2 → 2CO + 172 kJ/mol (Eq. 1)

Water-gas reaction: C + H2O → CO + H2 + 131 kJ/mol

Combustion similar to gasification also converts carbonaceous materials into product gas but is different in the sense that (1) the product gas of combustion has no useful heating value, (2) combustion releases energy from chemical bonds, and (3) combustion requires oxygen and releases heat (Basu 2010). However, combustion is an optional process as it depends on the purpose of the biomass plant on whether it is use to produce syngas or heat for power generation.

The general equation for the combustion reaction is shown as below,

C + O2 → CO2 – 394 kJ/mol (Eq. 2)

Partial combustion reaction (Eq. 3) occurs between the combustion and gasification zones as the oxygen concentration is lower because it is mostly consumed during the combustion process.

C + ½ O2 → CO – 111 kJ/mol (Eq. 3)

Table 1 shows a summary of comparison of the three main thermochemical conversion processes in a gasifier.

Process

Temperature (°C)

Pressure (MPa)

Drying

Pyrolysis

250 – 330

0.1 – 0.5

Necessary

Gasification

500 – 1300

> 0.1

Necessary

Combustion

700 – 1400

> 0.1

Not essential, but may help

Table 1. Comparison of Three Main Thermochemical