Offsetting particulate matter emissions has become a critical global aim as there are concerted efforts to deal with environmental and energy poverty challenges. This study consists of investigations of computing emissions of particulate matter from biomass fuels in various atmospheres and temperatures. The laboratory setup included a fixed bed electric reactor and a particulate matter (PM) measuring machine interfaced with the flue gas from the fixed bed reactor combustion chamber. The experiments were conducted at seven different temperatures (600°C–1200°C) and six incremental oxygen concentrations (21%–100%). Five biomass types were studied; A-cornstalk, B-wood, C-wheat straw, D-Rice husk, E-Peanut shell, each pulverized to a size of approximately 75 microns. The study shows that PM emitted during char combustion is consistently higher than that emitted during the de-volatilization. During de-volatilization, increase in temperature leads to linear decrease in PM emission between atmospheres of 21%O2 to 50%O2, thereafter, between 70%O2 to 100%O2; increase in temperature leads to a rise in PM emission. The average PM formation from all the five considered biomass is relatively comparable however, with differing atmospheres and temperatures, the fibrous and low-density biomass forms more PM. During char combustion, the study shows that at oxygen levels of 21%, 70%, 90% and 100%, increase in temperature leads to increased PM emission. The increase in oxygen concentration and temperature increases the rate of combustion hence diminishing the time of combustion.
Particulate Matter (PM) is directly associated with several health complications besides the prominent environmental effects. PM10 and PM2.5 is monitored in indoor spaces and outdoor environments as exposure to them has been linked to respiratory illnesses and carcinogenic effects [
Oxy-fuel combustion is a recent technology that has been recognized for multiple benefits. It increases the partial pressure of CO2 in the flue gas hence making CO2 sequestration more economical, moreover, the CO2 in the flue gas can be recycled for combustion purposes [
Several studies carried out have shown the impact of various conditions on PM emissions during both biomass and coal combustion investigations. Several studies have been done including the impact of solid fuel [
The objectives of this study were to find out how PM formation is related to the combustion temperature and combustion atmosphere and fuel type. In this study, the emission behavior of PM during oxy-biomass combustion was investigated using available materials such as cornstalk, wood, wheat straw, rice husk, and peanut shell.
Cornstalk is the tough fibrous stem of the corn plant. China is the second largest producer of cornstalk in the world. Cornstalk energy can be exploited to produce power therefore eliminating the effects of open burning and dumping.
Wood is composed of cellulose (40% to 50%), hemicellulose (15% to 25%) and lignin (15%–30%).
Wheat straw is the dry stalk of the wheat plant after the removal of grain and chaff. As gathered from the elemental analysis, the sulphur at 0.4 is the highest of the five biomass types, therefore pausing a significant pollution potential.
Rice husk is the outer sheath covering of the rice crop, it has low density, high moisture content and low energy density as opposed to wood.
Peanut shells are generally herbaceous biomass fuel with composition of raw peanut shell determined to be lignin, cellulose, minimal compositions of protein, ash and moisture. The shells have a fibrous ‘skeleton’ supporting a cellulose-lignin layer.
The ultimate analysis for carbon and hydrogen was done according to the ASTME 777 standard. Nitrogen and sulphur were done using the ASTME 778 and ASTME 775 standard, respectively. Oxygen was determined by difference.
Proximate analysis was performed on the biomass samples for the determination of the moisture, volatile and ash content according to the British and European standards respectively (BSI 2009 a, b, c). The fixed carbon content was determined by difference. The moisture content was determined using oven method at 105°C.
The heating values of the samples were both determined by bomb calorimeter measurements and calculated by formulae available in literature. The bomb calorimeter was used according to the ASTM D 2015 standard method.
Five types of biomass which are readily available: maize stalk(A), wood(B), wheat straw(C), rice husk(D), and peanut shell(E) were combusted to produce PM. Investigations were conducted in an electric fixed bed reactor with ability to perform in a constant pre-determined temperature and airflow for stable and complete combustion as illustrated in
Elemental |
Cornstalk(A) | Wood(B) | Wheat straw(C) | Rice husk(D) | Peanut shell(E) |
---|---|---|---|---|---|
N | 1.7 | 1.5 | 1.5 | 1.2 | 1.3 |
C | 42.2 | 45.1 | 40.9 | 34.1 | 43.1 |
H | 5.6 | 5.8 | 5.5 | 4.5 | 5.4 |
S | 0.2 | 0.2 | 0.4 | 0.1 | 0.1 |
O | 31.9 | 39.5 | 31.6 | 42.4 | 37.0 |
Proximate Analysis (wt.% adb) | |||||
Moisture content | 11.5 | 5.2 | 11.1 | 9.4 | 9.7 |
Ash content | 6.9 | 2.7 | 9.0 | 8.3 | 3.4 |
Volatile matter | 71.7 | 80.8 | 76.1 | 73.6 | 73.7 |
Fixed carbon | 10.0 | 11.3 | 3.7 | 8.7 | 13.1 |
Heat value | |||||
Gross calorific value (Mj/kg.db) | 19.98 | 20.79 | 19.13 | 20.21 | 21.64 |
Feature | Specifics |
---|---|
Biomass type | Maize stalk(A), Wood(B), Wheat straw(C), Rice husk(D), Peanut shell(E) |
O2/CO2 concentration | 21%O2/79%CO2, 30%O2/70%CO2, 50%O2/50%CO2, 70%O2/30%CO2, 90%O2/10%O2, 100%O2 |
Temperature(°C) | 600–1200 (Step size-100) |
Fuel particle size | Less than or equal to 75 microns |
After attaining the predetermined temperature in the combustion furnace at the set airflow of 20 ml/sec, a ceramic boat with approximately 30 mg of sample fuel was introduced and the combustion started instantly in the combustion zone. Both readings were simultaneously recorded by the portable soot analyser and flue gas analyser. After a considered period, the concentration of the flue gases in the exhaust stream decreased to normal at which point the experiment was terminated. All emissions parameters were recorded from the moment when the samples were introduced into the combustion furnace up to when the experiment was terminated. All samples were kept under the same ambient conditions. All PM measurements were done at least twice after which a mean emission value was calculated from results obtained.
The values for PM in mg/g were derived from the following equation:
All data was examined for outliers using MATLAB using the code as shown in
During the entire temperature profile considered in the investigation, the amount of PM emitted during char combustion is consistently higher than that emitted during de-volatilization (
The cornstalk trend shows little variation as the temperature increases, during both de-volatilization and char combustion (
Value | Cornstalk | Wood | Wheat straw | Rice husk | Peanut shell |
---|---|---|---|---|---|
Min | 0.04 | 0.07 | 0.06 | 0.05 | 0.02 |
Max | 0.12 | 0.61 | 0.32 | 0.31 | 0.42 |
Mean | 0.08 | 0.20 | 0.17 | 0.19 | 0.19 |
STDEV | 0.03 | 0.20 | 0.09 | 0.11 | 0.13 |
Range | 0.08 | 0.54 | 0.26 | 0.26 | 0.40 |
Min | 0.17 | 0.26 | 0.24 | 0.25 | 0.32 |
Max | 0.54 | 0.56 | 0.75 | 1.43 | 1.22 |
Mean | 0.30 | 0.42 | 0.50 | 0.65 | 0.62 |
STDEV | 0.15 | 0.13 | 0.17 | 0.40 | 0.30 |
Range | 0.37 | 0.30 | 0.51 | 1.18 | 0.90 |
During de-volatilization, wheat straw, rice husk and peanut shell show a linear decrease in PM emission as temperature increases specifically between 600°C and 1000°C, exceptionally high PM emission is experienced at 1100°C (
During de-volatilization there is a linear decrease as the temperature increases with inconsistent spikes at 1100°C (
There is significantly high PM emission during char combustion with mean values of approximately 0.5 mg/g of fuel which are quite high compared to values of 0.2 mg/g of fuel during de-volatilization as seen in
Value | Cornstalk | Wood | Wheat straw | Rice husk | Peanut shell |
---|---|---|---|---|---|
Min | 0.14 | 0.11 | 0.08 | 0.08 | 0.04 |
Max | 0.42 | 0.61 | 0.58 | 0.32 | 0.26 |
Mean | 0.27 | 0.26 | 0.22 | 0.20 | 0.15 |
STDEV | 0.12 | 0.17 | 0.17 | 0.08 | 0.08 |
Range | 0.28 | 0.5 | 0.5 | 0.24 | 0.22 |
Min | 0.34 | 0.41 | 0.37 | 0.37 | 0.34 |
Max | 0.76 | 0.63 | 0.70 | 0.70 | 1.58 |
Mean | 0.52 | 0.51 | 0.53 | 0.52 | 0.47 |
STDEV | 0.13 | 0.11 | 0.12 | 0.11 | 0.09 |
Range | 0.42 | 0.22 | 0.33 | 0.33 | 0.24 |
The increase in temperature leads to a decrease in PM emission during de-volatilization with exceptional trend at 1100°C and 900°C except for peanut shell where the decreasing trend is perfectly linear as seen in
The observation made during char combustion at
Value | Cornstalk | Wood | Wheat straw | Rice husk | Peanut shell |
---|---|---|---|---|---|
Min | 0.03 | 0.08 | 0.08 | 0.10 | 0.07 |
Max | 0.36 | 0.34 | 0.57 | 0.41 | 0.53 |
Mean | 0.15 | 0.18 | 0.23 | 0.17 | 0.22 |
STDEV | 0.12 | 0.09 | 0.17 | 0.11 | 0.15 |
Range | 0.33 | 0.26 | 0.50 | 0.31 | 0.46 |
Min | 0.33 | 0.29 | 0.30 | 0.29 | 0.35 |
Max | 0.59 | 0.54 | 0.59 | 0.59 | 0.75 |
Mean | 0.46 | 0.45 | 0.45 | 0.42 | 0.50 |
STDEV | 0.09 | 0.08 | 0.09 | 0.12 | 0.13 |
Range | 0.26 | 0.25 | 0.29 | 0.30 | 0.40 |
There is a linear increment in PM emission as temperature increases from 600°C to 800°C concurrently for the five biomass fuels during de-volatilization as shown in
Value | Cornstalk | Wood | Wheat straw | Rice husk | Peanut shell |
---|---|---|---|---|---|
Min | 0.06 | 0.06 | 0.06 | 0.06 | 0.10 |
Max | 0.22 | 0.27 | 0.18 | 0.24 | 0.24 |
Mean | 0.14 | 0.14 | 0.13 | 0.15 | 0.16 |
STDEV | 0.06 | 0.08 | 0.04 | 0.06 | 0.05 |
Range | 0.16 | 0.21 | 0.12 | 0.18 | 0.14 |
Min | 0.13 | 0.15 | 0.14 | 0.28 | 0.29 |
Max | 0.48 | 0.51 | 0.46 | 0.47 | 0.65 |
Mean | 0.35 | 0.38 | 0.39 | 0.41 | 0.48 |
STDEV | 0.15 | 0.12 | 0.11 | 0.06 | 0.14 |
Range | 0.35 | 0.36 | 0.32 | 0.19 | 0.36 |
There is significantly low PM at 700°C and 1200°C during de-volatilization as shown in
Value | Cornstalk | Wood | Wheat straw | Rice husk | Peanut shell |
---|---|---|---|---|---|
Min | 0.07 | 0.05 | 0.08 | 0.08 | 0.05 |
Max | 0.22 | 0.22 | 0.32 | 0.24 | 0.25 |
Mean | 0.14 | 0.15 | 0.16 | 0.16 | 0.16 |
STDEV | 0.06 | 0.07 | 0.09 | 0.06 | 0.08 |
Range | 0.15 | 0.17 | 0.24 | 0.16 | 0.20 |
Min | 0.18 | 0.25 | 0.20 | 0.28 | 0.24 |
Max | 0.53 | 0.44 | 0.49 | 0.53 | 0.71 |
Mean | 0.36 | 0.35 | 0.33 | 0.40 | 0.40 |
STDEV | 0.13 | 0.07 | 0.11 | 0.09 | 0.15 |
Range | 0.35 | 0.19 | 0.29 | 0.25 | 0.47 |
There is a consistent increase in PM emission as temperature increases from 600°C to 1100°C during char combustion as shown in
The trend shows higher PM potential for the herbaceous, low density fuels. During combustion, a fraction of the inorganic compounds in the fuel is volatilized, i.e., potassium. Some refractory species that are copious in burnt herbaceous plants like silica and calcium are also sublimed to gaseous state making the herbaceous plants more likely to form fly ash depending also on other contributing factors [
The increase in oxygen concentration and temperature increases the rate of combustion hence decreasing the time of combustion. Increasing the temperature from 600°C to 700°C decreases the time by approximately 40% in low oxygen content (21%/79%) and by 15% in high oxygen content (100%) as shown in
The decrease in time of combustion also decreases as the temperatures spiral upwards indicating a definite limit at a specific point. Studies have shown that oxygen concentration has a significant effect on coal particle combustion with experimental findings demonstrating that the peak temperature and the burnout temperature are both decreased with increased oxygen concentration, the same holds in biomass as observed in
In this paper, combustion was conducted in a fixed bed reactor. The experiments were conducted at seven different temperatures (600°C–1200°C) and six different oxygen concentrations. Five biomass types were studied: A-cornstalk, B-wood, C-wheat straw, D-Rice husk, E-Peanut shell each powdered to a size of approximately 75 microns. The main conclusions are:
PM emitted during char combustion is consistently higher than that emitted during the de-volatilization (
Considering all de-volatilization observations, increase in temperature leads to linear decrease in PM emission between atmospheres of 21%O2 to 50%O2 (
There average PM formation from all the five considered biomass is relatively comparable however, with differing atmospheres and temperatures, the fibrous and low-density biomass forms more emissions of PM. The mean PM emitted was 0.28 ± 0.019 mg/g of cornstalk, 0.30 ± 0.018 mg/g of wood, 0.31 ± 0.018 mg/g of wheat straw, 0.32 ± 0.023 mg/g of rice husk, 0.33 ± 0.023 mg/g of peanut shell.
During char combustion, the study shows that at oxygen levels of 21%, 70%, 90% and 100%, increase in temperature leads to increase PM emission with slight peculiar observations at some temperatures. At 30% and 50% oxygen the observation shows a lateral distribution with no consistent identifiable rise or fall.
The increase in oxygen concentration and temperature increases the rate of combustion hence decreasing the time of combustion.