Gasification is an important technology that will help
utilize agricultural waste products to meet growing energy demands. The high variability in biomass feedstocks causes
high variability in the composition and quantity of biogas produced.
The study analyzed in this post (citation and link at the bottom) derives a model to predict the
gasification products, and compares them with experimentally obtained data.
Why is this
important?
Gasification of biomass is becoming more widely used in
small scale local production. Besides
having a close to “net zero” carbon impact, it is widely available, and
under-utilized. The low energy density, which is inherent
in many agricultural and industrial waste products are ideal for
gasification. This makes proper utilization even more important. To be considered a clean technology, gasification must take place in a manner that does not create significant atmospheric contamination. This is done by achieving more complete utilization, as well as lowering concentrations of undesirable by-products. Having a computer model which can effectively estimate biogas components for specific conditions makes more ideal gasification easier to achieve. Models have been previously created for such situations, however discrepancies under certain conditions have been large. This work builds upon previous models, and aims to lower the variability between the simulation and experimental results.
What they did:
This took place in three distinct parts, which were modeling, gasification and comparing.
1)A mathematical model was derived to predict the constituents of the resulting biogas produced from gasification of pine wood.
2) A fluidized bed gasifier was used (utilizing pine wood) to create biogas for analysis in a gas chromatograph.
3) Results from each part were compared to each other to determine the efficacy of the model created.
Model
The model was mostly built upon the work of four previous studies. Without delving too deep into the derivation, the main assumptions were as follows:
- Gas in bubble and emulsion phase was assumed to act as a plug flow.
- Solids in emulsion phase were underwent complete mixing.
- Wake and cloud region of bubbles in fluidized bed were neglected.
- Gas in the emulsion phase ascends at the minimum fluidization velocity.
- Final products are calculated using ideal gas law.
With these assumptions, as well as equations deduced from previous research, the model was created. Matlab was used to solve the differential equations and obtain numerical values for the results.
Gasification
The apparatus (broken down into upstream, gasifier unit, and downstream parts)
Upstream:
The vapor feeding system consisted of water (combined with a heater for steam generation), high pressure air, high pressure nitrogen, as well as valves (to control flow) and flowmeters to measure. Biomass feeding was done though the top of the unit so the particles could almost instantaneously be introduced into the fluidized bed.
Gasifier:
The researchers used a small (80mm internal diameter) fluidized bed gasifier, with wood as the biomass. The gasifier was split into two zones which were physically separated, allowing the combustion zone to be fully excluded from the gasification. The zones were separated as a means to ensure the syngas produces was high quality, and nitrogen could be supplied without stifling the combustion zone, mitigating the need to use pure oxygen in the gasification environment.
Downstream:
Post gasification, the flow entered an array of heated ceramic candle filters to remove entrained particles. Post filtering the gas flowed through a cold trap to bring the stream to room temperature for analysis. At room temperature the composition was measure using a GC-MS unit.
Procedure
Charcoal was used to heat the bed (isolated zone) to 750°C-800°C. The testing took place in two phases, pyrolysis and combustion. The pine wood was first pyrolized, and subsequently the remaining material burned. First, during pyrolysis testing, nitrogen was used as the fluidizing agent. The downstream flow was measured for the composition of H2, CO, CH4, and CO2,, and time averaged values were reported. Once pyrolysis was complete, the fluidizing gas was switched to air, and the residual material was combusted. The resulting gas was then analyzed for CO and CO2 as well as TAR's (which were broken down into four sub-groups). Gasification was also completed using steam entirely.
Validation
The model was quite accurate at depicting constituents of the final gaseous mixture. The difference between experimental findings and numerical analysis for yield was less than 5%, and less than 2% for composition. Unfortunately at high temperatures the model has larger discrepancies (about 20%) for various TAR compounds, which are largely comprised of benzene.
Thoughts
The ability for the model to accurately calculate the quantity as well as composition of producer gas is quite impressive. As more biofuels are utilized, consistency will become an ever more important issue. Using gasifiers as a means to more cleanly utilize various forms of biomass is a great strategy, but having large differences in gasses produced is a problem if the content is not previously known. As with other models, surely in the future this will be refined and optimized.
One major problem with biofuels in general is reliability. The stringent requirements for petroleum fuels that are used commonly make reliability not a big issue currently. Drivers don't have to worry if their fuel is going to "gunk up" engines, or combust only within a specific temperature range. Unfortunately these issues are more common in biofuels. It seems one way to deal with the reliability in liquid fuels would be to gasify the components, then once they are broken down into their constituents, they can be reformed into a reliable fuel. Although this model looks at the products based on a biomass input, further work could be done to generalize the work. That would make it easier to turn biomass into reliable liquid fuel.
Citation
Vecchione,L., Moneti, M, Di Carlo,A., Savuto, E, Pallozzi, V., Carlini, M., Boubaker, K., Longo, L., Colantoni, A. (2015) Steam Gasification of Wood Biomass in a Fluidized Biocatalytic System Bed Gasifier: A Model Development and Validation Using Experiment and Boubaker Polynomials Expansion Scheme (BPES. Int. Journal of Renewable Energy Development, 4(2), 143-152, doi : 10.14710/ijred.4.2.143-152
Article Link
http://ejournal.undip.ac.id/index.php/ijred/article/view/8641/PDF
What they did:
This took place in three distinct parts, which were modeling, gasification and comparing.
1)A mathematical model was derived to predict the constituents of the resulting biogas produced from gasification of pine wood.
2) A fluidized bed gasifier was used (utilizing pine wood) to create biogas for analysis in a gas chromatograph.
3) Results from each part were compared to each other to determine the efficacy of the model created.
Model
The model was mostly built upon the work of four previous studies. Without delving too deep into the derivation, the main assumptions were as follows:
- De-volatilization time was assumed negligible (based on previous work).
- Fluidized bed only consists of emulsions and bubbles with a single coefficient governing mass transfer.- Gas in bubble and emulsion phase was assumed to act as a plug flow.
- Solids in emulsion phase were underwent complete mixing.
- Wake and cloud region of bubbles in fluidized bed were neglected.
- Gas in the emulsion phase ascends at the minimum fluidization velocity.
- Final products are calculated using ideal gas law.
With these assumptions, as well as equations deduced from previous research, the model was created. Matlab was used to solve the differential equations and obtain numerical values for the results.
Gasification
The apparatus (broken down into upstream, gasifier unit, and downstream parts)
Upstream:
The vapor feeding system consisted of water (combined with a heater for steam generation), high pressure air, high pressure nitrogen, as well as valves (to control flow) and flowmeters to measure. Biomass feeding was done though the top of the unit so the particles could almost instantaneously be introduced into the fluidized bed.
Gasifier:
The researchers used a small (80mm internal diameter) fluidized bed gasifier, with wood as the biomass. The gasifier was split into two zones which were physically separated, allowing the combustion zone to be fully excluded from the gasification. The zones were separated as a means to ensure the syngas produces was high quality, and nitrogen could be supplied without stifling the combustion zone, mitigating the need to use pure oxygen in the gasification environment.
Downstream:
Post gasification, the flow entered an array of heated ceramic candle filters to remove entrained particles. Post filtering the gas flowed through a cold trap to bring the stream to room temperature for analysis. At room temperature the composition was measure using a GC-MS unit.
Procedure
Charcoal was used to heat the bed (isolated zone) to 750°C-800°C. The testing took place in two phases, pyrolysis and combustion. The pine wood was first pyrolized, and subsequently the remaining material burned. First, during pyrolysis testing, nitrogen was used as the fluidizing agent. The downstream flow was measured for the composition of H2, CO, CH4, and CO2,, and time averaged values were reported. Once pyrolysis was complete, the fluidizing gas was switched to air, and the residual material was combusted. The resulting gas was then analyzed for CO and CO2 as well as TAR's (which were broken down into four sub-groups). Gasification was also completed using steam entirely.
Validation
The model was quite accurate at depicting constituents of the final gaseous mixture. The difference between experimental findings and numerical analysis for yield was less than 5%, and less than 2% for composition. Unfortunately at high temperatures the model has larger discrepancies (about 20%) for various TAR compounds, which are largely comprised of benzene.
Thoughts
The ability for the model to accurately calculate the quantity as well as composition of producer gas is quite impressive. As more biofuels are utilized, consistency will become an ever more important issue. Using gasifiers as a means to more cleanly utilize various forms of biomass is a great strategy, but having large differences in gasses produced is a problem if the content is not previously known. As with other models, surely in the future this will be refined and optimized.
One major problem with biofuels in general is reliability. The stringent requirements for petroleum fuels that are used commonly make reliability not a big issue currently. Drivers don't have to worry if their fuel is going to "gunk up" engines, or combust only within a specific temperature range. Unfortunately these issues are more common in biofuels. It seems one way to deal with the reliability in liquid fuels would be to gasify the components, then once they are broken down into their constituents, they can be reformed into a reliable fuel. Although this model looks at the products based on a biomass input, further work could be done to generalize the work. That would make it easier to turn biomass into reliable liquid fuel.
Citation
Vecchione,L., Moneti, M, Di Carlo,A., Savuto, E, Pallozzi, V., Carlini, M., Boubaker, K., Longo, L., Colantoni, A. (2015) Steam Gasification of Wood Biomass in a Fluidized Biocatalytic System Bed Gasifier: A Model Development and Validation Using Experiment and Boubaker Polynomials Expansion Scheme (BPES. Int. Journal of Renewable Energy Development, 4(2), 143-152, doi : 10.14710/ijred.4.2.143-152
Article Link
http://ejournal.undip.ac.id/index.php/ijred/article/view/8641/PDF
