Internal Thermal Biomass Carbonization Plant

タイトルまたは説明

Carbonization is one of the most important process in the production of activated carbon with gas-activating method. In the process, materials are heated with a small amount of air to eliminate non-carbon substances for producing carbonaceous materials required for activation. Stack of carbonization furnace consists of a material bin, a lifter, a feeder, stack, refractories, a rotator, a waste heat boiler, a temperature detector, a PLC cabinet, a cooler, a settling chamber, a fan, and a device for dedusting and end-gas treatment. The carbonization process is an integration of carbonization of materials and treatment of end gas.
 

Carbonization
Carbonization is a process in which materials going through destructive
distillation at low temperature. During the process, refractories are heated by
radiation heat generated through combustion of fumes in chlorine. Materials are
heated gradually in a low-temperature ambient isolated from air. At this point,
materials are distributed on bottom of the furnace with a minor exposure to
oxygen, and free of loss from ignition since the flames of fume combustion are on
top. Lower molecular substances in materials are vaporized first under radiation
heat, physical changes involved therein include dehydration, degassing and
drying, and chemical changes include pyrolysis and condensation polymerization.
Bonded groups of oxygen in organic compounds are broken, oxygen separated
in forms of H2O, CO and CO2, and aromatic compounds and cross-linked
high-hardness carbon- structures are formed. During carbonization, oxygen,
hydrogen and other non-carbon are discharged in pyrolysis, and ordered basic
graphite crystal structures are formed. The crystals are combinations of irregularly
arranged carbon atom hexagons, leaving gaps as initial pores of carbonized
materials. Therefore, carbonization forms secondary pores for activations, and
provides mechanical hardness required for activation. Requirements on material carbonization are results of carbonization.
Criterion on size and shape, initial pore structure and mechanical hardness
are set for carbonized materials. Carbonization may be divided into the following stages.
 (1) Drying stage below **0°C. Internal and external moisture contents of
materials are vaporized by external heat. No deformation of materials at this
stage.
 (2) Heat- absorption and pyrolysis stage between ******0°C. Materials are
decomposed, releasing gaseous products (CO, C*2, H2 S). Change in chemical composition of materials is started. Pyrolysis of various materials are triggered at different temperatures. Pyrolysis of material with lower metamorphic grade initiates at a lower temperature.
 (3) Carbonization stage between ******0°C. At this point, fume temperature
in furnace may reach ******0°C. The stage involves mainly condensation
polymerization and pyrolysis. A large quantity of volatile is separated from
materials. Almost all tars, pyroligneous liquid and ethylene are products of this
stage. Materials are softened and melted at this stage, forming tristate plastic
mass, which will turn into semicoke after flowing, condensation polymerization
and solidification.
Heating rate is the major operating condition in carbonization process.
Heating rate significantly influence yield of carbonized products. A higher
heating rate would increase tars and gas separated from materials, but reduce
yield of carbonized materials. A lower heating rate would lead to a prolonged
duration which offers pyrolysis with a high selectivity. Initial pyrolysis breaks weak bonds in material molecular for concurrent sequential condensation polymerization, forms structures with higher thermal stability, and thereby reduce yield of volatiles separated from high-temperature pyrolysis, achieving higher productivity of solid carbonized products (carbonized materials). Yield,
temperature and capacity are adjustable by controlling feeding amount and
chloride amount.
Volatiles, carbon content, moisture content, as content and hardness are major
indexes to evaluate quality of carbonized materials. Qualified carbonized materials
shall be with a volatiles content of *2%**6%, a moisture content of *5%- *5%, and
a ball-pan hardness of *0%.
As measurements of the above indexes are time-consuming, and immediate
evaluation on carbonized materials are required in on-site debugging for further
adjustments on parameters, visual inspection and evaluation are acceptable.
Qualified carbonized materials shall have smooth and crack-free surfaces, high
hardness and uniform sections.
Treatment of End Gas from Carbonization
End gas from carbonization consists of two parts, namely the high- temperature
gas generated from external fuel combustion, including
CO2, H2O, N2 and traces of SO and CO, and volatiles generated from pyrolysis of materials, including CO, H2, CH4, alkane, olefin and tar. Traces of harmful substances could be found in
end gas from carbonization, which could cause air contamination without proper
treatments. Therefore, such end gas has to be treated before discharging to
atmosphere. Negative pressure must be applied for all processes during such
adjustment, positive pressures may cause explosion.
With the burning method, end gas is induced into settling furnace for
sufficient burning in an environment with excessive air at *******0C, in which
combustible gases and toxic substances will be burnt into CO2 and dumped.
Heat generated by the such combustions may be recycled by the waste heat
boiler for vapor production. It is a low-cost simple method which could eliminate
most of environmentally harmful substances, and generate vapor required for
carbonization, therefore, the method is widely used in production of activated
carbons.