Method and System for Conversion of Waste into Fuel and Other By-Products

Field of the Invention

The present invention relates to a method and system for conversion of waste into fuel and other by-products, more particularly, to a method and system for conversion of municipal solid waste into light fuel, gaseous fuel and other by-products.

Background of the Invention

Waste disposal problems and high rising fuel prices are major push factors for advancement in the technology for conversion of waste into fuel, particularly for recovering valuable light fuel or gasoline from waste.

United States patent application publication number 2003/0019789 discloses a method and system for preparation of gasoline, kerosene and diesel oil from waste plastics comprising the steps of: subjecting a melt of waste plastics to a first catalytic reaction in which the waste plastic melt is in contact with a nickel or nickel alloy catalyst to be dehydrogenated while being decomposed; subjecting the dehydrogenated and decomposed waste plastic melt to fluid catalytic cracking, as a second catalytic reaction, to produce a cracked material and fractionating the cracked material into a gasoline-based fraction, a kerosene fraction and a diesel fraction. This method and system is suitable to be used on waste plastics but not on other kinds of waste.

United States patent number 6,000,639 discloses a process for treating municipal solid waste (MSW) materials containing organic and inorganic portions and producing clean hydrocarbon liquid products wherein the process comprises: sizing MSW materials to produce sized particulates; density separating sized particulates by float-sink action in a polar acidic organic liquid medium and removing a lower density organic material portion from a high density inorganic material portion; digesting the organic material portion in a polar acidic organic liquid medium to produce a digested organic effluent material and fractionating the digested organic effluent material into a gaseous fraction, a light hydrocarbon liquid fraction, a polar acidic organic liquid fraction and a heavy liquid slurry fuel product fraction. Although this process can be used on most kinds of waste for

producing clean hydrocarbon liquid products but the main product of this process is the heavy liquid slurry fuel product fraction but not valuable light fuel or gasoline.

A method that can address waste disposal problems effectively and at the same time convert waste into fuel: a method capable of handling most kinds of waste and converting them into fuel without producing much pollutants; is highly in need.

Summary

The present invention relates to a method for conversion of waste into fuel comprising the following steps: a) treating waste in an alkaline medium to form a waste stream; b) degrading the waste stream at a temperature ranging from 300 - 370 ⁰ C and under pressure ranging from 8.4 — 17.5 MPa to form a product stream; c) purifying the product stream by separating impurities including salts and alkaline substances from it; d) fractionating the purified product stream into a gaseous fuel fraction, a light fuel fraction and a water fraction.

Step (c) can proceed as follows: i) lifting the temperature and pressure conditions of step (b) from the product stream to effect gasification of the product stream and precipitation of salts; ii) separating the precipitated salts from the gaseous product stream; iii) cooling the gaseous product stream to effect condensation of an alkaline liquid; iv) separating the alkaline liquid from the residual gaseous product stream which now forms a purified product stream.

Step (d) can proceed as follows: i) cooling the purified product stream to effect condensation of a light fuel and water mixture; ii) separating the light fuel and water mixture from the residual purified product stream which now forms a gaseous fuel fraction;

iii) separating the light fuel and water mixture into a light fuel fraction and a water fraction.

Step (b) is preferably conducted at a temperature ranging from 300 - 350 ⁰ C, more preferably, ranging from 300 - 320 ⁰ C and for a preferable period of not more than 15 minutes.

The alkaline medium preferably has a pH value of at least 1 1 and a molarity of at least 1, more preferably, a molarity of at least 2.

Examples of the alkaline medium are a sodium hydroxide solution and a potassium hydroxide solution or a mixture thereof.

Depending on the nature of the raw waste used, pre-treatment steps of separating ferrous materials from raw waste and/or shredding the waste can be incorporated into the method of present invention. If required, the waste is preferably shredded to a size of not more than approximately 1.0 cm, more preferably, not more than approximately 0.5 cm.

The present invention also relates to a system for conversion of waste into fuel comprising the following: a) a setup for treating waste in an alkaline medium to form a waste stream including a vessel (1); b) a setup for degrading the waste stream at a temperature ranging from 300 - 370 ⁰ C and under pressure ranging from 8.4 - 17.5 MPa to form a product stream including a high pressure pump (15), a heat exchanger (17) and a high pressure reactor (19); c) a setup for purifying the product stream including a cyclone (23) for separating salts and a condensation column (27) for separating alkaline substances; d) a setup for fractionating the purified product stream into a gaseous fuel fraction, a light fuel fraction and a water fraction including a condensation column (31) and a separator (33).

The setup for treating waste in an alkaline medium to form a waste stream can further include a homogenizer (1 1) and a holding tank (13).

Depending on the nature of the raw waste used, a setup for pre-treatment of raw waste including a magnetic separator and/or a shredder can be incorporated into the system of present invention.

Brief Description of Drawings

Figure 1 is a diagram showing the method for conversion of waste into fuel of the present invention.

Figure 2 is a diagram showing an embodiment of the method for conversion of waste into fuel of the present invention.

Figure 3 is a drawing showing an embodiment of the system for conversion of waste into fuel of the present invention.

Detailed Description of the Invention

The present invention relates to a method and system for conversion of waste into fuel and other by-products, particularly for recovering light fuel and gaseous fuel from municipal solid waste (MSW).

The method of present invention for conversion of waste into fuel and other by-products (sometimes referred to as 'the process') involves the following steps: first, treating waste in an alkaline medium (hereinafter referred to as alkaline treating step) to form a waste stream; second, degrading the waste stream under high pressure and high temperature (hereinafter referred to as alkaline degrading step) to form a product stream; third, purifying the product stream by separating impurities including salts and alkaline substances from it (hereinafter referred to as purifying step); forth, fractionating the purified product stream into a gaseous fuel fraction, a light fuel fraction and a water fraction (hereinafter referred to as fractionating step).

Some techniques for conversion of waste into fuel require that the waste to be sorted extensively so that only plastic waste and/or organic waste are used as raw materials.

Some techniques even require that waste having low water content to be used as raw materials. These requirements have made such techniques impractical since exhaustive pre-treatment steps have to be conducted on raw waste to enable processing by such techniques and waste disposal problems still exist for waste portions that could not be processed by such techniques.

The method of present invention is able to convert most types of waste including municipal solid waste (MSW), sewage sludge (SS) and industrial waste (IW) into fuel and other by-products without requiring exhaustive pre-treatment steps to be conducted on raw waste. Raw waste only has to be sorted, so that it is substantially free from big pieces of ferrous material; and shredded, so that it would have more surface areas accessible by the alkaline medium before processing by the method of present invention. There is no need for sorting out non-ferrous inorganic material, such as aluminium, from raw waste before processing by the method of present invention.

The waste has to be substantially free from big pieces of ferrous material because such ferrous material can cause damages on the shredder used for shredding the waste. For effective processing by the method of present invention, the waste is preferably shredded to a size of not more than approximately 1.0 cm (<10 mm), more preferably, not more than 0.5 cm (<5 mm). Big pieces of ferrous material can be separated from raw waste by magnetic action and any types of shredder, as commonly used in the industry, can be employed for shredding the waste.

The shredded waste is then subjected to the alkaline treating step. The alkaline medium suitable to be used for treating the shredded waste preferably has a pH value of at least 11 and a molarity of at least 1, more preferably, a molarity of at least 2. Examples of suitable alkaline medium are sodium hydroxide (NaOH) solution and potassium hydroxide (KOH) solution or a mixture thereof. The weight ratio of alkaline medium to waste is preferably

2 to 1. This ratio is given only as illustration of the method of present invention and one using the method may find other ratios more economical or more suited to his/her needs.

Lower ratios of alkaline medium to waste can be used as the organic portion of the waste lessens. The alkaline treating step can be conducted at a temperature ranging from 25 -

90 ⁰ C. This step is conducted for digesting the organic portion of the waste and at the same time dissolving certain substances of the inorganic portion of the waste (hereinafter referred to as soluble inorganic substances) to form a waste stream. To have a faster rate of digestion and/or dissolution of the waste, a higher temperature can be used for conducting the alkaline treating step. As the water used to form the alkaline medium is normally originated from the water fraction obtained in the fractionating step, the water often has a temperature in the range of 40 - 50 ⁰ C and therefore the alkaline treating step is conducted under the same temperature range.

The ferrous material separated from the waste still carries residual waste and it can be sprayed with the same alkaline medium used for the alkaline treating step resulting in clean ferrous scrap. The used alkaline medium from this step can be reused in the alkaline treating step.

Plastic materials remain undigested in the alkaline treating step and they remain suspended in the waste stream until they melt when heat is applied. Certain substances of the inorganic portion of the waste which are insoluble in the alkaline medium (hereinafter referred to as insoluble inorganic substances), for example glass and silicate materials, would form sediments and be separated from the waste stream. These insoluble inorganic substances can be utilized as raw materials for the construction industry.

Next, the waste stream is subjected to the alkaline degrading step at a temperature ranging from 300 - 370 ⁰ C, preferably ranging from 300 - 350 ⁰ C, more preferably ranging from 300 - 320 ⁰ C; under pressure ranging from 8.4 MPa (84 bar) to 17.5 MPa (175 bar) and for a preferable period of not more than 15 minutes. This step is meant to be proceeding in liquid phase; hence the high pressure is maintained at all time during this step to keep the waste stream in liquid phase at the temperature range as specified above. In this step, the organic portion and plastic materials in the waste stream are completely degraded and converted into light fuel and gaseous fuel.

When the temperature and pressure conditions as mentioned above are lifted from the product stream after the alkaline degrading step, gasification of products from this step

along with precipitation of inorganic salts would occur. The precipitated inorganic salts together with any solid particulate materials present, for example minerals, are separated from the gaseous product stream. Then, the gaseous product stream is cooled down to a temperature of approximately 110 - 150 ⁰ C and its pressure is let down to approximately 0.3 - 0.5 MPa (3 - 5 bar) wherein condensation of an alkaline liquid would occur thereby separating alkaline substances from the gaseous product stream, which now forms the purified product stream.

The purified product stream is further cooled down to a temperature of approximately 5 - 10 ⁰ C wherein condensation of a light fuel and water mixture would occur thereby separating them from the residual gaseous product stream, which now forms the gaseous fuel fraction. Finally, the light fuel and water mixture is separated into the light fuel fraction and the water fraction by using a separator.

The alkaline liquid obtained in the purification step contains alkaline substances from the alkaline medium, NaOH and/or KOH, and aluminium hydroxide (Al(OHb), which forms when aluminium materials from the waste are dissolved in the alkaline medium. An A1(OH) ₃ solution can be separated from the alkaline liquid by using a membrane separator. The alkaline liquid left after separation of A1(OH)3 often has a temperature of about 60 ⁰ C making it suitable to be reused as alkaline medium in the alkaline treating step. Aluminium can be recovered from the A1(OH) ₃ solution in a separate electrolytic process. Despite that a high energy input is required for the electrolytic process; it is still profitable to do so as the aluminium recovered can be sold as high grade aluminium.

The gaseous fuel fraction contains hydrocarbon gases and other reaction gases such as methane (CH ₄ ), ethane (CH ₃ CH ₃ ), ethylene (CH ₂ CH ₂ ), propane (CH ₃ CH ₂ CH ₃ ), nitrogen (N ₂ ), hydrogen (H ₂ ) and oxygen (O ₂ ). This fraction can be used to provide heat for the alkaline degrading step. Its combustion results in safe emissions such as nitrogen (N ₂ ), carbon dioxide (CO ₂ ) and steam.

The light fuel fraction contains light hydrocarbon compounds with carbon chain length in the range of Cs - Cn and boiling range between 34 - 100 ⁰ C.

The system of present invention for conversion of waste into fuel and other by-products comprises a setup for the alkaline treating step (hereinafter referred to as alkaline treating setup), a setup for the alkaline degrading step (hereinafter referred to as alkaline degrading setup), a setup for the purifying step (hereinafter referred to as purifying setup) and a setup for the fractionating step (hereinafter referred to as fractionating setup).

The method and system of present invention will now be described in more details with reference to an embodiment as depicted in Figure 3.

Waste, which is substantially free from big pieces of ferrous material, is shredded and fed to a closed vessel (1). An alkaline medium is also fed to the closed vessel (1). The waste is treated in the alkaline medium wherein organic portion of the waste is digested and soluble inorganic substances from the waste are dissolved resulting in a waste stream. Hydrogen gas is released during this alkaline treating step and it can be trapped in a gas dome (3) being equipped on top of the closed vessel (1). The trapped hydrogen gas can be pumped by using a vacuum pump (5) to a gas tank (7) and be stored for further usage. Plastic materials from the waste remain suspended in the waste stream while insoluble inorganic substances from the waste settled down at the bottom of the closed vessel (1). The insoluble inorganic substances can be extracted and passed through a flow press (9) wherein most of the residual alkaline medium is removed from the insoluble inorganic substances. The residual alkaline medium is recycled back into the closed vessel (1) while the insoluble inorganic substances recovered are ready for use in the construction industry.

The waste stream is then transferred from the closed vessel (1) to a homogenizer (1 1) followed by a holding tank (13) before being transferred to a heat exchanger (17) for heating up the waste stream. By transferring the waste stream to a holding tank (13) before heating, the action time of the alkaline medium is lengthen to ensure that the waste is fully digested and/or dissolved.

After leaving the holding tank (13), the waste stream is let through a high pressure pump (15) for increasing pressure to the preferred pressure range and then the pressurized waste

stream is transferred to the heat exchanger (17) for heating it to the preferred temperature range. By then, the plastic materials are melted into the waste stream. Next, the heated pressurized waste stream is passed through a high pressure reactor (19) for effecting alkaline degradation of the waste stream.

The temperature and pressure conditions as mentioned above are lifted from the product stream as it exits the high pressure reactor (19) and let through an expansion pipe (21). The resulting gaseous product stream is then let through a cyclone (23) for separating precipitated inorganic salts and/or solid particulate materials from gaseous products. An extruder (25) is fitted at the bottom outlet of the cyclone (23) so that a layer of precipitated inorganic salts and/or solid particulate materials can be maintained there for preventing escape of gaseous products through the bottom outlet. The gaseous product stream is let through the top outlet of the cyclone (23) and then fed to a first condensation column (27) wherein an alkaline liquid is condensed at the bottom section and other gaseous products are let out from the column's head to a second condensation column (31). In the second condensation column (31), a light fuel and water mixture is condensed at the bottom section and a gaseous fuel fraction is let out of the column's head to a gas tank (7).

The light fuel and water mixture is directed to a separator (33) wherein it is separated into a light fuel fraction and a water fraction. The resulting clean and demineralised water can then be reused at various points of the process and be released into the environment without further treatment when it is in excess.

According to the embodiment as described above, a setup for the alkaline treating step includes a closed vessel (1) equipped with hydrogen gas recovering devices: gas dome (3), vacuum pump (5) and gas tank (7); insoluble inorganic substances recovering devices: flow press (7); a homogenizer (1 1) and a holding tank (13).

A setup for the alkaline degrading step includes a high pressure pump (15), a heat exchanger (17) and a high pressure reactor (19).

A setup for the purifying step includes a cyclone (23), an expansion pipe (21) connecting the high pressure reactor (19) and the cyclone (23) and a condensation column (27).

A setup for the fractionating step includes a condensation column (31) and a separator (33).

The ability of the alkaline treating step and the alkaline degrading step to act on both organic and inorganic materials renders the method of present invention applicable on most types of waste and maximizes capture of recyclable materials from waste. Residuum for disposal is minimized in the method of present invention thereby adequately solving waste disposal problems. On top of that, the residuum for disposal has minimal environmental implication as hazardous compounds, such as halogen-containing compounds, in the waste have been neutralized and converted into salts in the alkaline degrading step.