Process for Removal of SO.sub.2 and NO.sub.x From Combustion Flue Gases and an Apparatus Used Therefor

1. Field of Industrial Utility

The present invention concerns a process for removal of acid pollution such as SO.sub.2 and NO.sub.x from flue gases by a radiation method, in particular from heat and power generating stations and an apparatus for removal of SO.sub.2 and NO.sub.x from flue gases.

2. Discussion of the Background

Air pollution caused by gaseous products derived from combustion of coal and fossil fuels products in thermal-electric power stations present a problem of global proportions. A typical power station generating 500 MW of power emits about 3 to 5 tons pollution per hour which has a cumulatively detrimental effect on the environment. Several technologies have been developed to provide effective purification of flue gases.

The removal of acid pollution from flue gases by chemical methods is based on the absorption of acidic impurities in alkaline solutions i.e. lime suspensions. Those wet methods lead to deposition of large quantities of by-products, besides they allow elimination of SO.sub.2 only. Substantial amounts of NO.sub.x still remains in the flue gases and in particular NO, together with Freon compounds is considered to be responsible for the ozone hole. As such, it is necessary to build separate plants for NO.sub.x removal from flue gases. Those plants are based on different principles, mainly on catalytic reduction.

Radiation technology uses a stream of accelerated electrons to generate free radicals. This leads to simultaneous SO.sub.2 and NO.sub.x removal from flue gases.

The radiation method enables elimination of 95% of SO.sub.2 and 80% of NO.sub.x in one plant. In the radiation technology it is important to increase efficiency of reactions which depends on the amount, temperature and composition of the gas mixture. A higher efficiency can be achieved by introducing moisture and a quantity of ammonia before conducting irradiation process as described in Polish Patent No. 153259 and Polish Patent Application No. 284996 filed on 27 Apr. 1990. Those methods are based on simultaneous reactions initiated by radiation and result in the formation of solid products. These products are useful as fertilizers.

The flue gas irradiation in the presence of water aerosols leads to the formation of atomic and molecular radicals and free electrons. Radicals OH., O. and H.sub.2 O. are responsible for oxidation of SO.sub.2 and NO.sub.x to SO.sub.3 and NO.sub.2 and further in the presence of water H.sub.2 SO.sub.4 and HNO.sub.3 are formed. Finally these compounds react with ammonia to form solid products NH.sub.4 NO.sub.3 and (NH.sub.4).sub.2 SO.sub.4, which can be used as fertilizers. The temperature of this process is kept in the range of 65.degree. to 100.degree. C.

Optimalization of temperature, degree of watering and ammonia content depending on gas composition and its flow rate slightly change the efficiency of acid pollution removal from flue gas.

Research has also been conducted into improving the efficiency of radiation methods. Such improvements are based on additional use of electrostatic and electromagnetic fields, which could increase the amount of free electrons and free radials and change the chemical reaction process.

The known method described in Patent DD-243-216A, (87-170590) proposes using, (beside a beam of electrons with energy 5-500 KeV), an electrostatic field having an intensity up to 100 V/cm to reduce the consumption of electrical energy in the process. In this process purifying efficiency is increased. The disadvantage of this method is the necessity of using additional grid electrodes located in the reaction vessel. They are located at a distance of 16 cm one from the other to incorporate the electric field into the space where the reaction proceeds. The solid reaction products and fly ash formed during and after irradiation tend to be deposited on electrodes and block the reaction vessel.

The disadvantage of the method described above can be overcome by a method disclosed in Patent JO-1099-633-A, (89-156548/21) where an irradiation vessel laser beam is utilized (ArF laser with wavelength 193 nm) and CH.sub.3 OH added. CH.sub.3 OH is excited by light to generate OH. radicals which bond NO and SO.sub.2 to solid products and enables their removal. The use of a laser beam has a beneficial effect but industrial application is complicated and expensive. Efficiency is rather low because of limited penetration of UV light in a reaction vessel caused by the presence of water. It is also difficult to obtain good homogeneity of spatial distribution of the light beam, and it is necessary to the use CH.sub.3 OH compound.

The problem of reduction of electrical energy consumption is especially important in industrial scale installations because 2 to 4 per cent of total electrical energy produced in the power station is consumed for purifying flue gases from acid pollution.

The present invention uses along with an electron beam a microwave energy for increasing the effectiveness of the purifying process and for reducing the energy consumption for this purpose.

The essential feature of the process according to the invention is a secondary utilization of free electrons induced during irradiation and introduced to the system in the form of a beam of accelerated electrons for generating oxidizing radicals and the use of a microwave energy for increasing the number of free electrons and sustaining their energy to the optimum level. This results in a decrease in the dose rate average power of the electron beam and a reduction in costs of accelerators with the same removal efficiency.

Thus, the invention concerns a process for SO.sub.2 and NO.sub.x removal from flue gases wherein a stream of flue gases is subjected to radiation by an electron beam and microwaves are applied in the form of a steady, continuous and/or pulsating stream. The stream of flue gases irradiated by the electron beam in the radiation zone is subjected to the action of microwave energy in the whole cross section of the stream of flue gases, and the microwaves are introduced with an electric field intensity of E.sub.1 >300 V/cm and pulse duration .tau.=10.sup.-7 to 10.sup.-3 seconds at a frequency of 200 to 10,000 MHz. The frequency of repetition of the microwave pulses should be f>v/.alpha..sub.k, where v is a gas flow velocity, and .alpha..sub.k is the length of the irradiated zone.

Alternatively a stream of steady, continuous microwave energy E.sub.c may be used simultaneously, and its intensity may be in the range 100 to 300 V/cm at a frequency 200 to 10,000 MHz.

The maximum applicable electron beam dose in this invention is 1-20 kGy. Of course dose is set according to required results. For instance, in the case that a reduction of pollution of about 50% is desired then the dose may be in a range 5-10 kGy.

The electron beam utilizes in a process may also be of a pulsed type, with a pulse duration of .tau..sub.e =10.sup.-8 to 10.sup.-5 seconds.

The amount of ammonia added to the flue gases in the reaction vessel depends on the SO.sub.2 and NO.sub.x content and should be about a stoichiometric amount. Water content preferably amounts to 8-12% volume and may be optimalized according to the situation pertaining in the system.

A further aspect of the invention concerns the apparatus for SO.sub.2 and NO.sub.x removal from combustion flue gases in a radiation process in which a stream of accelerated electrons and microwave energy are used simultaneously. To achieve that purpose, the reaction vessel has to be provided with at least one electron beam source and at least one microwave source. The positioning of the microwave energy source and electron beam source is not critical. However, it is essential that an electron beam and microwaves are introduced to the same zone in the reaction vessel. The inlet of microwave energy may be installed at the axis of the reaction vessel and perpendicularly to the axis and in any suitable angle.

FIG. 1 is a schematic view showing an example of the reaction vessel with a concentrated electron beam according to the present invention.

FIG. 2 is a schematic plan view of FIG. 1.

FIG. 3 is a schematic side view showing an example of the reaction vessel with an electron accelerator a linear scanning system mounted thereon according to the present invention.

FIG. 4 is a flow diagram of a pilot plant according to the present invention.

The flue gas is processed before the entrance to a reaction vessel according to the standard procedures of the radiation method (filtering fly ashes particles, moistening, ammonia injection). Microwave energy having a frequency 200 to 10,000 MHz is additionally introduced to the reaction zone which is irradiated by electron beam and increases the number of free electrons and free radicals in this zone. This leads to better acid pollution removal from flue gases. The solid product of the process is collected by filtration. The electrical component of a microwave energy stream is involved in the processes in the reaction zone where flue gases are irradiated. The microwave energy is introduced to the reaction vessel, by inlet and outlet waveguides that are fixed on the side wall of the reaction vessel, into the stream of accelerated electrons. The connections are made in narrow sides of waveguides in the form of a rectangle. The apparatus which may be equipped with two additional waveguides, makes the purifying process more effective. In the preferred embodiment the microwave energy is introduced to the reaction vessel perpendicular to the axis of the reaction vessel, but it may be directed in any other angle depending on the geometry of a particular construction. As shown in FIGS. 1-3 the stream of microwave energy is incorporated into the reaction vessel 4 across its side wall in the same way as above mentioned to an electron beam by way of waveguides 5 where connections are made in narrow walls of waveguides. The presence of microwave energy pulses leads to an increased number of free electrons and free radicals in this volume, which makes the purifying process more effective.

To support the free electron energy, two additional waveguides 3 are installed on electron output chamber 2 between the reaction vessel 4 and accelerator 1 or 1'. The connections for the waveguides 3 are made in narrow walls of the waveguides 3 and the waveguides 3 are fixed to the two output arms of 3-dB device (microwave divider), where another arm is connected to a microwave fitted load (microwaves in this device are absorbed without being reflected) and the other one is fixed to a microwave generator. The steady stream of microwave energy may support the energy of free electrons taking part in the process. Generation of free radicals and purifying process are more efficient than in the prior art (German Patent DD 243 216 A1) process despite the fact that no additional electrodes are mounted in the reaction vessel.

According to the invention, beside the electron beam, the stream of microwave energy with frequency 200-10,000 MHz is used in the reaction vessel. The flue gas at the inlet of the reaction vessel is free of fly ash and moistened as in other radiation methods. The use of microwave energy increases the number of OH. radicals due to the presence of a higher number of free electrons as expressed by the following formula:

wherein:

n.sub.eo is a number of free electrons before use of the microwave energy

v.sub.i is a number of ionizing collisions depending on the intensity of electrical field within the microwave energy pulse, and

t is time.

The use of microwave pulses leads to a free electron multiplication effect whereas the steady stream of microwave energy sustains the energy of those free electrons at a desired level.

The invention was tested in the installation shown in FIG. 4. The unit for SO.sub.2 and NO.sub.x removal from flue gases has been built on the basis of an ILU 6 accelerator 1'. This installation was completed with two independent microwave generators. This arrangement allows testing of a combined removal concept based on the simultaneous use of the electron beam and streams of microwave energy to produce free radicals in a reaction vessel.

Two heating furnaces 6 each of them being a water-tube boiler were applied to produce combustion gas. The tested composition of flue gas has been obtained by introducing into the gas stream such components as SO.sub.2, NO and NH.sub.3.

The installation is composed of an inlet system--two boilers housing a heating furnace, boiler pressure regulator, SO.sub.2, NO and NH.sub.3 dosage system, analytical equipment--, a reaction vessel where an electron beam from an ILU 6 accelerator 1' and microwave streams from a pulse generator 7 and c.w. generator 8 can be introduced simultaneously or separately, and an outlet system--retention chamber 9, filtration Knit (e.g. bag filter), fan 11, offtake duct of gas 12 and analytical equipment. Temperature sensors are installed at several points in ducts and in the reaction vessel. The flow rate through the installation is 400 Nm.sup.3 /h. The gas temperature in the reaction vessel can be adjusted in the range 70.degree. to 100.degree. C. by the cooling water system of the boiler.

The basic parameters of the sources of accelerated electrons and microwave energy streams are given in Table 1.

The flue gas composition input to the reaction vessel is given in the Table 2 below.

The flue gas was fed to the reaction vessel constructed in the form of a cylinder with a diameter of 200 mm. The microwave streams were propagated axially. The electron beam was introduced to the reaction volume perpendicularly to the axis of the vessel, passing a titanium window 50 .mu.m thick. More than 75% of microwave energy was concentrated in the discharge volume. The inlet and outlet of the stream of the flue gas were situated on the wall side of the vessel. The stream of gas can flow directly or may be formed orbicularly. The temperature of the flue gas at the outlet of the reaction vessel was not higher than 100.degree. C.

The tests were carried out to estimate the effectiveness of the elimination SO.sub.2 and NO.sub.x from the flue gases and to estimate the reduction of the energy consumption to obtain the same purification effect with an electron beam only and electron beam combined with microwave energy applied as a continuous wave and/or in the form of pulsating source, using equal power levels of the microwave stream and electron beam deposited in the gas phase.

The efficiency of the flue gas purification effect at the same power consumption is shown in the Table 3 below.

Input power requirements for combination of electron beam and microwave is as follows:

SO.sub.2 95%--input power 5.5 kGy

NO.sub.x 80%--input power 7 kGy

Reduction of power requirement with reference to dose in kGy under the same conditions is shown in Table 4 below. The same conditions mean that during the experiments all parameters concerning the amount and the composition of the gas used as well as temperature and pressure were the same.

The obtained results show that both the purification effect and reduction in power consumption make this method valuable.

The invention is defined in the enclosed patent claims. It is possible to effect various modifications on the basis of the claims without departing from the spirit of the invention.

Industrial Applicability

In a process for removal of acid pollution such as SO.sub.2 and NO.sub.x from combustion flue gases, in particular, from heat and power generating stations, the present invention uses along with an electron beam a microwave energy for increasing the effectiveness of the purifying process and for reducing the energy consumption for this purpose.