Biomass gasification is a thermochemical conversion process that dates back to the 19th century. Nevertheless, designing and operating a gasifier system is not an easy task. Every biomass feedstock has different characteristics and the gasifier needs to be designed according to those qualities. Hence, many laboratory analyses on bamboo were carried out for this study. This study also concentrates on finding the best possible process variables for a bamboo fueled downdraft gasifier through a sensitivity analysis. A software program called Thermoflex was used for this purpose and the effect of gasifier temperature, air-fuel-ratio, moisture content of the fuel and temperature of pre-heated air on the syngas composition were simulated. The results show that bamboo is a decent gasification feedstock because of its low ash and sulfur content and satisfactory energy value. The simulations reveal that the best gas quality is obtained with the gasifier temperature between 700 ℃ and 800 ℃, A/F-ratio of 1.25 - 1.75 and dry basis moisture content between 10% and 15%.
The world is at turning point regarding energy issues. Especially in developing countries the demand for electricity is rising steeply because of growing population and energy-dependent technologies. This need of energy has been satisfied by fossil fuels but their reserves are finite and the price fluctuations are hard to foresee making their utilization more complicated and expensive in the future. Also the climate change is a concerning fact. Hence, there is an increasing need to look for alternative, clean and sustainable energy sources and to control the greenhouse emissions.
Biomass gasification is a thermochemical process where a limited amount of oxygen (air) reacts with feed- stock in high-temperature producing synthesis gas. This gas can be fed into an internal combustion engine (ICE) or micro turbine to generate electricity. Many different biomasses can be gasified but this study concentrates on gasification of bamboo.
Gasification was a commercial process in England already in the 19th century but it has been very little investigated in Mexico, and this study is the first one to research gasification of bamboo in the country. In general studies on bamboo gasification in the open literature are rather limited. Some investigations carried out in Asia and Africa exist [
To get an idea what kind of fuel bamboo is, its proximate and ultimate analysis together with calorific values was investigated and they are shown in the section of results. Because every biomass is different the gasifier should be designed according to the feedstock qualities. The qualities together with the desired engine output define not only the dimensions of the gasifier but also the process parameters. For this study a 50 kW gasifier was chosen, because it was calculated to be the most cost-efficient solution for the pilot project. Also the amount of available bamboo is limited but a 50 kW gasifier ensures that the plantations will not be over exploited.
In this study a series of sensitivity analysis is carried out by using a design and simulation software program called Thermoflex [
The thermal conversion processes of biomass gasification consist of a few different phases that are normally modeled consecutive (one after another). Nevertheless, no sharp boundaries between the phases exist and they happen partly simultaneously. These phases are preheating and drying of the matter, pyrolysis, gasification and combustion [
Depending on the type of biomass the moisture content can rise up to 90% (on dry basis) [
Pyrolysis is an essential and relatively fast reaction in a gasifier. It means thermal (=pyro) degradation (=lysis) of organic materials. They start to pyrolyze in elevated temperatures of 350˚C - 600˚C forming a hydrogen-rich fraction and a carbon-rich residue called char [
The combustion and gasification reactions can be summarized in seven chemical equations [
The gas phase reactions (6) and (7) are important for the final gas quality. The Equation (6) has an influence on the CO/H2 ratio whereas the Equation (7) increases the calorific value of the syngas [
To get an idea what kind of fuel bamboo is, its proximate and ultimate analysis together with calorific values were investigated in the laboratories of Electrical Research Institute in Cuernavaca Mexico. To determine ash content, volatile matter and fixed carbon of bamboo, the following ASTM standard methods were used, respectively: D3174, D3175 and D3172. For determination of carbon, hydrogen and nitrogen content, D5373 was applied and for determination of sulfur the method D4239 was used. The amount of oxygen was obtained by balance calculations [
To measure the higher calorific value (HCV) of bamboo in the laboratory the standard D5865 was utilized [
where Xi = w-% of the substance i on a dry basis [
All the analyses and simulations were carried out at the Institute of Electrical Research in Cuernavaca Mexico. The gasification system discussed in this study is currently being built in the municipality of Huatusco, state of Veracruz.
The average altitude of the municipality is 1344 meters above the sea level but there are lower valleys and higher peaks as well. The annual rainfall is approximately 1825.5 mm. Because of the high altitude the air pressure is lower than the standard value and because of the rain the air is usually very humid. The average temperature varies between 9 and 23 degrees centigrade but obviously the absolute values sink lower or rise higher [
For this study only domestic bamboo, which grows in Huatusco, was investigated. The qualities of five different bamboo species were examined and the results are listed in Section 4.
An engineering software program called Thermoflex [
Thermoflex utilizes thermodynamic equilibrium models (heat balances) to carry out the simulations. It concerns only the reactions without letting the user define the gasifier geometry. The program does not specify if the “gasification temperature” is the temperature in the reduction zone, combustion zone, the gas exit temperature, an average of all of these or something else. The equilibrium is reached in an infinite time and that is why the software might give ideal yields as results. In practice, only a finite time is available for the reactions [
The simulation model in Thermoflex was built using the components that exist in the library of the program. As seen in the process diagram presented in
Thermoflex configures automatically some of the operational parameters. For example the air pressure of the gasifier was set at 1014 mbar (see
The temperature of the gasifier and the air-fuel ratio (A/F) are each other’s functions in Thermoflex meaning that if temperature is fixed by the user, the software adjusts the A/F-ratio according to the temperature. Then again if A/F-ratio is fixed, the temperature will vary. Both variables cannot be fixed at the same time. One of the most important factors defining the gasifier’s operation is the equivalence ratio, ER, which is defined by using the A/F-ratios as follows:
Stoichiometric means the amount of air that is needed for complete combustion of the fuel.
The following inputs were used in the different simulations:
Gasification temperature:
• Ambient temperature = Tamb = 15˚C
• Ambient pressure = pamb = 881 mbar
• Relative humidity = Ψ = 60%
• Moisture content of the fuel = MCfuel = 15%
• Gasifier temperature = Tgasifier = 500˚C∙∙∙1000˚C
• Air-fuel-ratio = A/F = varies when temperature is changed
Air-fuel-ratio:
• Tamb = 15˚C
• Pamb = 881 mbar
• Ψ = 60%
• MCfuel = 15%
• Tgasifier = varies when A/F is changed
• A/F = 0.5∙∙∙2.0
Moisture Content of the fuel:
• Tamb = 15˚C
• Pamb = 881 mbar
• Ψ = 60%
• MCfuel = 0%∙∙∙25%
• Tgasifier = 800˚C
• A/F = 1.76
Temperature of pre-heated air:
• Tamb = 15˚C
• Pamb = 881 mbar
• Ψ = 60%
• MCfuel = 15%
• Tgasifier = varies when the air is heated
• A/F = 1.76
• Tair = 50˚C ∙∙∙600˚C
In this section the simulation results obtained in the software Thermoflex according to the chemical characteristics of bamboo are presented.
An analysis for bamboo can be seen in
The volatiles may also turn into harmful tars depending on the process temperature and the gasifier design. That is another reason why a downdraft gasifier was selected for this project. It produces only 0.015 - 3.0 g/Nm3 of tars [
Ashes can cause a lot of problems in gasifiers. The compounds may melt and agglomerate producing clinker and causing slagging. This slag has to be removed which increases the need for workforce, causes a break for operation and thus increases costs. The occurrence of slagging depends on the ash content of the fuel, the melting characteristics of the ash and the temperature profile of the gasifier. Usually slagging causes no troubles if the ash content of the fuel is lower than 5% - 6% [
Fixed carbon is the solid part of the fuel that remains when all the volatile material, humidity and ash is driven off or distilled. It is an important parameter for gasification analysis because in most gasifiers the conversion of fixed carbon into gases determines the rate of gasification and its yield. It is also the slowest conversion reaction of the gasifier [
Scientific Name | Volatile Matter w-% | Ash w-% | Fixed Carbon w-% |
---|---|---|---|
Bambusa Oldhamii Munro | 78.80 | 3.28 | 17.9 |
Bambusa vulgarisVitata | 76.70 | 5.14 | 18.1 |
Bambusa vulgaris Schrader | 75.89 | 4.76 | 19.3 |
Dendrocalamus strictus | 79.07 | 3.41 | 17.5 |
Dendrocalamus asper | 77.49 | 3.25 | 19.2 |
Pseudotsuga menziesii (Wood) | 86.20 | 0.10 | 13.70 |
Phalaris Arundinacea (Reed Canarygrass) | 74.00 | 5.50 | 20.50 |
Pittsburg Seam Coal | 33.90 | 10.30 | 55.80 |
Source for bamboo: [
The ultimate analysis for bamboo is presented in
Higher calorific value (HCV) is a measure of energy content of biomass without any “free” water (=oven-dry basis). Still, the biomass contains chemically bound water and water that will arise as a result of combustion reactions. The HCV includes the latent heat of the water and that is why the result is higher [
The lower calorific value (LCV) is obtained when the latent heat of water is excluded from the results. The LCV is usually used when comparing results of different materials [
As seen in Equation (8) in Section 3.1 the carbon, hydrogen and sulfur content have a positive effect on the calorific value of the fuel whereas nitrogen, oxygen and ash lower the value. For example wood has more carbon and less ash than bamboo and hence the calorific value is higher. All biomasses include a high amount of oxygen as seen in
A sensitivity analysis was carried out by using a software program called Thermoflex. Different parameters were varied and the results are listed and discussed next.
Gasification temperature: because the A/F-ratio and the gasifier temperature are each other’s functions in the program, two y-axes are illustrated in
In
If the growing ER and its effects are forgotten, the volume fractions seem rational. The fraction of CO in- creases affecting positively the LCV of the gas. This increase is expected within higher temperature because
Scientific Name | C w-% | H w-% | O w-% | N w-% | S w-% |
---|---|---|---|---|---|
Bambusa oldhamii Munro | 46.70 | 6.09 | 43.80 | n/d | 0.05 |
Bambusa vulgarisVitata | 45.90 | 5.81 | 43.10 | n/d | 0.05 |
Bambusa vulgaris Schrader | 46.00 | 5.95 | 43.10 | 0.15 | 0.05 |
Dendrocalamus strictus | 46.70 | 5.91 | 43.80 | 0.12 | 0.05 |
Dendrocalamus asper | 47.00 | 5.93 | 43.80 | n/d | 0.05 |
Pseudotsuga menziesii (Wood) | 52.30 | 6.30 | 40.50 | 0.10 | 0.00 |
Phalaris arundinacea (Reed Canarygrass) | 44.60 | 5.55 | 40.11 | 0.00 | 0.22 |
Pittsburg Seam Coal | 75.50 | 5.00 | 4.90 | 1.20 | 3.10 |
Source for bamboo: [
Scientific Name | HCV MJ/kg | LCV MJ/kg |
---|---|---|
Bambusa oldhamii Munro | 18.50 | 17.20 |
Bambusa vulgarisVitata | 18.19 | 16.91 |
Bambusa vulgaris Schrader | 18.36 | 17.07 |
Dendrocalamus strictus | 18.50 | 17.20 |
Dendrocalamus asper | 18.19 | 16.91 |
Wood | 21.00 | 19.50 |
Phalaris arundinacea (Reed Canarygrass) | 18.49 | 16.83 |
Black Coal | 29.60 | 28.70 |
Source for bamboo: [
Boudouard reaction (Equation (3)), C + CO2↔2CO, needs a certain amount of heat in order to occur. The reaction consumes CO2 and produces CO which explains their behavior in
The quantity of hydrogen first increases but then starts consuming itself after 700˚C. This affects negatively the LCV of the gas. The gas would have its highest heating value at 500˚C because of the very high methane content but it has to be kept in mind that lignin (tars) needs 800˚C - 900˚C of temperature to decompose [
Considering all three factors (gas composition, temperature and the ER) it seems that the optimal values would be between 700˚C - 800˚C when the ER is 0.34 - 0.37 and the LCV of the gas 4320 - 3866 kJ/kg at 25˚C.
A/F-ratio (the ER): the air-fuel-ratio (A/F) for the values 0.5 - 2.0 was varied next. The two A/F-values (simulated and stoichiometric) are listed in
As already explained, the gasification temperature and the A/F-ratio are dependent on each other in Thermoflex. That is why the
The moisture content: The third simulation was done by varying the moisture content of bamboo for the range 0% - 25% as seen in
According to
A/F (simulated) | A/F (calculated) | ER |
---|---|---|
0.50 | 4.64 | 0.11 |
0.75 | 4.64 | 0.16 |
1.00 | 4.64 | 0.22 |
1.25 | 4.64 | 0.11 |
1.50 | 4.64 | 0.16 |
1.75 | 4.64 | 0.22 |
2.00 | 4.64 | 0.27 |
would be energy and time consuming. A small amount of water is also needed as steam to react with volatiles and char and to take part in the water-gas shift reaction (Equation (4)) that produces hydrogen. Thus 0% is not convenient for the process. With reasonable effort the moisture content can be reduced to 10% - 15% which is considered as a suitable amount of humidity in the fuel according to the sensitivity analysis and the literature. Thus 15% will be considered a satisfactory value.
Temperature of pre-heated air: the last simulation is done with a slightly different system. A heat exchanger is added into the system to utilize the hot exhaust gases of the ICE in order to pre-heat the air that is fed into the gasifier. Normally the air is at ambient temperature but now it is heated from 50˚C up to 500˚C. It can be observed in
Although there are no big changes in the gas com-position according to
be investigated using different software or observing the process in practice.
Bamboo in general seems to be a satisfying feed-stock for a downdraft gasifier because of its acceptable heating value, low ash content and low sulfur content. According to this study Bambusa oldhamii Munro and Dendrocalamus strictus seem to have the best qualities for gasification.
Finding the optimal parameters and obtaining good quality synthesis gas are challenging tasks because of the many affecting variables. According to Thermoflex simulations, the following parameters should be applied for the process:
• MCfuel = 10% - 15%
• Tgasifier = 700˚C - 800˚C
• A/F = 1.25 - 1.75 (ER = 0.27 - 0.38)
This would set the lower calorific value of the gas approximately between 3647 and 5046 kJ/kg at 25˚C. Beside the optimal process parameters discussed in this study, the Thermoflex simulations also helped to estimate the needed mass flows for bamboo and air so that enough gas could be produced to generate a 50 kW power. The future investigation should concentrate on real experiments on the field in order to validate the simulated results.
The authors would like thank the Sustainable Energy Fund of SENER-CONACYT (Project 152364, Trust 2138) for funding this project. They also thank the Ministry of Foreign Affairs of Mexico (Secretaría de Relaciones Exteriores), Tampere University of Technology and International Centre for Mobility (CIMO) who gave a grant for the Master Thesis of J. Salovaara. Special thanks are also given to the whole department of Renewable Energy-IIE.