**1.2. Fluidised Bed Combustion (FBC) technology**

Fluidised bed combustion technology is one of the most significant recent developments in both coal and biomass incineration over conventional mass burning incinerator designs. This technology has been accepted by many industries because of its economic and favoura‐ ble environmental consequences.

The major advantages of fluidized bed combustors are [8,9]:


Sets against these advantages are the following disadvantages:


#### **1.3. Combustion studies**

mass with coal must be recognized as one of the most important source of energy for the

A recent study shows that Malaysia has been one of the world's largest producers and ex‐ porters of palm oil for the last forty years. The Palm Oil industry, besides producing Crude Palm Oil (CPO) and Palm Kernel Oil, produces Palm Shell, Press Fibre, Empty Fruit Bunch‐ es (EFB), Palm Oil Mill Effluent (POME), Palm Trunk (during replanting) and Palm Fronds (during pruning). Almost 70% of the volume from the processing of fresh fruit bunches (FFB) is removed as waste.Malaysia has approximately 4 million hectares of land under oil palm plantation. Over 75% of total area planted is located in just four states, Sabah, Johor, Pahang and Sarawak, each of which has over half a million hectares under cultivation. The total amount of processed FFB was estimated to be 75 million tons while the total amount of EFB produced was estimated to be 16.6 million tons. Around 58 million tons of POME is produced in Malaysia annually, which has the potential to produce an estimated 15 billion

Rice husk is another important agricultural biomass resource in Malaysia with good potential for power cogeneration. An example of its attractive energy potential is biomass power plant in the state of Perlis which uses rice husk as the main source of fuel and generates 10 MW pow‐ er to meet the requirements of 30,000 households. The US\$15 million project has been under‐ taken by Bio-Renewable Power Sdn. Bhd in collaboration with the Perlis state government, while technology provider is Finland's Foster Wheeler EnergiaOy. Under the EC-ASEAN Co‐ generation Program, there are three ongoing Full Scale Demonstration Projects (FSDPs) – Titi‐ Serong, Sungai Dingin Palm Oil Mill and TSH Bioenergy – to promote biomass energy systems in Malaysia. The 1.5MW TitiSerong power plant, located at Parit Buntar (Perak), is based on rice husk while the 2MW Sungai Dingin Palm Oil Mill project make use of palm kernel shell and fibre to generate steam and electricity. The 14MW TSH Bioenergy SdnBhd, located at Ta‐ wau (Sabah), is the biggest biomass power plant in Malaysia and utilizes empty fruit bunches,

Fluidised bed combustion technology is one of the most significant recent developments in both coal and biomass incineration over conventional mass burning incinerator designs. This technology has been accepted by many industries because of its economic and favoura‐

**•** Uniform temperature distribution due to intense solid mixing (no hot spots even with

**•** FBC systems have a very short residence time for their fuels (making these systems

foreseeable future.

390 New Developments in Renewable Energy

m3

**1.1. Biomass as potential renewable resources**

of biogas can be produced each year [6].

palm oil fibre and palm kernel shell as fuel resources [7].

The major advantages of fluidized bed combustors are [8,9]:

**1.2. Fluidised Bed Combustion (FBC) technology**

ble environmental consequences.

**•** strongly exothermic reactions **•** High combustion efficiencies

Fluidized bed combustion of alternative solid fuels (including biomass) are attractive as a result of the constantly increasing price of fossil fuels, the presence of high quantities of wastes to be disposed of and global warming issues. Extensive experimental investigation has been carried out to date on the feasibility and performance of different biomass fuels FB combustion such as rice husk [10-13], animal waste [14-15], municipal solid waste (MSW) [16-19] and Refuse Derived Fuel (RDF) [5]. In whatever form biomass residues are fired (loose, baled, briquettes, pellets), a deeper understanding of the combustion mechanisms is required in order to achieve high combustion efficiency and to effectively design and oper‐ ate the combustion systems. The combustion properties and their effect on combustion mechanisms are all important information required to understand the combustion charac‐ teristics of biomass residues and their co-combustion with coal in FBC.

In general when a single coal or biomass particle enters a fluidised bed furnace, then three phenomena occur namely [20-22]:


Fuel properties and combustion operating parameters significantly influenced the combus‐ tion efficiency and emissions. Armesto et al (2002) has stated that the bed temperature has an effect on combustion efficiency, which improves from 97% to 98% as bed temperature in‐ creased from 840 to 880°C. Also, they found that the efficiency increased with decreasing fluidising velocity. They claimed that when fluidisation velocity increased above 1.0 m/s, the combustion efficiency decreased. This behaviour was attributed to an increase in the elutria‐ tion of unburned carbon. On the contrary, Suthum (2000) found that the combustion effi‐ ciency increased from 88% to 92% with increasing excess air up to 30% during combustion of oil palm waste in a 10 kW FBC with over-bed feeding. Saxena et al., 1993 reported similar results. It was suggested that there is an optimum balance between the carbon to CO conver‐ sion rate and increased elutriation with high excess air [22]. Furthermore, Fahlstedt et al (1997) performed a series of tests on co-firing wood chips, olive pit and palm nut shell with coal in 1MW FBC facility. It was noted that the co-combustion had a slightly higher carbon combustion efficiency based on flue gas emissions (97.2 -98.1%) than coal-only combustion (97.1%) [22]. The reason is likely due to the higher volatile matter content of the biomass fuels. Increased volatile matter will also increase the fuel reactivity and hence reduce the un‐ burned carbon. In contrast, a decrease in combustion efficiency was obtained by Armesto et al (2003) and Suksankraison et al (2004) during co-combustion of Lignite-olive waste and Lignite-MSW mixture, respectively, even though the volatility of the fuel used quite similar (60-70% VM). The decrease was mainly attributed to a drop in the bed temperature. Since most fixed carbon generally burns in the bed while the volatile gas burns in the freeboard, there is insufficient chance for CO conversion to CO2. As the freeboard temperature is main‐ tained at a higher value, devolatilisation occurred rapidly and produced more volatile gases. As the biomass fraction increased, the reduced fixed carbon gives more chance for the vola‐ tiles to escape combustion.

However, there was a significant improvement in CO emissions, particularly when the air to the freeboard was introduced at different heights (air staging). The CO levels were brought

Sustainable Power Generation Through Co-Combustion of Agricultural Residues with Coal in Existing Coal Power

ted by EU directives; 50 mg/N m3 at 11% O2.The trends observed during single fuel combus‐ tion are reflected also in co-combustion: in the practically important cases with moderate amounts of biomass (an energy fraction of less than 25%) the properties of the base fuels dominate the emission obtained. Suksankraisorn et al (2004) reported that for 100% lignite combustion, CO drops significantly as excess air increases due to the increased CO to CO2 conversion [16]. The emission of CO is relatively insensitive to changes in excess air and waste fraction, which further strengthen the argument that co-combustion is dominated by

The feasibility of FBC technologies has been widely demonstrated for the combustion of a variety of fuels. As a drawback, severe problems of agglomeration in the bed as well as foul‐ ing and slagging may sometimes occur, especially during combustion of biomass fuels as some agricultural residues have high contents of alkali oxides and salts, the low melting points of which may lead to various problems during combustion. Werther et al (2000) en‐ countered the problems of sintering and agglomeration during the combustion of coffee

Although there are many potential benefits associated with co-combustion, there are several combustion related concerns associated with the co-combustion of coal and biomass. Utiliza‐ tion of solid biomass fuels and wastes sets new demands for boiler process control and boil‐ er design, as well as for combustion technologies, fuel blend control and fuel handling systems. For example, the different mineral matter composition (high alkali levels) and mode of occurrence (mostly mobile forms) in biomass results in concerns over enhanced fouling and slagging of pulverized coal boilers, particularly when firing agricultural resi‐ dues or herbaceous materials. The economics of co-combustion in pulverized coal boilers are closely tied to the biomass preparation costs (i.e. drying and milling), so an improved un‐ derstanding of the effect of biomass particle size and moisture content on combustor per‐ formance is needed (e.g. in the areas of flame stability, flame shape, and carbon burnout).Thus, this research was carried out with the objective to characterise biomass prop‐ erties that affect the co-combustion of biomass with coal, in particular biomass that is availa‐

This research was performed with the objective to determine the combustion efficiency of the existing coal-fired combustor during co-firing agricultural residue with coal. The effi‐ ciency is calculated mainly based on the carbon monoxide and carbon dioxide emissions. Furthermore, this works also to demonstrate the technical feasibility of a fluidized bed as a clean technology for burning agricultural residues. In addition, the effect of biomass proper‐ ties such as such as particle size, particle density and volatility as well as influences of oper‐

at 11%O2 in the flue gases, which is very close to what is permit‐

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down to about 60 mg/N m3

**1.4. Problem statement**

ble in large quantities in Malaysia.

**1.5. Research scope and application**

the combustion of the volatiles in the freeboard zone.

husks in a 150 mm diameter fluidized bed combustor [24].

Significant fluctuations of CO emissions were reported during co-combustion of biomass in a FBC. The value of the CO concentration in the flue gas has been found to depend on the type of fuel, fuel properties (volatility, particle size and density) and the operating condi‐ tions (bed and freeboard temperature, excess air, secondary air). In addition to the expected immediate ignition and the high volatile matter contents, the volatiles consist mainly of the combustibles (CO, H2, CxHy). These factors together indicate that the combustion of the vol‐ atiles would be the dominant step during the biomass combustion. At higher temperatures, the combustibles (CO, H2, CH4) accounted for more than 70–80 % of the gas components [23]. Saxena et al (1994) found that the hydrodynamic activity in the bed is related to the sol‐ id mixing and gas-solids contacting and these in turn are directly related to CO emissions. Higher bed temperature seems to provide optimum conditions for rapid de-volatilisation and hence increased conversion CO to CO2 [19]. They found that in the turbulent regime, the carbon utilisation efficiency reached a maximum and a further increase in the fluidisation velocity had an insignificant influence on the bed hydrodynamics and hence CO emissions. Similarly, as most of the biomass combustion was observed to take place in the freeboard, the supply of oxygen to this zone in amounts sufficient to achieve satisfactory combustion had to be ensured. Furthermore, Sami et al (2001) found that if the level of CO was within acceptable limits, then approximately 10% excess air and a temperature of 650°C provided optimum conditions for the combustion of manure in a fluidised bed unit [17].

However, there was a significant improvement in CO emissions, particularly when the air to the freeboard was introduced at different heights (air staging). The CO levels were brought down to about 60 mg/N m3 at 11%O2 in the flue gases, which is very close to what is permit‐ ted by EU directives; 50 mg/N m3 at 11% O2.The trends observed during single fuel combus‐ tion are reflected also in co-combustion: in the practically important cases with moderate amounts of biomass (an energy fraction of less than 25%) the properties of the base fuels dominate the emission obtained. Suksankraisorn et al (2004) reported that for 100% lignite combustion, CO drops significantly as excess air increases due to the increased CO to CO2 conversion [16]. The emission of CO is relatively insensitive to changes in excess air and waste fraction, which further strengthen the argument that co-combustion is dominated by the combustion of the volatiles in the freeboard zone.

The feasibility of FBC technologies has been widely demonstrated for the combustion of a variety of fuels. As a drawback, severe problems of agglomeration in the bed as well as foul‐ ing and slagging may sometimes occur, especially during combustion of biomass fuels as some agricultural residues have high contents of alkali oxides and salts, the low melting points of which may lead to various problems during combustion. Werther et al (2000) en‐ countered the problems of sintering and agglomeration during the combustion of coffee husks in a 150 mm diameter fluidized bed combustor [24].

### **1.4. Problem statement**

an effect on combustion efficiency, which improves from 97% to 98% as bed temperature in‐ creased from 840 to 880°C. Also, they found that the efficiency increased with decreasing fluidising velocity. They claimed that when fluidisation velocity increased above 1.0 m/s, the combustion efficiency decreased. This behaviour was attributed to an increase in the elutria‐ tion of unburned carbon. On the contrary, Suthum (2000) found that the combustion effi‐ ciency increased from 88% to 92% with increasing excess air up to 30% during combustion of oil palm waste in a 10 kW FBC with over-bed feeding. Saxena et al., 1993 reported similar results. It was suggested that there is an optimum balance between the carbon to CO conver‐ sion rate and increased elutriation with high excess air [22]. Furthermore, Fahlstedt et al (1997) performed a series of tests on co-firing wood chips, olive pit and palm nut shell with coal in 1MW FBC facility. It was noted that the co-combustion had a slightly higher carbon combustion efficiency based on flue gas emissions (97.2 -98.1%) than coal-only combustion (97.1%) [22]. The reason is likely due to the higher volatile matter content of the biomass fuels. Increased volatile matter will also increase the fuel reactivity and hence reduce the un‐ burned carbon. In contrast, a decrease in combustion efficiency was obtained by Armesto et al (2003) and Suksankraison et al (2004) during co-combustion of Lignite-olive waste and Lignite-MSW mixture, respectively, even though the volatility of the fuel used quite similar (60-70% VM). The decrease was mainly attributed to a drop in the bed temperature. Since most fixed carbon generally burns in the bed while the volatile gas burns in the freeboard, there is insufficient chance for CO conversion to CO2. As the freeboard temperature is main‐ tained at a higher value, devolatilisation occurred rapidly and produced more volatile gases. As the biomass fraction increased, the reduced fixed carbon gives more chance for the vola‐

Significant fluctuations of CO emissions were reported during co-combustion of biomass in a FBC. The value of the CO concentration in the flue gas has been found to depend on the type of fuel, fuel properties (volatility, particle size and density) and the operating condi‐ tions (bed and freeboard temperature, excess air, secondary air). In addition to the expected immediate ignition and the high volatile matter contents, the volatiles consist mainly of the combustibles (CO, H2, CxHy). These factors together indicate that the combustion of the vol‐ atiles would be the dominant step during the biomass combustion. At higher temperatures, the combustibles (CO, H2, CH4) accounted for more than 70–80 % of the gas components [23]. Saxena et al (1994) found that the hydrodynamic activity in the bed is related to the sol‐ id mixing and gas-solids contacting and these in turn are directly related to CO emissions. Higher bed temperature seems to provide optimum conditions for rapid de-volatilisation and hence increased conversion CO to CO2 [19]. They found that in the turbulent regime, the carbon utilisation efficiency reached a maximum and a further increase in the fluidisation velocity had an insignificant influence on the bed hydrodynamics and hence CO emissions. Similarly, as most of the biomass combustion was observed to take place in the freeboard, the supply of oxygen to this zone in amounts sufficient to achieve satisfactory combustion had to be ensured. Furthermore, Sami et al (2001) found that if the level of CO was within acceptable limits, then approximately 10% excess air and a temperature of 650°C provided

optimum conditions for the combustion of manure in a fluidised bed unit [17].

tiles to escape combustion.

392 New Developments in Renewable Energy

Although there are many potential benefits associated with co-combustion, there are several combustion related concerns associated with the co-combustion of coal and biomass. Utiliza‐ tion of solid biomass fuels and wastes sets new demands for boiler process control and boil‐ er design, as well as for combustion technologies, fuel blend control and fuel handling systems. For example, the different mineral matter composition (high alkali levels) and mode of occurrence (mostly mobile forms) in biomass results in concerns over enhanced fouling and slagging of pulverized coal boilers, particularly when firing agricultural resi‐ dues or herbaceous materials. The economics of co-combustion in pulverized coal boilers are closely tied to the biomass preparation costs (i.e. drying and milling), so an improved un‐ derstanding of the effect of biomass particle size and moisture content on combustor per‐ formance is needed (e.g. in the areas of flame stability, flame shape, and carbon burnout).Thus, this research was carried out with the objective to characterise biomass prop‐ erties that affect the co-combustion of biomass with coal, in particular biomass that is availa‐ ble in large quantities in Malaysia.

### **1.5. Research scope and application**

This research was performed with the objective to determine the combustion efficiency of the existing coal-fired combustor during co-firing agricultural residue with coal. The effi‐ ciency is calculated mainly based on the carbon monoxide and carbon dioxide emissions. Furthermore, this works also to demonstrate the technical feasibility of a fluidized bed as a clean technology for burning agricultural residues. In addition, the effect of biomass proper‐ ties such as such as particle size, particle density and volatility as well as influences of oper‐ ating parameters such as excess air, fluidizing velocity on axial temperature profile, the combustion efficiencies and CO emissions are also being investigated.

measured at 8 different heights above the distributor plate by means of sheathed Ni/Cr-Ni thermocouples (TC) type K. Fuel was fed pneumatically into the bed surface from a sealed hopper through an inclined feeding pipe the flowrate for which flow rate was controlled by a screw-feeder. A cyclone was fitted to the combustor exit and the carryover from the bed was collected for analysis. CO and O2 were measured using a Xentra 4904 B1 continuous emissions analyzer, whereas CO2 was measured using a non-dispersive infrared absorption spectrometry analyser. A fly ash sample was collected from the catch pot after finishing the combustion run. The fly ash sample was then weighed and analysed to determine the total amount of unburned carbon of the fuels in the test. The percentage of combustion efficiency was computed using Equation 1. Based on the values of combustion efficiency from experi‐ ments where duplicate runs were conducted under almost identical conditions, the combus‐

Sustainable Power Generation Through Co-Combustion of Agricultural Residues with Coal in Existing Coal Power

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**Figure 1.** Schematics diagram of the laboratory scale fluidized bed combustor (Tc =Thermocouple).

tion efficiency values should be within ±2%.
