Method and Device for processing Carbon Soot Waste

The invention relates to a method and a device for processing a slurry comprising water and solid residues from a gasification process of solid and/or liquid hydrocarbon feeds.

Carbon monoxide- and hydrogen-containing gas mixtures, termed synthesis gases, are important starting materials for producing a multiplicity of products, such as ammonia, methanol or else synthetic fuels. A widespread industrial process by which synthesis gases are produced is the gasification of solid and/or liquid carbonic feeds by Partial Oxidation (POX). In this case a generally preheated carbonic feed is reacted with steam and an oxidising agent in a reaction chamber (POX reactor) at temperatures between 1300 and 1500°C and pressures up to 150 bar to give a crude synthesis gas which mostly consists of hydrogen, carbon monoxide, carbon dioxide and water. The heat required for the reaction is generated by incomplete (partial) oxidation of carbon resp. hydrocarbons present in the feed. For this, oxygen is fed to the POX reactor in an amount which is insufficient for complete reaction of the oxidisable components.

In Partial Oxidation, in addition to gaseous components, solids are also produced, such as soot and ash, which must be removed from the crude synthesis gas before this can be further treated. The crude synthesis gas exiting from the POX reactor having a temperature above 1300°C is first cooled and subsequently subjected to water scrubbing. If the crude synthesis gas is cooled in direct contact with water (quenching), already in this process step, a large part of the solids is scrubbed out of the crude synthesis gas and pass into the quench water. In the water scrubbing, fine purification proceeds in which the solids content of the crude synthesis gas is reduced typically to approximately 1 mg/m3. The thus received slurry, commonly termed as Carbon Soot Waste, which typically consists from 70-85%w water and 1-5%w metals and ash, cannot simply be released into the environment, but has to be processed in order to destroy hazardous components or bring such components in a form ready for disposal. It is therefore an object of the present invention to specify method and a device for carrying out the method which enable the economic and safe processing of Carbon Soot Waste.

The object in question is achieved according to the invention in terms of the method in that the slurry, referred to as Carbon Soot Waste, is passed into a rotary kiln furnace where contained water is evaporated in a drying section and combustible substances are oxidised, whereby heat is provided for the drying section by a burner or lance firing co-current to the Carbon Soot Waste and combustion air is conducted through the rotary kiln furnace counter current to the Carbon Soot Waste in an amount sufficient to supply oxygen in excess to the process.

Rotary kiln furnaces are well known in the state of the art for thermal treating of solid materials. A rotary kiln furnace consists of a cylindrical, rotating body mounted between stationary material feed and outlet housing. The inclined axes and the constant rotation of the kiln body provide for the transport and intense mixing of the material, ensuring that it is processed homogeneously. In a direct-fired rotary kiln furnace, a burner is situated inside the kiln body, that is, inside the reaction chamber. The material to be treated is heated directly by the burner flame and the stream of hot gas produced by the burner. These kilns, usually lined with refractory material, are highly robust, easily scalable and can be used to achieve high throughput rates with relatively low production costs.

According to the invention, the Carbon Soot Waste is fed into the reaction chamber of the rotary kiln furnace at the upper end of the cylindrical body, while the combustion air is introduced via the lower end. The burner or lance over which an auxiliary fuel such as natural gas, diesel or waste solvent is injected, is installed at the upper end of the cylindrical body, to provide highest radiation heat input to the upper section of the reaction chamber. The Carbon Soot Waste enters the reaction chamber with highest water content and is dried under the influence of the radiation heat, wherefore the upper section of the reaction chamber forms the drying section. Preferably the water content is reduced in the drying section to approx. 60%w, below which the Carbon Soot Waste is self-combustible, and combustion will continue without support of auxiliary fuel providing sufficient combustion air is added to the reaction chamber. Preferably the combustion takes place under an oxidising environment with an oxygen content exceeding the stoichiometrically required quantity by 2-10%v and at temperatures in the range of 600-900°C.

Although the oxidising environment in the Rotary Kiln furnace are expected in the range 2-10%v oxygen, in particular if vanadium is present in the Carbon Soot Waste, it is favourable to operate the furnace in flow direction of the Carbon Soot Waste downstream the drying section under substoichiometric conditions, with lower or zero partial pressure of oxygen in range 0-2%v. Under these conditions the formation of Vanadaium Pentoxide (V2O5) in the furnace is avoided, which is molten and therefore has a higher propensity to carry-over in the fly ash and cause downstream corrosion. Under reducing conditions V2O4 is expected to be formed with other lower oxidation states of vanadium, with lower volatility and corrosivity to the downstream plant.

Control of the stoichiometry of the burner or lance and the directional flow of the flame can be adjusted to produce substoichiometric conditions over the Carbon Soot Waste.

Additional air or oxygen is added either over the flame region above the Carbon Soot Waste or in the secondary combustion chamber to allow for complete combustion of unburnt hydrocarbons and carbon monoxide, before the flue gas is conditioned and released to atmosphere via the stack.

The control of the burner stoichiometry and the addition of combustion air is critical to the staging of reducing and oxidising conditions within the furnace. A burner management system with feedback control from flue gas oxygen monitoring devices will be used in order to allow such control. Oxygen monitoring devices can be installed at the exit of the Rotary Kiln and at the exit of the Secondary Combustion Chamber.

The rotary kiln furnace is operated with a rotating characteristic such as rotational speed and holding time that allows proper mass reduction of the Carbon Soot Waste of more than 90%. Residual carbon in the ash can be reduced to less than 30%w via controlling e.g. the combustion temperature, the excess oxygen and the residence time of the material in the cylindric body of the rotary kiln furnace. A high level of mass reduction and low carbon content in the ash provide a higher quality soot for landfill, with low propensity for leaching, negating ned to stabilise material or for further downstream metals recovery. In some cases, it can be energetically and economically more favourable to feed the Carbon Soot Waste not with its full water content directly into the rotary kiln furnace. In an embodiment of the invention the Carbon Soot Waste is treated in a pre-drying device upstream of the rotary kiln furnace in order to reduce the water content. Preferably steam or combustion flue gas exiting the rotary kiln furnace is used as drying agent in the pre-drying device. The gas phase produced in the pre-drying device, comprising water and hazardous components, such as NH ₃ , H ₂ S, HCN, is sent to the rotary kiln furnace or a combustion chamber.

The combustion flue gas produced in the rotary kiln furnace is extracted from the upper end of the cylindrical body, while slag and ash are withdrawn from the lower end. The extracted flue gas is treated in a flue gas treating system where it is first passed through a post-combustion chamber for further combustion of residual hazardous components, such as NH ₃ , H ₂ S, HCN, and quenched by means of air or water to below 500°C. Dioxins and furans that can form during the quenching are removed in a suitable treatment system, before the treated flue gas is passed through a filter unit to capture any residue solids, producing a flue gas emission that is compliant with international regulations. The ash extracted from the rotary kiln is cooled via a water cooled device such as a water cooled screw conveyer (or similar) and residual solids collected from the flue gas are disposed, if necessary or appropriate after stabilisation e.g. by encapsulation with cement, or the recovery of metals like vanadium or nickel. In preferred embodiments of the invention, activated carbon is injected into the quenched flue gas to adsorb dioxins and furans, and/or bicarbonates are added to the flue gas to react with S0 ₂ .

To destroy dioxins and furans and combust carbon dioxide, the post-combustion chamber is operated at temperatures in the range of 900-1100°C with a residence time of the flue gas of at least 2 seconds.

Preferably, nitrogen oxides contained in the combustion flue gas from the rotary kiln furnace are reduced by a urea based selective non-catalytic reduction system.

In an embodiment of the invention, the rotary kiln furnace and the flue gas treating system are operated under negative pressure, preventing the release of toxic gases to the atmosphere. The negative pressure is created by a fan installed upstream of the flue gas stack. The system will automatically shut down when the pressure inside the rotary kiln furnace approaches atmospheric pressure.

Recovery of heat from the flue gas is possible downstream of the quenching section via the use of a waste heat boiler for the production of steam, which is preferably used as drying agent in the pre-drying device and/or for preheating the combustion air to a temperature in the range of 120-250°C. In case of any blockage from fly-ash carryover or corrosion, the flue gas can be conducted in a bypass around the waste heat boiler.

In another embodiment of the invention, direct recovery of heat from the flue gas is possible using a heat exchanger to preheat the combustion air to the Rotary Kiln to a temperature in the range 120-500°C. Where exposed directly to the flue gas the heat exchanger is refractory coated to avoid corrosion. In case of any blockage from fly-ash carryover or corrosion, the flue gas can be conducted in a bypass around the waste heat boiler or designed for easy removal and replacement.

The invention also relates to a device for processing a slurry referred to as Carbon Soot Waste and comprising water and solid residues from a gasification process of solid and/or liquid hydrocarbons.

The object in question is achieved according to the invention in terms of the device, in that it comprises a rotary kiln furnace for incinerating the Carbon Soot Waste, the rotary kiln furnace is equipped with a burner or lance firing co-current to the Carbon Soot Waste and providing heat for a drying section where water contained in the Carbon Soot Waste can be evaporated, and a combustion air supply via which combustion air can be conducted through the rotary kiln furnace counter current to the Carbon Soot Waste in an amount sufficient to supply oxygen in excess to the process.

The rotary kiln furnace is lined with refractory material suitable for corrosion attack from alkali and vanadates. The refractory material is either castable or refractory brick of a thickness in the range of 200-400mm, providing sufficient insulation to protect the metallic furnace shell from high temperature and the furnace from excessive heat loss. Refractory bricks provide the advantage that they can be installed pre-dried avoiding in-situ refractory dry-out and do not require anchor bolts for attachment. Castable provides the advantage that it can be easily maintained by application of new material and prevents carbon soot material from passing through gaps causing damage between heat up and cool down of the rotary kiln furnace.

Typically, the rotating cylindrical body of the rotary kiln furnace has a length between 10 and 25m and a diameter in the range of 2-3, 5m depending on the required throughput of the furnace.

The inventive device also comprises a flue gas treating system for treating the combustion flue gas exiting the rotary kiln furnace with at least a post-combustion chamber lined with refractory material suitable for corrosion attack from components in the combustion flue gas. The post-combustion chamber is designed as a cylinder with vertical axis, allowing slag formed from fly ash carried over with the combustion flue gas to flow down into a catch pot for collection and disposal.

A filter unit used to capture solids in the combustion flue gas advantageously comprises at least one filter bag consisting of a ceramic material eliminating the risk of fire brought about by glowing embers carried over by the flue gas.

The inventive device preferably comprises a system for collecting slag from the rotary kiln furnace including a water or air quench system to cool the residues that may still be combusting while being removed from the furnace.

Hereafter, the invention will be described in more detail on the basis of two exemplary embodiments shown schematically in Figures 1 and 2.

Figure 1 shows a rotary kiln furnace suitable to perform the inventive method.

Figure 2 show a preferred embodiment of the invention.

In Figure 1 Carbon Soot Waste 1 is fed into reaction chamber 2 of the rotary kiln furnace 3 at the upper end of the rotating cylindrical body 4, while the combustion air 5 is introduced via the lower end. Over a burner or lance 6 an auxiliary fuel 7 such as natural gas or diesel is injected into reaction chamber 2 co-currant to the Carbon Soot Waste 1 at the upper end of the cylindrical body 4. The flame 8 of the burner or lance 6 provides highest radiation heat input to the upper section 9 of the reaction chamber 2. The Carbon Soot Waste 1 enters the reaction chamber 2 with highest water content of up to 85%w and is dried under the influence of the radiation heat, wherefore the upper section 9 of the reaction chamber 2 forms a drying section. Preferably, the water content is reduced in the drying section to approx. 60%w, below which the Carbon Soot Waste is self-combustible, and combustion will continue in the carbon oxidation section 10 without support of auxiliary fuel providing sufficient combustion air 5 is added to the reaction chamber 2. The combustion takes place under an oxidising environment with an oxygen content exceeding the stoichiometrically required quantity by 2-10%v and at temperatures in the range of 600-900°C. In particular if vanadium is present in the Carbon Soot Waste, it is favourable to operate the carbon oxidation section 10 under substoichiometric conditions to avoid the formation of V2O5, which melt and therefore has a higher propensity to carry-over in the fly ash and cause downstream corrosion. In this case additional air or oxygen is added either over the flame region 8 above the Carbon Soot Waste or in the post-combustion chamber (not shown). The combustion flue gas 11 produced in the rotary kiln 3 is extracted from the upper end of the cylindrical body 4, while slag 12 and ash 13 are withdrawn from the lower end.

In Figure 2 Carbon Soot Waste 1 is fed into a pre-drying device P, where a first part of the combustion flue gas 2 exiting the rotary kiln furnace R is used as drying agent to reduce the water content before the pre-dried Carbon Soot Waste 3 is fed into the rotary kiln furnace R for combustion. The gas phase 4 produced in the pre-drying device P, comprising water and hazardous components, such as NH ₃ , H ₂ S, HCN, is mixed with the second part 5 of the combustion flue gas exiting the rotary kiln furnace R and sent to the post-combustion chamber C where at temperatures in the range of 900-1100°C within a residence time of the flue gas of at least 2 seconds hazardous components are destroyed. Downstream of the post-combustion chamber C the hot treated flue gas 6 is quenched with water or air to below 250°C. The cooled flue gas 7 is conducted into the treatment unit T where activated carbon and bicarbonates are added to adsorb dioxins and furans that have formed during quenching and to react S0 ₂ created in the post-combustion chamber C. The flue gas 8 exiting the treatment unit T is passed through the filter unit F to capture any residue solids, producing a flue gas emission 9 that is compliant with international regulations, wherefore it can be released to the atmosphere via the chimney K. Bottom ash 10 from the rotary kiln furnace R and residual solids 11 from the filter unit F are sent to the ash collection unit S where they are brought e.g. by encapsulation with cement into a stabilised form 12 ready for disposal.