Pyrogasification Plant

In thermal degradation processes, fuels can undergo several kinds of transformations according to the conduction of the oxidation reaction.
When oxygen content keeps its concentration levels low inside the zone of reaction, some reactions occur, causing the formation of chemical species which can be used in specific engines turning the mechanical work into thermal and electric one.
Both typical pyrolytic and gasification reactions happen in the oven; for this reason this process is defined pyrogasification. Joining the benefits of the different energy balances, we obtain, from the biomass degradation, other chemical compounds useful to feed cleanly the engine connected to the electric generator.

After the starting phase, production is mainly formed by a synthesis gas , defined in various ways, and containing essentially CO; H2; light hydrocarbons (C1-C3); CO2, N2, H2O. The residues consist of charcoal and other solid inerts.

DESCRIPTION OF THE PROCESS

Biomass, (with 15-20% of max.wet in input) reduced in pieces 30/50 mm large, is sent inside the reactor through the top of the gasifier where there is a temperature gradient and the possibility to add an air mixture with adjustable flow.
This controlled quantity of combustive agent has to start and conduct the oxidation reactions providing the energy sufficient for the next thermal dissociation.

This thermal degrading process creates, at normal performance, a gaseous energy phase, a solid mineralized phase and a liquid phase, even if in lower quantities. When coming out from the pyrogasification reactor, the produced gas has still an high temperature and its enthalpic content has to be lowered by an air/fumes exchanger.

With a sufficiently proportioned ventilator and a series of finned tubes, the output gas temperature is led to approx. 80-65°C. The deducted heat can be partly used ,for example to dry the supply biomass storage tank.
Any condensed gas and low tar quantities, that can be formed during the thermal exchange process , are collected in a purge pipe, placed in the collector of the exchanger distribution.

 

These condensed gases can be stored or disposed. Any other impurities, dragged the next phase, are purified in a cooling tower. In fact, cooled gases, carrying some impurity residues, are introduced and washed with basic water (pH 8-10) (basic for NaHCO3, NaOH, CaO, DEA, and others, depending on availability) in a scrubber with filling elements, increasing the exchange surface and with a pump to circulate the washing liquid.

The washing solution, collected in the tank under the scrubber, reduces the last particulate residues, dragged by the gases. Since, over time the washing solution gets rich of polluting agents, it is necessary to treat periodically the effluent or organize its transfer. Finally, the cooled and purified gas, passes into another dehumidification unit with high absorbing power to optimize the combustible mixture for the endogenous engine.

The processes are controlled, during the various phases, by thermocouples, pressure transducers, flow meter and a gas analyser to calculate the percentage of CO or other gases, if necessary (CH4, CO2, O2, H2, etc.).

DESCRIPTION OF THE PLANT

The pyrogasification plant is formed by machinery and reactors connected to reach the set goal.

The core of the plant is the reactor where the main reactions take place and it is formed by 2 cylindrical and concentric rooms: a steel internal one with high resistance,where the above mentioned reactions take place, and an external one that conveys the synthesis gases to the cooling system and to the next processes.
The external cylinder is covered on the outside with refractory panels, thick enough to create a sufficient thermal insulation.
The reactor has a safety valve on the upper part and a liquid seal flame arrestor for the discharge on the bottom. Moreover, we can introduce a nitrogen flow in the system, that makes the extinguishing operations easier.
The nitrogen cylinders are stored next to the plant.

The next plant sections have to cool and to purify the gaseous mixture obtained in the reactor, and they are:

• an air/fumes exchanger with a bottom condensate collector vessel, finned tubes and proportioned ventilator.
• Washing tower with pH basic solution and filling elements
• dehumidifier
• endogenous engine (equipped with filters)

In start-up phases or when needed, , the plant has a torch with a pilot light where the gases produced in the reactor can be conveyed and burnt, bypassing the other sections.

CONCLUSIONS

Compared to incineration, this technology for energy exploitation of natural biomass, enables an important energy valorisation, minimising the pollution risks and environmental impact.
Due to the relative homogeneity of the used raw material, the system can be calibrated to obtain an excellent energy potential, keeping the capital ecological needs in this application as well. This technology for the energy valorisation is considered among the best ones, with an important ratio of biomass used to kW produced.

This technology can produce approx. 1 kW gasifying 1,1-1,7 kg of dry woody biomass (depending on the kind of wood) and emitting, at the final discharge, a lower concentration of polluting agents than the actual law limits for the emissions into the atmosphere