Method and system for cooling hot gases by water injection   

The invention relates inter alia to a method and a system for cooling and cleaning a hot gas, e.g. flue gas, produced in a thermal reactor, or—more precisely—using water injection to cool gases, released by thermal conversion (gasification or combustion) of fuels e.g. biomass, waste, coal, oils, gases or mixtures of these by evaporating part of or all the injected water.

Furthermore, the invention can be applied for cooling and cleaning hot gases, released from industrial plants; like cement kilns, refineries, metals works, etc.

BACKGROUND OF THE INVENTION

Hot gases are cooled and cleaned for environmental purposes as particulates and other pollutants, such as sulphurs, chlorides etc. hereby can be removed before the gas is let into the atmosphere. Hot gases are also cooled and cleaned in energy plants where the energy of the gas is transferred to a media that heated, for instance water, steam or oil.

Reference is made to Perry Chemical Engineer\'s Handbook, 7. Edition, especially pp. 17, 39-40, WO 2007/036236 A1 and Autojet Spraying Systems, Bulletin 540, rev. 1, 2004.

Further reference is made to:

JP 2004061024, which describe a gas cooling tower where the warm gas inters a narrow top of the cooling tower, with a diameter of 0.05-0.25 in respect of the internal diameter (D) of the tower main housing. The tower main housing is equipped with the cooling unit to cool the gas supplied from the gas supply duct.

EP 1 325 773 A1 which describe a flue gas cooling apparatus, which cool the flue gas and collect the dust in a single apparatus

JP 09-178367, which describe a gas cooler where the gas is turning and go upwards.

WO 2007/036236 A1, which describe that heat can be recovered from hot gas produced in a thermal reactor, by injecting water into the gas at one or more injection zones in such an amount and in such a way that the gas temperature due to water evaporation is reduced to below 400° C.

Often cooling is considered in relation to two types of cooling, namely “shell and tube heat exchangers” and “evaporative cooling” and the such two types of cooling will be discussed briefly in the following.

It is well known, that hot gases can be cooled from high temperatures (400-1000° C.) to lower temperatures (100-400° C.) by using shell and tube heat exchangers carrying the hot gas inside tubes, which are in external contact with a cooling medium (water, air, thermal oil, etc.). Such installations are typically built in sizes ranging from a hundred (kg/h) hot gases up to capacities of several 100 thousands (kg/h).

It is also well known, that the aforementioned hot gases can be cooled by utilising the heat of evaporation from water, injected into the gas for cooling.

There are several advantages of such cooling method, for instance: It is more simple and cost effective than shell- and tube heat exchangers The water vapour content of the gas increase which result in several advantages for instance: a following condensation unit can produce more energy at higher temperatures than normal condensation units (ref: WO 2007/036236 A1) and electrostatic precipitators have a better particle separation. Cooling and gas cleaning can be integrated in one unit as gas cleaning agents such as lime, activated carbon etc can be injected into the gas cooling system.

Two main principles can be used: A. An excess amount of water injection to produce cooled gas, containing residual water droplets (ref: Perry above) 1. By using a scrubber column, which may be oriented horizontally, obliquely or vertically and where the water spray is injected counter current, cross current or co-current with the hot gas 2. By using a vertical column, where the gas is conducted in a rotating/axial direction and subject to a radial water spray from the centre 3. A water ejector, where the gas is moved through the equipment by a co-current jet of water

Such installations are typically built in sizes ranging from a few hundred (kg/h) hot gases up to capacities of several 100 thousands (kg/h). B. A reduced amount of water injection to produce a cooled dry gas (ref: Autojet above) The main advantage of this system is that dry gas cleaning systems (e.g. cyclones, bag filters and electrostatic filers) can be applied after the cooler.

Other advantages of dry evaporative cooling are: A wide range of materials can be used for the cooling reactor, including low cost steels Contaminants such as sulphurs, chlorides, dioxins etc. can be removed form the gas by injection of gas cleaning agents followed by removal of solids.

Typically a dry evaporative cooler is built as a vertical tower.

The inlet of the gas into the cooling tower can be tangentially to the cooling reactor (ref: JP 2004061024) which will create a swirl in the cooler or the gas flow can be directed so it become as close to plug flow as possible in the cooling section.

When the gas reaches the cooling section, water is sprayed into the gas through one or several nozzles placed in a lance that is mounted to the cooler and spraying into the gas. The water nozzles are oriented in a fashion, which avoids wetting of the walls.

The water droplets are atomised preferable to droplets, having Sauter diameter less than 100 μm. Pressurized air can be used as atomizing agent.

During the cooling section the water droplets are evaporated completely and the gas leave the exit of the cooling tower in a dry state. Also, the avoidance of wetting the walls will prevent evaporation of drops at the walls and possible deposition of harmful salts absorbed in the water drops.

Typically such dry evaporative coolers cool gases from 400C-1200C to 100-400C Such installations are typically built for large gas capacities up to several million (kg/h).

It is well known, that in the case of an cement, waste or energy plant, where the gas is released from thermal reactions with biomass, waste or coal, the raw gas will contain alkali metal salt vapours, which during a normal dry cooling system (shell and tube heat exchanger) will condense and cause heavy fouling and corrosion, and it is well known, that this will often result in component failure by general fouling of gas passages and nozzles and high maintenance costs replacing corroded sections. Typically alkali salts in this connection are chlorides of potassium and sulphates of potassium and calcium.

It is also well known, that similar problems occurs with other (chemical plant) processes mentioned previously.

It is well known, that technologies based on heat exchangers—because of the many narrow gas passages—have inherent tendency for fouling, and are generally avoided, when the gas to be cooled is heavily contaminated with tars, salts and particles. The same problems are observed in the technologies of evaporative cooling, described above under paragraph A.

It is also well known, that the fouling and corrosion problems are generally avoided by the use of the technologies of evaporative cooling, described above under paragraph B.

However, these cooling systems will have a typical ratio of length/diameter for the vertical tower in excess of 5 and often a height of 20-30 m, which is considered inappropriate for smaller plants—e.g. district heating and gasification plants, where building legislation very often limits the height to typically below 10 m.

The reason for the large height in standard coolers is that the system consists of three sections: inlet, drying and outlet that each represents a considerable building height.

Inlet section: In the inlet zone the gas shall develop a flow profile, which takes several diameters. Often it is also needed to make a diameter expansion until the dryer diameter is reached. This inlet section can be as high as 10 m long or even more.

Dryer section: The length of the dryer section depend on inlet and outlet temperatures, flow profile and on the droplet size of the water e.g. a 100 μm droplet at temperature difference between gas and droplet of about 250° C. needs about 0.3 (s) to evaporate, while 500 μm drops needs about 7.0 (s) for complete evaporation. Typically the dryer section is more than 5 m long.

Outlet section: The outlet section is followed by the dryer section. To avoid disturbance of the flow profile in the dryer the outlet duct is placed well after the drying section. Normally the outlet section is more than 3 meters. Therefore, for cleaning problematic hot gases—especially for smaller plants, but also for very large plants with capacities of more than 1 million (kg/h) gas—there is a need for a technology, which is considerably more compact.

In a first aspect of the invention, a method is provided. The method is for cooling hot gasses produced in a thermal reactor, and the method comprising leading of the hot gas into a cylindrical evaporative cooler in one or more tangential gas channels inlets with such a velocity that to carry out a rotary movement in the cylindrical evaporative cooler injection of water in the hot gas at one or more injection zones in such an amount and droplet sizes that the water droplets will fully evaporate inside the evaporative cooler.

Typically and preferably, the gas evaporative cooler has a height/diameter (similarly:length/width) ratio less than 5 m. The height and diameter of the evaporative cooler are preferably defined relatively to an in situ arrangement of the evaporative cooler: height being the internal vertical length of the evaporative cooler and diameter being the internal horizontal diameter of the evaporative cooler. Accordingly, the cylindrical evaporative cooler may be given some kind of box-shaped packing.

In addition, the injection of water may preferably be provided by at least one nozzle arranged on the top of the evaporative cooler.

In a second aspect of the invention, a system for cooling hot gasses is provided. The system for cooling hot gasses comprises: an inlet duct for the hot gas, placed tangentially on a vertically arranged cylindrical evaporative cooler with at least one water injection device, e.g. in the form of nozzles, for injection of water into the gas preferably on top of the evaporative cooler, and an outlet duct.

In a third aspect a plant is provided, the plant utilises the system according to the second aspect of the invention and the method according to the first aspect of the invention.

Accordingly, the invention provides inter alia a method, a system and a plant for cooling hot gases in a compact dry evaporative cooler, as the gas enters a cylindrical drying reactor tangentially. Furthermore, only one or at least only a few water injection nozzles may be needed for spraying water, said nozzles being preferably arranged on the top of the evaporative cooler and spray down into the gas to be cooled, which considerably simplifies the equipment. As indicated, the evaporative cooler is compact typically considered in the sense that the height/diameter ration is below 5.

Further aspects and embodiments of the invention are presented in the accompanying claims. In the following the invention and in particular preferred embodiments thereof are presented with reference to the accompanying drawings in which:

FIG. 1 schematically shows the first design of the system, according to the invention, where fuel is burned or gasified in a thermal reactor and where the hot gas released from the reactor is cooled,

FIG. 2 shows a typical design for the evaporative cooler used in the system,

FIG. 3 shows a typical design for the evaporative cooler used in the system including the anticipated flow pattern inside the cooler,

FIG. 3a show another typical design for the evaporative cooler used in the system including the anticipated flow pattern inside the cooler,

FIG. 4 shows a typical design for the evaporative cooler used in the system including the anticipated total pressure in the central plane of the evaporative cooler,

FIG. 5 shows the invention applied to an energy plant, based on solid fuel combustion and use of the cooled gas for production of district heating,

FIG. 6 shows the invention applied to an energy plant, based on solid fuel gasification and use of the cooled gas in a gas engine,

DETAILED DESCRIPTION

In FIG. 1, 1 reference a thermal reactor, which is supplied with fuel, air or oxygen and water and/or steam. Item 2 is a channel for transporting the hot gases from the thermal reactor to the evaporative cooler 4.

The evaporative cooler is oriented vertically. Item 3 is one or more water inlet nozzle (s). The nozzle(s) can use air/gas assisted atomisation.

Some particles (dust and condensed alkali salts) can be separated in the evaporative cooler and can be removed from the bottom, and the evaporative cooler exit channel 5 conducts the dry, cooled and relatively clean gas to a residual particle filter—which may be a scrubber, a bag house filter, a cyclone or an electrostatic precipitator etc. A fan 7 handles the pressure drop throughout the system.

A sensor S placed downstream the evaporative cooler 4 measures temperature and/or the humidity of the gas exiting the evaporative cooler 4. The measurement of the temperature and/or the humidity is used by a water injection system that controls the amount of water being injected into to the evaporative cooler 4 so that the temperature and/or the humidity of the gas exiting the evaporative cooler 4 is within pre-selected limits.

Due to the water injection the gas is now very suitable to cool and condensate in a condensation unit 8.

Agents, for instance lime and/or activated carbon can be sprayed into the hot gas and/or the cooled gas in order to adsorb contaminants such as acids and dioxins etc.

In FIG. 2, 2 is the inlet channel of the evaporative cooler for the hot gases from the reactor. The gas enters at a sufficient high velocity to ensure that it will move in a helical motion along the wall of the evaporative cooler. Therefore, the angular velocity of the gas close to the wall is considerably larger than the axial velocity through the evaporative cooler and the gas will carry out 6-12 revolutions, before reaching the tangential exit channel 5. It should be pointed out that the coil-like structure shown in FIG. 2 is not intended to show a tube, but illustrates that the fluid inside the evaporative cooler flows in a helical motion occupying the interior of the evaporative cooler 4.

At the centre of the evaporative cooler end cover 9 (or arranged symmetrically around the centre) one or more nozzles 3 are located, through which water droplets are sprayed along the evaporative cooler axis preferably having Sauter diameters below 100 μm with a total amount adjusted by a valve at the nozzle(s) and controlled by a sensor in combination with water injection system as disclosed above, which monitors the relative humidity at the evaporative cooler exit channel.

After a short distance from the nozzle exits, the droplets will begin drifting in a radial direction because of centrifugal forces induced by the rotating gas. The droplets will evaporate while entering the rotating gas and thereby cool the gas by using the heat of evaporation of the water droplets.

During the cooling of components that are in gaseous state in the inlet (Chlorine, potassium, sodium and maybe some organic components (tars) in the case of the thermal reactor being a gasifier) will form solid particles and be carried along with the gas in a dry state (salts or tar droplets/solid organics in the case of the thermal reactor being a gasifier).

The heavier salt particles and solid organics will be deposited at the bottom of the evaporative cooler and be removed. (Particle removal system is not shown in this figure)

The smaller particles (and tar droplets in the case of the thermal reactor being a gasifier) will follow the cooled dry gas from the evaporative cooler through the tangential exit channel 5.

Calculations for a 10 MW woodchips fuel input energy plant have been carried out and presented below:

The flue gas from the thermal reactor enters through the inlet channel of the evaporative cooler at a temperature of 900° C. and a mass flow rate of 5.9 (kg/s).

At the exit channel of the evaporative cooler the temperature has decreased to 200° C. being cooled by the evaporation of 2.1 (kg/s) water injected through the nozzle(s). Because of the evaporation of water, the dew point of the dry flue gas increases from 69° C. to 83° C. in the evaporative cooler having a diameter of 4-5 (m) and a height of 6.0 (m). The height is less than 50% as compared to a cooler, where the invention is not applied.

In FIG. 3, a fluid dynamics calculation reproduces the anticipated flow pattern inside a typical design of the evaporative cooler at a gas inlet velocity of 10 m/s. This calculation is a “cold calculation” in the sense, that no droplet evaporation (and therefore no gas cooling) is considered. It is noticed, that the tangential velocity increases from the axis (1 to 3 m/s) and toward the wall (10-15 m/s).

FIG. 3a, a fluid dynamic calculation of a evaporative cooler with an inlet flow rate of 16.000 Nm3 at a temperature of 600C, with a inlet gas velocity of 6 m/s is shown. The flue gas is after combustion of moist wood and therefore having 20% water vapours already at inlet. Only a single nozzle is used in the calculation: a FM 25 of Spraying Systems. This nozzle results in small droplets where the majority (by weight) is below 100 μm. The water flow rate is 38 litres/minutes which then cool the gas to 300C. The scale beside the simulation shows the retention time in seconds. It is seen that the water droplets are fully evaporated in less than 4 seconds.

In FIG. 4, the fluid dynamics calculation of FIG. 3 is depicted with respect to the total pressure at the central plane of the evaporative cooler. It is noticed, that the total pressure decreases slightly from the evaporative cooler axis toward the wall. However, as noticed in FIG. 3, the velocity (and therefore also the dynamic pressure) also increases in this direction and consequently, the static pressure will decrease from the axis toward the wall, which will result in the water droplets drifting in this direction into the hot gas layer.

In FIG. 5, the hot flue gases from a solid fuel combustion process are cooled and preliminary cleaned in the evaporative cooler according to the invention. The flue gas from the evaporative cooler exit channel is here applied in an energy plant with district heating scrubber system based heat recovery and preheating/moistening of combustion air for the base plant 8 as described in PCT international patent application WO 2007/036236 A1.

In FIG. 6, the hot gases from a solid fuel gasification process 1 are cooled and preliminary cleaned in the evaporative cooler according to the invention. The gas from the evaporative cooler exit channel is let to a bag filter where particles are removed. The cleaned gas in further cooled and water vapours condensate in a condenser 8 and hereafter let to a gas engine which produce electricity, hot water and a hot flue gas.