**2. Materials and experimental**

### **2.1. Raw materials and characterizations**

In this study British coal, rice husk and palm kernel shell originated from Perlis were em‐ ployed as fuel. These fuels were open air dried for 2 to 3 days to remove moisture. The proxi‐ mate and ultimate analyses were performed on coal and rice husk are summarized in Table 1.


**Table 1.** Coal and biomass characterization

#### **2.2. Experimental set up and procedures**

Figure 1, is a schematic diagram of the Atmospheric Fluidised Bed combustor used in this investigation. The system comprises of a 0.15m diameter and 2.3m high combustion cham‐ ber, allows for bed depths of up to 0.3m using 850μm sand, cyclone, screw feeder and gas analyzer. The combustor body is constructed from 1 cm thick 306 stainless steel and covered in Kaowool insulation to prevent excessive heat loss during operation. Fluidising air was in‐ troduced at the base of the bed through a nozzle distributor and provided fluidisation and combustion air. Start up of the bed was achieved using an in-bed technique; Propane was introduced directly into the distributor plate by injectors and mixed with air in the nozzles, providing a combustible mixture at the nozzle exit. Bed and freeboard temperatures were 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‐ tion efficiency values should be within ±2%.

ating parameters such as excess air, fluidizing velocity on axial temperature profile, the

In this study British coal, rice husk and palm kernel shell originated from Perlis were em‐ ployed as fuel. These fuels were open air dried for 2 to 3 days to remove moisture. The proxi‐ mate and ultimate analyses were performed on coal and rice husk are summarized in Table 1.

Fixed carbon 38.20 72.50 60.70 Ash 2.90 8.90 24.30

Carbon 80.10 49.5 36.20 Hydrogen 5.30 6.74 5.71

British Coal

0.90 0.70 13.00

31.1 1.4-4.8mm 1200

Figure 1, is a schematic diagram of the Atmospheric Fluidised Bed combustor used in this investigation. The system comprises of a 0.15m diameter and 2.3m high combustion cham‐ ber, allows for bed depths of up to 0.3m using 850μm sand, cyclone, screw feeder and gas analyzer. The combustor body is constructed from 1 cm thick 306 stainless steel and covered in Kaowool insulation to prevent excessive heat loss during operation. Fluidising air was in‐ troduced at the base of the bed through a nozzle distributor and provided fluidisation and combustion air. Start up of the bed was achieved using an in-bed technique; Propane was introduced directly into the distributor plate by injectors and mixed with air in the nozzles, providing a combustible mixture at the nozzle exit. Bed and freeboard temperatures were

Palm Kernel shell

58.90 18.60 15.00

1.85 0.00 41.91

18.0 3x6 mm 435

Rice Husk

0.10 0.00 57.99

13.5 0.8x1.00 mm 98

combustion efficiencies and CO emissions are also being investigated.

**2. Materials and experimental**

394 New Developments in Renewable Energy

Proximate Analysis (wt % dry basis)

**Ultimate Analysis (wt % dry basis)**

**Table 1.** Coal and biomass characterization

**2.2. Experimental set up and procedures**

Volatile matter

Nitrogen Sulfur Oxygen

Calorific value (MJ/kg) Particle size (mm) Particle density (kg/m3)

**2.1. Raw materials and characterizations**

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

#### **2.3. Operating conditions**

In this experiment, baseline data was first obtained for single combustion of 100% British bi‐ tuminous coal. Also, single combustion of other biomass fuels was carried out to investigate their combustion characteristics in comparison to coal during the co-combustion study. Cocombustion tests at biomass fractions of 30%, 50%, and 70% were performed. For each bio‐ mass fraction, excess air was varied from 30% to 70% at 20% intervals. For each excess air condition, air staging combustion was applied where the total secondary air is maintained at 65 l/min (about 10-20% to total air ratio). In order to study the impact of fuel property changes (volatiles, ash, and combustibles), heat input was fixed at the design value of the experimental rig i.e. 10 kW. The combustion tests were operated in the bed temperature range of 700-950 °C and superficial velocity range of 0.63 – 1.12 m/s.

#### **2.4. Carbon combustion efficiency calculation**

The carbon combustion efficiency of a system has been expressed as:

$$\text{Tr}\!\!\!\!\!C = \frac{B}{C} \text{tr}\, 100\%\tag{1}$$

*Mass of CO in the flue gas* =(28 / 12)*PB* (7)

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*O*<sup>2</sup> *consumed to produced CO*<sup>2</sup> + *CO* =(32 - 16*P*)*B* / 12 (8)

*O*<sup>2</sup> *consumed* = (16 / 2)*H* + (16 / 14)*N* + (32 / 32)*S* = *X* 1 (9)

(32 / 12)*C* + (16 / 2)*H* + (16 / 14)*N* + (32 / 32)*S* – *O* = *X* 2 (12)

*Mass of N* 2 *in the flue gas* = (79 / 12)(28 / 32) *X* 2(1 + *Z*) (14)

*O*2 *consumed during combustion* = (32 - 16*P*)*B* / 12 + *X* 1 (15)

*Y* = {44 *CO*<sup>2</sup> + 32 *O*<sup>2</sup> + 28 *N*<sup>2</sup> }/ 12{ *CO* + *CO*<sup>2</sup> } (17)

*Y* = {4 *CO*<sup>2</sup> + *O*<sup>2</sup> + 7} / 3{ *CO* + *CO*<sup>2</sup> } (18)

Let *Z* be the fractional excess air supplied, which is defined as the excess air divided by the

Let *F* be the mass of dry flue gas can also be estimated from the flue gas composition. The flue gas flow rate and composition are not appreciably influenced by neglecting the pres‐ ence of *SO <sup>2</sup>*and *NO* in the flue gas. Hence the flue gas may be taken as consisting of *CO,*

The square brackets represent the volume fraction of the particular chemical species in the

*Mass of <sup>O</sup>*<sup>2</sup> *in the flue gas* =( *<sup>X</sup>* 2(1 <sup>+</sup> *<sup>Z</sup>*) - (32 - <sup>16</sup>*P*)*<sup>B</sup>*

Let Y be the mass of dry flue gas per unit mass of C burnt in the fuel. Then,

*SO*<sup>2</sup> *produced* = (64 / 32)*S* (10)

*NO produced* = (30 / 14)*N* (11)

*O*2 *supplied* = *X* 2(1 + *Z*) (13)

<sup>12</sup> + *X* 1 (16)

Assuming that *H, N,* and *S* present in the fuel are completely converted to *H <sup>2</sup> O, NO* and *SO*

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

Therefore, total *O <sup>2</sup>* required for stoichiometric combustion of fuel

*<sup>2</sup>*respectively,

stoichiometric air. Therefore,

*CO2, N <sup>2</sup>* and *O <sup>2</sup>*.

flue gas and Y can be simplified to

where*B* and *C* are *the mass fractions of burnt and total carbon in the fuel*, respectively. Knowing the flue gas composition, the flue gas composition, fractional excess air and the fuel ultimate analyses of the fuel, *B* can be determined [20].

This method is particularly appropriate for solid fuels and is described as follows:

Let *C, H, O, N,* and *S* be the mass fractions of *carbon, hydrogen, oxygen, nitrogen and sulphur,* respectively, in the feed.

Further, let *A* and *B* be the *mass fractions of unburned and burnt carbon*, respectively, in the fuel. Then,

$$A + B = \mathbb{C} \tag{2}$$

Further define

$$P = \frac{\text{C converted to CO}}{\text{C converted to CO} + \text{CO}\_2} = \frac{\text{C converted to CO}}{\text{B}}\tag{3}$$

$$\text{C}\begin{aligned} \text{C}\begin{aligned} \text{over}\text{\" \(\text{to}\; \text{CO}=\text{PB}\end{aligned} \end{aligned} \tag{4}$$

$$\subset\text{corrected to }\text{CO}\_2 = \{1 - P\}B\tag{5}$$

$$\text{Mass of CO}\_2 \text{ in the flue gas} = \binom{44}{12} (\text{1} - P)B \tag{6}$$

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$$\text{Mass of CO in the flue gas} = \text{(28/12)} \text{PB} \tag{7}$$

$$\text{CO}\_2 \text{ consumed to produced } \text{CO}\_2 + \text{CO} = \text{(32 - 16P)B}/12 \tag{8}$$

Assuming that *H, N,* and *S* present in the fuel are completely converted to *H <sup>2</sup> O, NO* and *SO <sup>2</sup>*respectively,

$$\text{O}\_2 \text{ consumed} = \text{(16/2)H} + \text{(16/14)N} + \text{(32/32)S} = X1 \tag{9}$$

$$SO\_2 \, produced = \{64/32\} \,\text{S} \tag{10}$$

$$\text{NO} \text{ produced} = \text{(30/14)N} \tag{11}$$

Therefore, total *O <sup>2</sup>* required for stoichiometric combustion of fuel

**2.3. Operating conditions**

396 New Developments in Renewable Energy

In this experiment, baseline data was first obtained for single combustion of 100% British bi‐ tuminous coal. Also, single combustion of other biomass fuels was carried out to investigate their combustion characteristics in comparison to coal during the co-combustion study. Cocombustion tests at biomass fractions of 30%, 50%, and 70% were performed. For each bio‐ mass fraction, excess air was varied from 30% to 70% at 20% intervals. For each excess air condition, air staging combustion was applied where the total secondary air is maintained at 65 l/min (about 10-20% to total air ratio). In order to study the impact of fuel property changes (volatiles, ash, and combustibles), heat input was fixed at the design value of the experimental rig i.e. 10 kW. The combustion tests were operated in the bed temperature

range of 700-950 °C and superficial velocity range of 0.63 – 1.12 m/s.

The carbon combustion efficiency of a system has been expressed as:

*<sup>P</sup>* <sup>=</sup> *<sup>C</sup> converted to CO*

*Mass of CO*<sup>2</sup> *in the flue gas* =( <sup>44</sup>

<sup>η</sup>*CE* <sup>=</sup> *<sup>B</sup>*

This method is particularly appropriate for solid fuels and is described as follows:

where*B* and *C* are *the mass fractions of burnt and total carbon in the fuel*, respectively. Knowing the flue gas composition, the flue gas composition, fractional excess air and the fuel ultimate

Let *C, H, O, N,* and *S* be the mass fractions of *carbon, hydrogen, oxygen, nitrogen and sulphur,*

Further, let *A* and *B* be the *mass fractions of unburned and burnt carbon*, respectively, in the

*<sup>C</sup> converted to CO* <sup>+</sup> *CO*<sup>2</sup> <sup>=</sup> *<sup>C</sup> converted to CO*

*<sup>C</sup> x*100*%* (1)

*A* + *B* =*C* (2)

*C converted to CO* = *PB* (4)

*C converted to CO*<sup>2</sup> =(1 - *P*)*B* (5)

*<sup>B</sup>* (3)

<sup>12</sup> )(1 - *P*)*B* (6)

**2.4. Carbon combustion efficiency calculation**

analyses of the fuel, *B* can be determined [20].

respectively, in the feed.

fuel. Then,

Further define

$$\text{(\ $2/12)C + (16/2)H + (16/14)N + (\$ 2/32)S - O = X \,\Omega} \tag{12}$$

Let *Z* be the fractional excess air supplied, which is defined as the excess air divided by the stoichiometric air. Therefore,

$$\text{CO2 supplied} = \text{X2(1+Z)}\tag{13}$$

$$\text{Mass of N} \,\Omega \text{ in the flue gas} = \text{(79/12)(28/32)} \,\text{X} \,\Omega (1+\text{Z})\tag{14}$$

$$\text{O2 consumed during combustion} = \text{(32 - 16P)} \text{B/12 + X1} \tag{15}$$

$$\text{Mass of O2 in the flue gas} = \left(X\,2(1+Z) \cdot \frac{(32 \cdot 16P)B}{12} + X\,1\right) \tag{16}$$

Let *F* be the mass of dry flue gas can also be estimated from the flue gas composition. The flue gas flow rate and composition are not appreciably influenced by neglecting the pres‐ ence of *SO <sup>2</sup>*and *NO* in the flue gas. Hence the flue gas may be taken as consisting of *CO, CO2, N <sup>2</sup>* and *O <sup>2</sup>*.

Let Y be the mass of dry flue gas per unit mass of C burnt in the fuel. Then,

$$Y = \left\{ 44\text{[CO}\_2\text{]} + 32\text{[O}\_2\text{]} + 28\text{[N}\_2\text{]} \right\} / 12\text{[CO]} + \left\{ \text{CO}\_2\text{]} \right\} \tag{17}$$

The square brackets represent the volume fraction of the particular chemical species in the flue gas and Y can be simplified to

$$Y = \left[ 4\text{[CO}\_2\text{]} + \text{[O}\_2\text{]} + 7\right] / 3\left[\text{[CO]} + \text{[CO}\_2\text{]}\right] \tag{18}$$

By substituting

$$\text{[CO]} + \text{[N}\_2\text{]} + \text{[CO}\_2\text{]} + \text{[O}\_2\text{]} = 1\tag{19}$$

mass of dry flue gas per unit mass of the fuel is

$$F = \text{'YB} \tag{20}$$

combustion efficiency. However, when the combustion is stabilized, increasing fluidizing velocity contributed to a greater particle elutriation rate than the carbon to CO conversion rate and hence increased the unburned carbon [5]. However, when the combustion is stabi‐ lized, increasing the fluidizing velocity contributes to a greater particle elutriation rate than carbon to carbon monoxide conversion rate and increases the amount of unburned carbon. This phenomenon can be seen in Figure 2 where the carbon combustion efficiency is lower than expected for 50% rice husk mixtures when the fluidizing velocity increases beyond the optimum value. Apart from solid mixing, increasing the fluidizing velocity also influences the fuel particle settling time during the combustion process in the FBC. Increasing fluidiz‐ ing velocity drives the lighter fuel particles upwards and into the freeboard region, which is indicated by higher freeboard temperatures. Thus, the settling time for the biomass to reach the bed will be greater and a significant portion of the combustion will be completed before the particles return to the bed is reached, although this is dependent upon fuel particle size and density. This settling time depends on the fuel particle size and particle density. The greater settling time the higher the freeboard temperature due to greater volatile combus‐ tion contributing to higher combustion efficiency providing the bed temperature is main‐

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tained within the range of 800-900˚C.

**Figure 2.** Combustion efficiency during co-combustion of coal with rice husk and palm kernel shell

Substituting *F* in (A-6.18) into (A-6.17), then the fraction of C burnt, B, can be written as fol‐ lows:

$$B = \frac{\prod\_{4,29} (1+Z) \left(\frac{32}{12}\right) \Gamma\_s + \left(\frac{16}{2}\right) H\_s + \left(\frac{16}{4}\right) N + \left(\frac{32}{32}\right) S - O\left[-8H\_s + N + S\right]}{Y \cdot 1} \tag{21}$$

$$
\eta \text{CE} = \left( B(21) + \text{Luburned carbon in ash} \right) / \text{C} \times 100\% \tag{22}
$$

#### **3. Results and discussion**

This section describes the combustion of agricultural residue in a fluidized bed combustor. The influences of fuel properties such as particle size, particle density and volatility as well as influences of operating parameters such as excess air, fluidizing velocity on axial temper‐ ature profile, the combustion efficiencies and CO emissions are discussed.

#### **3.1. Carbon combustion efficiencies**

The combustion tests were performed using different coal mass fraction; 0, 50 and 100%, corresponding to heat input of 10kW under optimum excess air conditions. Figure 2 shows the effect of different mixtures of rice husk and palm kernel shell with coal on carbon com‐ bustion efficiency with the same heat input. Generally, Carbon combustion efficiency for single biomass (rice husk and palm kernel shell) but increases with increasing coal addition and experimental runs. The following carbon combustion efficiencies, from Eqn. (1), range between 67-75% for burning 100% rice husk and 80-83% for burning 100% palm kernel shell, 83-88%, and 86-92% for 50% of coal addition to rice husk and palm kernel shell, respectively. The improved carbon combustion efficiency by co-combustion of rice husk with coal can be attributed to an increase in bed temperature, Figure 3, which is caused by the addition of fixed carbon content in the mixture. This fixed carbon, from coal, burns in the bed while the volatile gas burns in the freeboard region. Thus, there is more chance for fuel conversion carbon to carbon dioxide as the coal fraction increases and less volatile and tend to escape combustion, because of the reduced biomass concentration [16].

In addition, increasing the fluidizing velocity increases the turbulence in the bed leading to better solid mixing and gas-solid contacting and so as the amount of carbon in the bed is burnt at higher rate. Consequently, higher carbon burn out obtained leads to higher carbon By substituting

398 New Developments in Renewable Energy

lows:

mass of dry flue gas per unit mass of the fuel is

*<sup>B</sup>* <sup>=</sup> 4.29(1 <sup>+</sup> *<sup>Z</sup>* ) ( <sup>32</sup>

**3. Results and discussion**

**3.1. Carbon combustion efficiencies**

<sup>12</sup> )*<sup>C</sup>* <sup>+</sup> ( <sup>16</sup>

<sup>2</sup> )*<sup>H</sup>* <sup>+</sup> ( <sup>16</sup>

ature profile, the combustion efficiencies and CO emissions are discussed.

combustion, because of the reduced biomass concentration [16].

*CO* + *N*<sup>2</sup> + *CO*<sup>2</sup> + *O*<sup>2</sup> = 1 (19)

<sup>32</sup> )*<sup>S</sup>* – *<sup>O</sup>* – <sup>8</sup>*<sup>H</sup>* <sup>+</sup> *<sup>N</sup>* <sup>+</sup> *<sup>S</sup>*

*ηCE* = (*B*(21) + *Unburned carbon in ash* ) / *C* × 100*%* (22)

Substituting *F* in (A-6.18) into (A-6.17), then the fraction of C burnt, B, can be written as fol‐

<sup>4</sup> )*<sup>N</sup>* <sup>+</sup> ( <sup>32</sup>

This section describes the combustion of agricultural residue in a fluidized bed combustor. The influences of fuel properties such as particle size, particle density and volatility as well as influences of operating parameters such as excess air, fluidizing velocity on axial temper‐

The combustion tests were performed using different coal mass fraction; 0, 50 and 100%, corresponding to heat input of 10kW under optimum excess air conditions. Figure 2 shows the effect of different mixtures of rice husk and palm kernel shell with coal on carbon com‐ bustion efficiency with the same heat input. Generally, Carbon combustion efficiency for single biomass (rice husk and palm kernel shell) but increases with increasing coal addition and experimental runs. The following carbon combustion efficiencies, from Eqn. (1), range between 67-75% for burning 100% rice husk and 80-83% for burning 100% palm kernel shell, 83-88%, and 86-92% for 50% of coal addition to rice husk and palm kernel shell, respectively. The improved carbon combustion efficiency by co-combustion of rice husk with coal can be attributed to an increase in bed temperature, Figure 3, which is caused by the addition of fixed carbon content in the mixture. This fixed carbon, from coal, burns in the bed while the volatile gas burns in the freeboard region. Thus, there is more chance for fuel conversion carbon to carbon dioxide as the coal fraction increases and less volatile and tend to escape

In addition, increasing the fluidizing velocity increases the turbulence in the bed leading to better solid mixing and gas-solid contacting and so as the amount of carbon in the bed is burnt at higher rate. Consequently, higher carbon burn out obtained leads to higher carbon

*F* = *YB* (20)

*<sup>Y</sup>* - <sup>1</sup> (21)

combustion efficiency. However, when the combustion is stabilized, increasing fluidizing velocity contributed to a greater particle elutriation rate than the carbon to CO conversion rate and hence increased the unburned carbon [5]. However, when the combustion is stabi‐ lized, increasing the fluidizing velocity contributes to a greater particle elutriation rate than carbon to carbon monoxide conversion rate and increases the amount of unburned carbon. This phenomenon can be seen in Figure 2 where the carbon combustion efficiency is lower than expected for 50% rice husk mixtures when the fluidizing velocity increases beyond the optimum value. Apart from solid mixing, increasing the fluidizing velocity also influences the fuel particle settling time during the combustion process in the FBC. Increasing fluidiz‐ ing velocity drives the lighter fuel particles upwards and into the freeboard region, which is indicated by higher freeboard temperatures. Thus, the settling time for the biomass to reach the bed will be greater and a significant portion of the combustion will be completed before the particles return to the bed is reached, although this is dependent upon fuel particle size and density. This settling time depends on the fuel particle size and particle density. The greater settling time the higher the freeboard temperature due to greater volatile combus‐ tion contributing to higher combustion efficiency providing the bed temperature is main‐ tained within the range of 800-900˚C.

**Figure 2.** Combustion efficiency during co-combustion of coal with rice husk and palm kernel shell

#### **3.2. Temperature profiles**

Figure 3 illustrates the axial temperature distributions along the FBC height for fuel studied at 50% excess air. As can be seen from the figure, coal combustion gives higher bed tempera‐ ture (y = 0-40cm) but lower freeboard temperature (y = 450-120cm) in comparison to bio‐ mass. Then, all the temperatures shows start to fall from 120 cm above distributor plate indicating that most of the combustion was completed. This significant combustion behav‐ iour can be explained by the devolatilization process of the fuel [17]. With high volatility (more than 50%) and low ignition temperature (250-350°C), biomass (rice husk and palm kernel shell) will start to devolatilize upon feeding at 45 cm of the FBC height (freeboard re‐ gion) and was mostly burned before it reached the bed region. While coal with low volatility (30%) and higher ignition temperature (400-600°C) will travel down to the bed and complet‐ ed combustion in the bed region. This was also greatly influenced by settling velocity of the fuel particles which correspond to the fuel particle size and fluidizing velocity [5]. Those ex‐ plain why palm kernel shell has higher bed temperature than rice even though the volatility is almost similar (see Table 1).This was due to the fact that greater particle size contributed to a greater devolatilization time and settling time. The greater settling time, the higher the freeboard temperature due to greater volatile combustion contributing to higher combustion efficiency providing the bed temperature is maintained within the range of 800-900˚C.

Significant increment of carbon combustion efficiencies was noted with coal addition to bio‐ mass fraction (see Figure 2).The improvement can be attributed to an increase in bed tem‐ perature, Figure 3, which is caused by the addition of fixed carbon content in the mixture. This fixed carbon, from coal, burns in the bed while the volatile gas burns in the freeboard region. Thus, there is more chance for fuel conversion carbon to carbon dioxide as the coal fraction increases and less volatile and tend to escape combustion, because of the reduced biomass concentration. Furthermore, this can be explain by the fact that biomass fuels with lower density (about half) compared to coal tend to burn in freeboard and coal tends to burn in the bed region. Therefore, the addition of coal in biomass increases the amount of fixed carbon reaching the bed resulting in higher bed temperatures. This observation agrees with the results of Abelha et al. (2003) and Suksankraisorn et al. (2003) who investigated the cofiring of coal and chicken litter and co-firing of lignite with municipal solid waste in a flui‐

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In order to enable comparison of CO from all tests were converted to CO emitted 6% flue gas oxygen. Figure 4, it is evident that there are significant fluctuations in CO emissions, which between 200 and 900 ppm under the same conditions. The orders of fluctuation were similar to those observed by Abelha et al. (2003) and W.A.W.A.K. Ghani et al. (2009). The fluctuations are caused by slight variations in feed composition and this effect is reflected in the temperature profiles. It is noted that the addition of coal has no significant influence on CO emissions during all co-combustion cases, except at coal (50%) / rice husk (50%) where emissions tend to be lower than expected in reference to the other rice husk fractions. This phenomenon is due to the synergistic nature of the coal and rice husk mixture, which en‐ hances the fuel reactivity and lowers the CO emissions [10]. In most cases the emission of CO seems relatively insensitive to changes in excess and fluidising air. This insensitivity is due to increased segregation of fuels in the combustor between the feed point and the bed. If the combustor receives a batch with a relatively high amount of fuel pellets, then as burning CO2 is produced since the pellets need to be heated and dried first. While this occurs, oxy‐ gen is not consumed and results in high CO emissions. The decrease in CO levels at low per‐ centages of excess air, not below than 50%, can be attributed to low excess air, relatively high bed temperatures (about 900°C) causing rapid enhances and ignition of volatiles from rice husk. Thus, higher CO to CO2 conversion rates was observed which enhanced the reac‐

Tables 2 and 3 present the ash collection and unburned carbon analyses during combustion tests. Generally, the mass balance on the ashes particles accounted for over 90% of the ash input from the fuel. The analyses of the ash collected in all tests for unburned carbon dem‐ onstrates that with biomass only, there was the least amount of unburned carbon detected in ash collected from the cyclone. However, the unburned carbon content increased when coal was added which suggested that some fine particles were elutriated with the fluidising gas‐

dised bed combustor, respectively.

tivity of the mixture [13].

**3.4. Analyses of carryover**

**3.3. Carbon monoxide (CO) emissions**

**Figure 3.** Axial temperature profile for co-combustion of coal with rice husk and palm kernel shell combustion in the case of excess air = 50%

Significant increment of carbon combustion efficiencies was noted with coal addition to bio‐ mass fraction (see Figure 2).The improvement can be attributed to an increase in bed tem‐ perature, Figure 3, which is caused by the addition of fixed carbon content in the mixture. This fixed carbon, from coal, burns in the bed while the volatile gas burns in the freeboard region. Thus, there is more chance for fuel conversion carbon to carbon dioxide as the coal fraction increases and less volatile and tend to escape combustion, because of the reduced biomass concentration. Furthermore, this can be explain by the fact that biomass fuels with lower density (about half) compared to coal tend to burn in freeboard and coal tends to burn in the bed region. Therefore, the addition of coal in biomass increases the amount of fixed carbon reaching the bed resulting in higher bed temperatures. This observation agrees with the results of Abelha et al. (2003) and Suksankraisorn et al. (2003) who investigated the cofiring of coal and chicken litter and co-firing of lignite with municipal solid waste in a flui‐ dised bed combustor, respectively.

### **3.3. Carbon monoxide (CO) emissions**

**3.2. Temperature profiles**

400 New Developments in Renewable Energy

case of excess air = 50%

Bed Fuel feed point

Figure 3 illustrates the axial temperature distributions along the FBC height for fuel studied at 50% excess air. As can be seen from the figure, coal combustion gives higher bed tempera‐ ture (y = 0-40cm) but lower freeboard temperature (y = 450-120cm) in comparison to bio‐ mass. Then, all the temperatures shows start to fall from 120 cm above distributor plate indicating that most of the combustion was completed. This significant combustion behav‐ iour can be explained by the devolatilization process of the fuel [17]. With high volatility (more than 50%) and low ignition temperature (250-350°C), biomass (rice husk and palm kernel shell) will start to devolatilize upon feeding at 45 cm of the FBC height (freeboard re‐ gion) and was mostly burned before it reached the bed region. While coal with low volatility (30%) and higher ignition temperature (400-600°C) will travel down to the bed and complet‐ ed combustion in the bed region. This was also greatly influenced by settling velocity of the fuel particles which correspond to the fuel particle size and fluidizing velocity [5]. Those ex‐ plain why palm kernel shell has higher bed temperature than rice even though the volatility is almost similar (see Table 1).This was due to the fact that greater particle size contributed to a greater devolatilization time and settling time. The greater settling time, the higher the freeboard temperature due to greater volatile combustion contributing to higher combustion efficiency providing the bed temperature is maintained within the range of 800-900˚C.

0 20 40 60 80 100 120 140 160 180 200 **Heigh above distributor plate (cm)**

**Figure 3.** Axial temperature profile for co-combustion of coal with rice husk and palm kernel shell combustion in the

100% coal 100% rice husk 100% Palm Kernel Shell 50% rice husk / 50% coal 50% palm kernel shell/ 50% coal In order to enable comparison of CO from all tests were converted to CO emitted 6% flue gas oxygen. Figure 4, it is evident that there are significant fluctuations in CO emissions, which between 200 and 900 ppm under the same conditions. The orders of fluctuation were similar to those observed by Abelha et al. (2003) and W.A.W.A.K. Ghani et al. (2009). The fluctuations are caused by slight variations in feed composition and this effect is reflected in the temperature profiles. It is noted that the addition of coal has no significant influence on CO emissions during all co-combustion cases, except at coal (50%) / rice husk (50%) where emissions tend to be lower than expected in reference to the other rice husk fractions. This phenomenon is due to the synergistic nature of the coal and rice husk mixture, which en‐ hances the fuel reactivity and lowers the CO emissions [10]. In most cases the emission of CO seems relatively insensitive to changes in excess and fluidising air. This insensitivity is due to increased segregation of fuels in the combustor between the feed point and the bed. If the combustor receives a batch with a relatively high amount of fuel pellets, then as burning CO2 is produced since the pellets need to be heated and dried first. While this occurs, oxy‐ gen is not consumed and results in high CO emissions. The decrease in CO levels at low per‐ centages of excess air, not below than 50%, can be attributed to low excess air, relatively high bed temperatures (about 900°C) causing rapid enhances and ignition of volatiles from rice husk. Thus, higher CO to CO2 conversion rates was observed which enhanced the reac‐ tivity of the mixture [13].

### **3.4. Analyses of carryover**

Tables 2 and 3 present the ash collection and unburned carbon analyses during combustion tests. Generally, the mass balance on the ashes particles accounted for over 90% of the ash input from the fuel. The analyses of the ash collected in all tests for unburned carbon dem‐ onstrates that with biomass only, there was the least amount of unburned carbon detected in ash collected from the cyclone. However, the unburned carbon content increased when coal was added which suggested that some fine particles were elutriated with the fluidising gas‐ es. The amount of unburned carbon was, however, quite low, corresponding to about less than 5% of the total carbon input. Such observations seem to suggest that the large particle size and lower heating value of the biomass fuel did not adversely affect combustor per‐ formance, probably due to the higher volatile matter content of the biomass fuel. The vola‐ tile matter burns rapidly and the higher volatile matter content of the biomass can also result in a highly porous char, thus accelerating the char combustion as well. In all cases the amount of unburned carbon in the ash increased as the percentage s of coal increased which is due to the low volatility of coal. For the biomass materials the low density of palm fibre and rice husk are also led to increased carbon content in the ash. The initial particle size of the biomass does not appear to be significant.

**Fuel** Feed

**4. Conclusion**

(kg/h)

Superficial Velocity (m/s)

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

Coal (100%) 1.20 0.67 0.900 0.039 23.0 90.25 Palm kernel shell (100%) 1.97 0.59 0.898 0.028 5.0 80.67 Coal (30%) : Palm kernel shell (70%) 1.74 0.74 0.949 0.030 11.7 80.73 Coal (50%) : Palm kernel shell (50%) 1.59 0.65 0.962 0.031 14.9 89.86

**Table 3.** Ash analysis for single and co-combustion of coal and palm kernel shell at varies percentage of excess air.

The conclusions obtained in the present investigation on the temperature profile, carbon combustion efficiency and CO emissions in a 10 kW FBC can be summarised as that biomass combustion behaves differently in comparison to coal due to the significant difference in volatile matter content and variations of particle size and particle density. The carbon com‐ bustion efficiency was influenced by the operating and fluidising parameters in the follow‐ ing order: a) settling velocity; b) coal mass fraction; c) fluidising velocity; d) excess air and e) bed temperature (Tb). The maximum carbon combustion efficiency increased in the range of 3% to 20% as the coal fraction increased from 0% to 70%, under various fluidisation and op‐ erating conditions. Generally, the carbon combustion efficiency increased with increases of excess air and peaks at 50%. The corresponding increasing carbon combustion efficiency with excess air from 30-50% was found to be in the range of 5 – 12 % at 50% coal mass frac‐ tion in the biomass mixture. Further increase of excess air to 70% reduced the carbon com‐ bustion efficiency. Increasing the fluidising velocity increases the turbulence in the bed leading to better solid mixing and gas-solid contacting and shows as the amount of carbon in the bed is burnt at higher rate. However, when the combustion is stabilised, increasing fluidising velocity contributed to a greater particle elutriation rate than the carbon to CO conversion rate and hence increased the unburned carbon. Apart from solid mixing, increas‐ ing fluidising velocity also influenced settling time of fuel particle during the combustion process in FBC. Increasing fluidising velocity brought the lighter fuel particle upward to the freeboard region and completed before they reached the bed surface.The bed temperature had a small effect on carbon combustion efficiency for the biomass fuels. The turbulence cre‐ ated by increasing excess air related with increases in fluidising velocity had a greater influ‐ ence than reduced bed temperature. Significant fluctuations of CO emissions observed when coal was added into almost all biomass mixtures depending upon excess air ranges between 200-1500 ppm.The analyses of the ash collected in all tests for unburned carbon demon‐ strates that with biomass only, there was less unburned carbon detected in the ash collected from the cyclone indicating that the combustion of fixed carbon was almost complete. The percentages of unburned carbon increased in the range 3 to 30% of the ash content with the increases of coal fraction in the coal/biomass mixture. This can be explained by the fact that

Carbon feed (kg/h)

Ash (kg) **Carbon in Ash(%)**

http://dx.doi.org/10.5772/52566

Efficiency (%)

Plant

403

**Figure 4.** CO emissions as a function of excess air and Rice husk fraction combustion at heat input 10KW


**Table 2.** Ash analysis for single and co-combustion of coal and rice husk at varies percentage of excess air

Sustainable Power Generation Through Co-Combustion of Agricultural Residues with Coal in Existing Coal Power Plant http://dx.doi.org/10.5772/52566 403


**Table 3.** Ash analysis for single and co-combustion of coal and palm kernel shell at varies percentage of excess air.
