Gasification Technology
In the frame of the project MTF developed a new, oxygen-steam-air driven biomass gasification system and process capable of producing syngas with calorific value up to 8 MJ/m3 on a tar free basis. This device is an improvement of the biomass gasification units successfully used in many industrial plants in Poland, intended for energy production using waste from technological processes as a fuel. The operating scheme of the system is presented on Figure 1. The fuel is fed into the gasifying chamber at the upper part of the device through two separate dosing systems located symmetrically on both sides of the unit to secure more uniform level of bed formation. The chamber is 3,5 m in width a 6.5m in height. The bed height is 3,5 m while operating in high bed mode to accommodate the dry wood chips feed, separated graphically on Figure 1 into ash, gasifying, pyrolysis and drying zones. At the bottom we placed a sophisticated ash removal system coupled with steam and air injection in the form of four rotating conical grates each equipped with two ploughshares moving ash to the receiving auger presented on Figure 2. A combination of hot air-steam-oxygen rich air is fed into the gasifier through each of the four rotating elements made of cast iron with very high chrome content to withstand the temperatures of the process.
Slow rotating movement of the grate mixes the materials and pushes the ash into the receiving auger. The heat removed from the producer gas by the gas cleaning system is used to heat up the primary and secondary air to 250°C. The 186°C overheated steam with 4bar pressure is produced in electrically heated steam generator at a rate up to 100kg/h. The oxygen rich air, containing up to 40% of oxygen and flow speed standing at 100 is produced in a separate unit utilizing the molecular sieves method. These air streams are mixed with steam prior to being applied in gasification process. The role of steam is twofold. First it performs as an oxygen dispersive medium preventing occurrences of high temperature spots at the bottom of the gasifier, where the char and gas burning processes are the most intense.
The hot gases move up in the chamber and in the upper part of the gasification unit the overheated steam and CO2 can react with fixed carbon and tars to produce CO and H2 in a water shift reaction, efficient in high temperatures: C + H2 O → CO + H2 +131.28 kJ/kmol (endothermic) CO + H2 O → CO2 + H2 - 41.15 kJ/kmol (exothermic) C + CO2 → 2CO +172kJ/kmol (endothermic) improving significantly the producer gas quality and lowering temperatures in the carbon burning zone.
We constructed a unit allowing for various combinations of gasification gases delivered via a multilayer and multipoint injection system. At the bottom of the gasifier four rotating cones equipped with multiple inlets serve as main injection ports. The oxygen rich air mixed with steam is delivered into the bottom part of the unit through these cones. Approximately 50% of the oxygen necessary for the gasification process is provided through these four injection ports and most of the necessary hot CO2 and H2 O, is produced in the bottom area of the reactor. The remaining air necessary for the process is injected by 32 nozzles located above the cones, close to the bottom of the pyrolysis zone. The delivered air reacts with the hot pyrolytic carbon producing CO – rich gases and extending the high temperature zone in the gasifier - intensifying the process of wood drying and carbonizing. Such construction guarantees a more uniform distribution of gases and wood chips in the chamber, and therefore improves the mixing of carbonized material with gasifying agents. This enhances the effectiveness of gas production and unification of the temperature’s distribution in the gasification process. The control unit allows us to continuously change the parameters and the composition of gasifying gases leading to better control of the producer gas parameters.
We designed an absorptive gas cleaning system presented on Figure 3. The gas cleaning system consists of a cyclone integrated with the gasifier outlet (not shown on the drawings), with ceramic lining capable of withstanding temperatures of up to 1200°C, which removes part of carbon dust and as we see on Figure 3 an absorptive unit composed of an air-gas exchanger, water scrubber for dust and impurities removal, two water operated coolers able to lower the syngas temperature to 60°C, an oil scrubber an active carbon filter.
The syngas cleaning takes place by precipitation and removal of tars and heavy hydrocarbons contained therein by adequate cooling of the gas in several stages and absorption of light impurities in the syngas by directing the gas flow through two absorption devices (scrubbers). Water is used as absorbent in the first scrubber, oil in the second one. Both scrubbers are equipped with demisters placed immediately before the outlet, which keeps the scrubbing liquids inside the apparatus
Fig.1. The scheme of gasification system
Fig. 2. The receiving auger.
Fig. 3. The absorptive cleaning unit.
Our numerical model in Open Foam environment enables volumetric implementation of thermal processes by introduction of a porous medium into the computational domain. Modeling the flow inside a porous medium uses the Immerse Boundary Method, maintaining the same equations inside and outside the medium, and a medium boundary where the physical parameters in these equations have discontinuities. New sets of numerical libraries were developed for these processes describing the thermal, chemical, and radiative properties of the porous medium, scalar porosity and the viscous flow resistance tensor present in the set of viscous hydrodynamics equations describing gas flows. Treating gasification as a volumetric process allows for numerical simulation of this process as the front of the reaction moving inside the biomass bed. This allows the simulation of both thermal and chemical processes. The three-dimensional time-dependent mathematical model for thermal conversion of biomass is based on conservation laws determined independently for the gas phase and the solid phase. For the gas phase, these are the viscous hydrodynamics equations with the sources, and for the solid phase, the equations of component conservation and energy conservation. Thermal phenomena include evaporation of moisture, pyrolysis, gasification, tar gasification and gas combustion, which transfer energy and mass from a solid phase to gas, the volume and temperature of which increases at the expense of the mass of the solid phase. A functional numerical model was made in the Open FOAM environment, based on the above mathematical model, which was used to calculate the gasification process in the gasification chamber. The program has been supplemented with two modules describing pyrolysis and gasification of large particles.
These program codes are open and can be downloaded from following addresses:
CFD OpenFOAM:
https://github.com/pjzuk/porousGasificationFoam
https://github.com/btuznik/porousGasificationFoam
Piroliza
https://github.com/pjzuk/porousGasificationFoam
https://github.com/btuznik/porousGasificationFoam
As part of the work on expanding the base of biomass materials that can be used in gasification installations, laboratory tests on the gasification of leather waste were carried out in cooperation with the Institute of Fluid-Flow Machinery of the Polish Academy of Sciences in Gdansk, and they were compared with the results of the industrial gasification installation. The research results were published in the International Journal of Energy Research in 2021.
Technical and economic viability of an MTF gasification system was also analyzed – capital expenditure, operating expenditure, and the cost of producing 1 MWh of electricity and GJ of heat energy at different purchase prices of raw materials were all determined. To model the full CCHP system built by our partners in RISE, we wrote an open architecture economic calculator which allows the user to determine the profitability of the investment.
Bibliography:
Dudyński, M. (2018). Gasification of Selected Biomass Waste for Energy Production and Chemical Recovery. Chemical Engineering Transactions 65, 391-396. https://doi.org/10.3303/CET1865066 Dudyński, M. (2019). Novel oxygen-steam gasification process for high quality gas from biomass. Detritus. Vol.06. pp.68-76. https://doi. org/10.31025/2611-4135/2019.13814
Marek Dudyński (2021) WOOD GASIFICATION. INFLUENCE OF PROCESS PARAMETERS ON THE TAR FORMATION AND GAS CLEANING. Detritus in press.
Gasification of leather waste for energy production: Laboratory scale and industrial tests
Marek Dudyński, Karol Dudyński, Jacek Kluska, Mateusz Ochnio, Paweł Kazimierski, Dariusz Kardaś
International Journal of Energy Research
First published: 15 June 2021
