The use of coking byproducts - in particular, organic additives - in coal batches for coking is of great interest.
It not only permits the safe use of chemical wastes of the coking industry but also increases coke extraction from the coking chamber, without impairing its physical, physicochemical, or mechanical properties, thanks to the greater packing density of the batch.
The influence of organic additives on the thermochemical transformation of the batch includes the following aspects.
With heating, the additives have two main effects:
- they increase the packing density of the coal, which affects coke formation;
- and they enter directly into chemical reaction with the organic mass of the coal grains at the contact surface.
The additives change the initial temperature at which the coal passes to the plastic state; this may expand the temperature range at which the plastic state exists, thereby promoting reaction with the softening coal.
In this case, the main effect of the additive is to cake together the softened coal grains.
The additive must both modify the surface of the heated coal grains and create an intergrain layer of strength no less than that of the coke and the coal grains. This evidently explains why the best additives are organic compounds whose decomposition temperature is close to that of the coal batch, so as to further promote plasticization within the grains and hence the production of strong coke pieces by the mechanism of bulk sintering.
Research at Bagleisk Coke plant shows that byproducts intensify the chemisorptive reaction with the filler and initiate polycondensation processes in the thermochemical transformations of the organic material within the coal.
It has also been established that products such as acidic tars and boiler residues from the rectification shop are surfactants, while boiler residues and polymers from the benzene department contain up to 50–75% unsaturated compounds.
Both are initiators of condensation reactions and participate in sintering and coking. It is also found that oily additives to the batch increase the coke yield by 0.4–0.5%, reduce the sulfur content by 0.15–0.20%, and reduce its reactivity.
Carbon- bearing byproducts of the coke industry that do not
impair the quality of the coke produced may expediently be added to the coal batch.
When it is necessary to used carbon-bearing additives with a low yield of volatiles, which make the batch more lean, the deterioration in sintering properties of the batch may be compensated by adding a component with good sintering properties.
When using byproducts which potentially might impair the quality of the final coke, they must be supplied to the coking chamber at the end of the coking period, when the thermal transformation of the coal batch is almost over.
Obviously, it is necessary to optimize the additives employed for the sake of rational coke production, taking account of the properties and preparation of the batch, the coking conditions, and the properties (physical and chemical) of the organic additives.
The basic requirements on the organic additives to the coal batch are as follows:
- rational viscosity, consistency, ease of transportation and addition to the coal batch;
ability to influence the packing density of the coal charge and the possibility of regulating this effect by means of the quantity of additive;
- a content of neutral components (pH 7) such that the cladding of the coking-chamber walls remains intact;
- the presence of aromatic compounds capable of intensifying the polycondensation processes that accompany the thermal transformation of the coal on coking.
Currently, the most common additives at coke plants are coking byproducts (heavy coal tars from mechanized thinning systems, acidic tars from the sulfate department, tars and oils from biochemical equipment, boiler residues from various tanks, etc.).
The supply of additives is organized by means of special equipment with dosing systems. Dosing is necessary on account of the influence of organic additives on the batch density, which enhances coke production, in particular, by increasing the coal charge in the coking chambers.