MXPA98010127A - Controlled thermal oxidation process for organi waste - Google Patents

Abstract

A controlled thermal oxidation process for solid fuel waste. The process comprises a first combustion stage wherein the waste burns in a downward direction from top to bottom. A first fixed air flow of predetermined volume passes from top to bottom of the waste. A second modulated air flow of predetermined smaller volume passes over the waste and through the combustion flame. The process further comprises a second combustion stage wherein the combustion products of the first stage are exposed to high temperature conditions for a short period under global stoichiometric air conditions of 135% to 20%.

Description

CONTROLLED THERMAL OXIDATION PROCESS FOR ORGANIC WASTE BACKGROUND OF THE INVENTION The present invention relates to thermal oxidation of waste, and more particularly to a controlled process for thermal oxidation of two stages of selected solid waste to significantly reduce identified air emissions. The two-stage combustion process is an old technique where combustible materials are normally burned under substoichiometric conditions in the first stage chamber to produce combustible gases and ash. The resulting combustible gases are further mixed with air and burned under super-quatoiometric conditions in the second stage. The two-stage combustion control is typified in the patents of E. U.A. Nos. 4, 013, 023 and 4, 182,246 where reverse-action air control and auxiliary fuel burners are used to control operating temperatures of the first stage within a specific scale at the same time ensuring substoichiometric conditions when passing above the air and auxiliary burner requirements, when necessary, to maintain a certain oxygen content in the combustible gases that pass in the secondary stage. The second stage temperature is controlled by direct mode since an increase in the secondary temperature results in an increase in air flow which causes extinction effects in combustion gases and lower temperature. There is another complication in the control of temperature when they pass over and increase the requirements of air flow and when a certain minimum level of oxygen in the secondary exhaust gases is not maintained. The improvements for the control of two-stage combustion systems are documented in the Patent of E. U.A. No. 4,474, 121 which concentrates on securing substoichiometric conditions in the first stage and controlled superstoichiometric air velocities in the second stage which essentially eliminates any requirement to control oxygen from first stage exhaust gases and provides substantially better control of the combustion process compared to previous technologies. Other patents of general background interest, which describe and illustrate waste incineration methods and apparatus, include: E. U.A. No. 3,595, 181 Anderson July 27, 1971 E. U.A. No. 3,610, 179 Shaw October 5, 1971 EE .. UA .. NNoo .. 33,, 665511 ,, 777711 Eberic March 28, 1972 E. U.A. No. 3,664,277 Chatterjee et al May 23, 1972 E. U.A. No. 3,680,500 Pryor August 1, 1972 É. U .A. No. 4,517, 906 Lewis et al May 21, 1985 E. U.A. No. 4,800,824 DiFonzo January 31, 1989 USA ... AA .. NNoo .. 44 ,, 887700,, 991100 Wright et al 3 October 1989 E. U.A. No. 4,941, 415 Pope et al July 17, 1990 E. U.A. No. 4, 976,207 Richard et al 1 December 1, 1990 E. U.A. No. 5, 095, 829 Nevéis March 17, 1992 E. U.A. No. 5, 123,364 Gitman et al June 23, 1992 E. U.A. No. 5,222,446 Edwards et al June 29, 1993 These standardized control systems do not address air emission problems associated with highly variable air flow rates that pass through the combustion materials within the first stage which can cause dramatic increases in the entry of ash particles and the need to use particulate removal systems before the extinguishing gases can escape into heat exchangers or the atmosphere. The constant dirtiness of analytical instruments used to control the composition of existing first stage gases results in inaccurate readings and needs constant monitoring and maintenance to provide the desired process control. Accordingly, an object of the present invention is to provide a combustion oxidation process that is adapted to meet the internationally acceptable, specific air quality aspects without the need to distill and filter costly extinguishing gas to remove organic compounds and particulates. solid.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the present invention a controlled thermal oxidation process for solid fuel waste is provided. The process comprises a first combustion stage where the waste is burned in a downward direction from top to bottom. A first fixed air flow of predetermined volume passes from bottom to top of the waste. A second modulated air flow of predetermined smaller volume passes over the waste and through the combustion fiama. The process further comprises a second combustion stage wherein combustion products of the first stage are exposed to high temperature conditions for a short period under global stoichiometric air conditions of 135% to 200%. It is preferred that in the second combustion stage, the combustion products are exposed to a temperature of at least 1000 ° C for at least two seconds. The process is particularly well suited for solid waste where the waste has a maximum moisture content of approximately 60% by weight and a minimum average superior heating value of approximately 4000 BTU per pound and a maximum combined moisture and non-combustible content of about 57% by weight. The process according to the present invention provides substantially complete oxidation of organic compositions released from the burn of solid waste materials and those inherently synthesized during the combustion process, i.e., dioxins and furans.

BRIEF DESCRIPTION OF THE DIB UJOS These and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings wherein: Figure 1 is a schematic view of a combustion chamber arrangement for carrying out the process of the present invention. Although the invention will be described in conjunction with an exemplary embodiment, it will be understood that the invention should not be limited to said embodiment. On the contrary, all alternatives, modifications and equivalents must be covered as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES As illustrated in Figure 1, the process of the present invention makes use of a stationary waste incinerator 2 of two-stage air waste where, in the primary stage, a primary stage 4 combustion chamber is charged with waste solid of specific minimum and maximum properties with respect to the average Higher Heating Value, moisture content and total non-combustible content. After one hour of the initial ignition cycle, the primary stage operates only under substoichiometric conditions (less than 100% air) until the burn cycle has been completed. The combustion chamber 4 is fitted with two different fresh air supplies, and means for measuring and controlling each air flow independently. The first air flow 6 is of a fixed volume and enters the lower region of the chamber 4 and passes through the waste material 8 to be burned, in the upper region 10 of the chamber 4. The second flow of air 12 is of variable volume and enters the upper region 10 of the chamber above the waste material 8. The volume of air for the second air flow 12 must not exceed 50% of the first air flow 6. The temperature (T1 ) of the upper region 10, on the waste to burn 8 where both air flows are combined before leaving the secondary chamber 14, is measured and recorded by means 16. This upper temperature (T1) is limited to a temperature maximum of 732 ° C and a lower limit of 454 ° C as the extinction limits for the second air flow in the upper region of the chamber. Also provided, for chamber 4, and upper area 10, a burner lit with auxiliary fuel 18 to provide initial ignition of the solid waste material at its upper limits and ensure that the burn continues in an unconventional downward direction until the end. . The combustion process in chamber 4 ends substantially when the combustion gases in the upper area 10 of chamber 4 have reached a temperature T1 of 621 ° C, after the first hour of cycle time and after another period, the temperature T1 has dropped to 454 ° C. For the second stage combustion in the secondary chamber 14, means 20 are provided to mix fresh air with combustion gases entering the primary chamber 4. Those mixed gases are exposed to a temperature, in the secondary chamber 14, of less 1000 ° C of the burners 21, and thus more combustion is caused. A minimum of two seconds residence time is provided for all products in the secondary chamber 14, before leaving the heap 22. The process according to the present invention provides global stoichiometric air conditions ranging from 135% to 200% as normally expected from two-stage combustion. The waste to be used in accordance with the process of the present invention is restricted to waste categories that demonstrate a warming value of sufficient average, including water and non-combustible materials, to withstand self-contained sub-stoichiometric combustion. within the primary stage combustion chamber 4, without a requirement for supplementary heat energy from burners ignited with auxiliary fuel, different to initiate combustion. More particularly, it is preferred that the solid waste materials have maximum and minimum characteristics identified as: • having a maximum moisture content of 60% by weight • that have a minimum average upper heating value of about 4,000 BTU / liter • that have a maximum combined moisture and non-combustible content of about 57% by weight It has been found that the air quality of heap emission when burned said waste according to the process of the present invention, has an improved quality represented by: • entry of solid particles in extinguishing gases of less than 10 mg / dcsm • organic compounds of TOC (as C) in extinguishing gases of less than 10 mg / dscm • dioxins and furans in extinguishing gases of less than 0.10 ng / dscm as l-TEQ (toxic equivalents) • CO content of extinction gases of less than 50 mg / dscm • NOx content of extinguishing gases of less than 210 mg / dscm The process according to the present invention can be processed economically up to 50 tons of solid waste over a period of twenty-four hours and produce up to 25 million BTU per hour of useful heat energy, clean per combustion unit. The process according to the present invention provides two different air flows in the primary chamber 4, the first air flow being fixed and of higher volume and entering through the bottom of the chamber and passing through the solid waste 8. and subsequent cenisa layer. The second airflow is modulated and is of a lower volume that enters from above the chamber so as not to pass through the waste or any layer of ash but passes through the flame, causing gas combustion. and providing additional heat release in the primary chamber. The result of these two different air flows improves the combustion control in a significant way by: (a) reducting the entry of particles due to low fixed volumes of air that passes through the waste and ash layer its higher for a wide scale of combustion gas temperatures before leaving the primary stage. (b) decreasing the temperature of the combustion zone within the waste due to low fixed volumes of air that prevents the formation of slag and fused materials and that facilitates the recycling of ash components. (c) increasing the temperature of combustion gas within the upper area of the primary chamber by using a second variable air flow, without increasing the air flow through the waste. (d) provide a more consistent volume and temperature of combustion gases that leave the primary chamber and enter the secondary chamber.

EXAMPLES: An existing two-stage thermal oxidizer manufactured by Eco Waste Solutions Inc., which has an internal primary stage capacity of 9.71 cubic meters and measuring 2.13 meters x 2.13 meters x 2.13 meters, was modified to provide two separate entrances of fresh air at the first stage combustion chamber 4, as in Figure 1 and with means 26 and 28 for measuring, recording and controlling each air flow independently in accordance with the present invention. The first stage combustion chamber had the means to measure and record the gas temperature (T1) in its upper region. The second stage chamber 14 had a total internal volume of 5.60 cubic meters and was able to provide a residence time for all combustion products exceeding 2 seconds at a temperature of 1000 ° C before leaving the pile. The inlet temperature of the heap (T2) was measured, and recorded at 30, and controlled by two burners ignited with oil 21 located at the opposite end of the secondary chamber.

All test burns were carried out using the incineration / oxidation system just described and illustrated in Figure 1. Initial burns were carried out using heterogeneous municipal solid waste (MSW) pre-mixed with a higher value of approximately 4,000 BTU / liter and without upper air, to determine the maximum background air flow velocity that would produce levels of Heap extinction particles below 10 mg / dscm when calculated at 1 1% oxygen content for the heap. A total of three burns were evaluated for heap particle levels for periods of more than 3 hours during each burn with the results in Table 1.

TABLE 1 From Table 1 a normal lower air flow rate of 30 scfm or less was considered to provide a sufficient margin to ensure heap particle levels less than 10 mg / dscm. The lower air flow rate of 30 scfm corresponds to an air flow velocity of 0.61 dscf per square foot of primary chamber floor area (floor area was 4.55 square meters). A second series of test burns was carried out using MSW as the waste material to determine the differences in process conditions when. (a) Burn # 4, no lower air flows were controlled and determined by natural heap extraction and no upper air was added. (b) Burn # 5, lower air was established at a fixed speed and no upper air was added. (c) Burn # 6, lower air was established at a fixed speed and superior air was added in increasing volumes to a maximum of 50% lower air. The time, temperature (T1) and air flows for test burns # 4, # 5 and # 6 are delineated in Table 2, noting that all the waste consumed in these burns was pre-mixed to provide reasonable consistency with respect to a thermal value of approximately 4,000 BTU / pound and load weights of 385 kilograms per burn.

TABLE 2 NOTE: The substantially complete burn cycle was considered when T1 reached a minimum of 621.1 ° C for a period after the first hour of the time cycle and after yet another period reached 454.4 ° C. The time, temperature, and air flow conditions as established during burns # 4 to # 6 clearly indicate the following: 1. A combination of lower and upper air in the primary combustion stage as in burn # 6, significantly increased the rate at which the solid waste was consumed and resulted in a reduction of 15% to 20% in cycle time when It was compared to burns # 4 and # 5. 2. Operating temperatures T1 were obtained in burn # 6, for this category of waste, much earlier in the burn cycle # 6 and possibly contributed significantly to the reduced cycle time of that burn. 3. The particle levels had heap extinction gases, taken over a period of 3 hours during each burn (# 4, # 5 and # 6) and starting at a point three hours in each cycle showed the average particle levels of the following way In Heap Particle Level Burn # 4 - 17.3 mg / dscm calculated for 1 1% oxygen Burn # 5 - 8.6 mg / dscm calculated for 1 1% oxygen Burn # 6 - 9.2 mg / dscm calculated for 1 1% oxygen 4. The results indicated here, comparing burn # 4 and # 5, show that the supply of air from the lower feed primary combustion stage contributes significantly to the amount of particle contained in heap extinguishing gases. 5. By comparing particle levels measured in burns # 5 and # 6, it is also shown that when the lower air flow rate is set, it is possible to add an additional amount of air in the upper area of the stage chamber. primary combustion equivalent to at least half the amount of air fed lower without severely affecting the particle extinction levels.

A third series of test burns was performed to determine that when no upper air and a maximum lower air flow rate of 30 scfm is added (equivalent to 0. 61 scfm per square foot of primary stage floor area) and at T1 temperatures on the scale of 454.4 to 732.2 ° C, a significant scale of solid waste materials, which have different Average Higher Heating Values, could support self-substoichiometric combustion. held in an up-and-down direction through the waste within the primary stage and further establishing an appropriate fixed lower air flow for each waste material. Table 3 lists the materials expended during this series of individual test burns # 7 to # 12 and the individual properties of each waste. Table 4 lists the conditions established during burns # 7 to # 12 and the results of the heap air emission test obtained during each burn.

TABLE 3 TABLE 4 Test burns # 7, # 8, # 9, # 10 demonstrated the ability to burn a variety of waste materials under the primary stage parameters and conditions as previously established, and were considered applicable to the invention due to their conformity to the basic requirements of the invention of: 1. substoichiometric combustion 2. total lower air flow volume of less than or equal to 30 scfm 3. self-contained combustion and in a downward direction through waste and within the T1 temperature range of 454.4 to 732.2 ° C 4. maximum in heap particle levels of 10 mg / dscf or less. Test burns # 11 and # 12 required multiple ignitions of the primary stage auxiliary fuel burner to maintain a minimum T1 temperature of 454.4 ° C during the first 3 hours of the burn cycle and therefore did not meet the required parameter of self-sustained combustion. These two burns required multiple adjustments of lower airflow volumes in an attempt to maintain temperatures within the desired range and a fixed lower airflow velocity could not be achieved until approximately half through the cycle. It was also observed that on several occasions during both burns it was necessary to provide superestoichiometric conditions (greater than 100% air) within the primary stage to maintain combustion. The properties of the solid waste used in burns # 11 and # 12 were considered as not suitable for the process of this invention and these properties were determined as: 1. a solid waste having a moisture content of about 60% or more 2 a solid waste having an average Upper Heating Value of about 3,500 BTU / lb or less 3. a solid waste having a combined moisture and non-combustible content of more than about 57% by weight. Other series of seven test burns were carried out to provide examples in accordance with the invention and the solid waste parameters developed from burns # 7 to # 12 were used. Table 5 outlines the properties of each waste material used in the examples of the invention.

TABLE 5 Table 6 illustrates the conditions observed and measured during each one of the example burns # 1 to # 7.

CUAD RO 6 Table 7 classifies the heap emission levels recorded for example 1 to 7.

EXAMPLE 7 In Examples 1 to 7 it is clearly shown that the claimed two-stage combustion process and as described above, has provided for combustion of a variety of solid waste materials having certain minimum and maximum characteristics identified as: 1. having a maximum moisture content of 60% by weight 2. having a minimum average Upper Heating Value of about 4,000 BTU / pound 3. which have a maximum combined moisture and non-fuel content of approximately 57% by weight and furthermore said two-stage combustion process has provided certain improvements in the quality of heap air emission as claimed from: 1. solid particle emissions of less than 10 mg / dscm 2. TOC, organic compounds such as carbon emissions of less than 10 mg / dscm 3. Dioxin and furan emissions of less than 0.10 ng / dscm as toxic equivalents l-TEQ 4. CO, carbon monoxide emissions of less than 50 mg / dscm 5. NOx, nitrogen oxides emissions of less than 210 mg / dscm and such low levels of air emissions have been achieved without the use of systems to disturb and filter conventional extinguishing gas.

These air emissions comply with all current international standards for particle levels, NOx, CO, organic components (such as carbon), and dioxin / furan levels without the aid of baghouse or carvers. In this way, it is evident that a controlled process for two-stage thermal oxidation of selected solid waste that fully satisfies the objects, attempts and advantages set out above has been provided in accordance with the invention. Although the invention has been described in conjunction with the specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in view of the foregoing description. Accordingly, such alternatives, modifications and variations should be embraced as they fall within the spirit and broad scope of the invention.

Claims (5)

1 .- A process of thermal oxidation controlled for solid waste fuel, the process comprising: a first stage of combustion where the waste burns in an upward direction from above to below, a first fixed air flow of predetermined volume passes from bottom to top of the waste and a second flow of modulated air of predetermined smaller volume passes over the waste and through the combustion flame; and a second combustion stage wherein the combustion products of the first combustion stage are exposed to high temperature conditions for a short period under overall stoichiometric air conditions of 135% to 200%.

2. A process according to claim 1, wherein in the second combustion stage the combustion products are exposed to a temperature of at least 1000 ° C for at least two seconds.

3. A process according to claim 1, wherein the waste has a maximum moisture content of about 60% by weight and a minimum average upper heating value of about 4000 BTU per pound and a maximum combined moisture content. and non-combustible of approximately 57% by weight.

4. - A process according to claim 2, wherein the waste has a maximum moisture content of about 60% by weight and a minimum average upper heating value of about 4000 BTU per pound and a maximum combined moisture and non-combustible content of about 57% by weight.

5. A process according to claim 1, wherein the first air flow of the first stage of combustion has a maximum flow velocity of approximately 0.61 cubic feet. SUMMARY A controlled thermal oxidation process for solid fuel waste. The process comprises a first combustion stage wherein the waste burns in an upwardly downward direction from low to low. A first fixed air flow of predetermined volume passes from top to bottom of the waste. A second modulated air flow of predetermined smaller volume passes over the waste and through the combustion flame. The process further comprises a second combustion stage wherein the combustion products of the first stage are exposed to high temperature conditions for a short period under global stoichiometric air conditions of 135% to 200%.