GB2033366A

GB2033366A - Process for Producing Energy from Low Grade Fuels

Info

Publication number: GB2033366A
Application number: GB7934362A
Authority: GB
Original assignee: Sterling Drug Inc
Current assignee: STWB Inc
Priority date: 1978-10-12
Filing date: 1979-10-03
Publication date: 1980-05-21
Also published as: FR2438628A1; NL7907408A; DE2940689A1; BE879247A; JPS5594993A

Abstract

Energy is obtained from biomass by wet oxidation followed by anaerobic digestion (biogasification) of the oxidized liquid phase. Optimum efficiency is obtained by recycling of effluents and solid residues.

Description

SPECIFICATION Process for Producing Energy from Low Grade Fuels This invention relates to an improvement in production of energy by means of a novel combination of wet oxidation and biogasification procedures as applied to organic material.

With the decline in the world wide supply of high grade fuels such as natural gas and petroleum there is a need to develop resources of abundant fossil fuels such as coal, lignite, and peat, as well as renewable fuel sources, commonly known as biomass. Biomass is meant to include materials such as plant matter, crop residues, agricultural animal manure, municipal refuse, and sewage sludge.

There are problems in using the above fuels directly. They generally contain water which makes efficient combustion difficult. They create pollution problems when used directly.

Two proposed methods to eliminate or alleviate some of the problems are the use of wet oxidation, for example, as described in U.S. Patent No. 4,100,730, and a technique generally known as biogasification or anaerobic digestion which converts at least a part of the organic material to a clean gaseous fuel.

These two techniques both have disadvantages. In wet oxidation the useable energy is contained in a gas stream which is at relatively low temperature since the operation of wet oxidation is limited to the critical temperature of water. According to well known principles of thermodynamics this temperature limitation results in a limitation in efficiency of energy recovery. There is also an unavoidable, if small, liquid effluent from the wet oxidation system.

Biogasification at best converts about 50% of the organic material to fuel gas, and there is a residue to be disposed of.

U.S. Patent 3,256,179 describes a sewage treatment process which involves subjecting sewage sludge to anaerobic digestion followed by wet oxidation in which the wet oxidation is regulated to cause less than 55% reduction in chemical oxygen demand (COD). The purpose of the process is primarily waste disposal rather than energy production. In the process of the patent the wet oxidation is carried out at temperatures in a preferred range of 120 ~165 C. providing substantial residual organic solids in oxidized mixture which is then subjected to solid separation by settling or filtration techniques. The resulting liquid phase is then recycled to the anaerobic digestion stage.

U.S. Patent 3,060,118 relates to the wet oxidation of sewage sludge to reduce the COD of the sludge from 60 to 85%. The wet oxidation is preceded by a sludge settling process which may involve aerobic or anaerobic digestion. The wet oxidized effluent is returned to the sludge settling process to serve as nutrient for the microorganisms. U.S. Patent 3,256,179 states (column 1, lines 64-70) that the system of U.S.

Patent 3.060,118 works well when the biological treatment is aerobic, but that the effluent of wet oxidation process of the latter patent is a relatively poor nutrient medium for digester organisms.

U.S. Patent 4,010,098 describes a process for treatment of solid waste and sewage sludge which involves the steps of subjecting the sludge to wet oxidation to reduce the COD by 50-85%, and then combining the solids from the wet oxidation with the original solid waste and subjecting the combined solids to pyrolysis under non-oxidizing conditions. The sludge, prior to wet oxidation, may be subjected to aerobic or anaerobic digestion.

U.S. Patent 3,959,125 describes a process of heat treatment of sewage sludge at 65-1 5000., without addition of air, followed by biological digestion, anaerobic and/or aerobic, to obtain a fluid sludge suitable for distribution on land.

An article entitled "Effect of thermal pretreatment on digestibility and dewaterability of organic sludges" by Roger T. Haug, David C.

Stuckey, James M. Gossett and Perry L. McCarty, Journal of Water Pollution Control Federation, January 1978, pup.73~85, describes the effect of thermal pretreatment of sewage sludge, preferably at 1 750C. and without addition of air, upon subsequent anaerobic digestion. In the case of activated sludge, gas production from the digestion was substantially improved.

The process of the invention is one for producing energy which comprises mixing organic material with water to form a slurry, subjecting said slurry to wet oxidation so as to oxidize more than about 55 grams per liter of the chemical oxygen demand of said slurry, separating the wet oxidized slurry into a solid ash and supernatant liquid phase, subjecting said supernatent liquid phase to bio-gasification, separating the biogasified material into an effluent and a solid residue, and recovering thermal or mechanical energy from the wet oxidation step and fuel gas as a source of energy, from the biogasification step.

According to a particular embodiment at least a portion of the effluent and/or solid residue from the biogasification step is recycled to one or more earlier points in the process.

Reference is made to the accompanying drawing which is a flow-sheet representation illustrating preferred embodiments of the invention.

Organic material 1 is slurried with water and the resulting slurry is subjected to wet oxidation 2 wherein more than about 55 g/l of the COD is removed. The oxidized effluent is then subjected to conventional anaerobic digestion in a biogasifier 3. The contents of the biogasifier are transferred to a settler~ and separated into a solid residue and liquid effluent. Energy is obtained in the form of a hot gaseous effluent from the wet oxidation and fuel gas, methane, etc., from the biogasifier. More efficient energy production can be obtained by recycling the products from the biogasifier. The solids residue can be recycled 5 to the wet oxidation reactor.

The liquid effluent can be recycled 6 to the original slurry or organic material to provide nutrients for biomass; or 7 to the wet oxidation step; or,2 to the oxidized effluent prior to biogasification. In this way a maximum amount of the COD will be converted to useful energy. If desired, a portion of the fuel gas produced in the biogasifier can be used in the wet oxidation system to increase the efficiency thereof.

The wet oxidation preferably takes place at a temperature between about 200 and 300 C. and a pressure sufficient to maintain a substantial amount of the water in the liquid phase, and for a time sufficient to effect removal of more than about 55 g/l of the COD.

The organic material which serves as the ultimate source of the energy production can be any low grade fuel, for example, a fossil fuel such as coal, lignite or peat; as well as biomass derived from plant life or microorganisms including animal manure and sewage sludge. When biomass is used, a variation in the process of the invention is useful where the effluent from the biogasification step is utilized as nutrient for the biomass, thereby producing further fuel source material in a cyclic process.

There are many differences between the process of U.S. Pat. 3,256,179 and the process of the present invention, but the most significant is that the former limits the process to reducing the COD of the sludge by less than 55% during the wet oxidation step. Degree of oxidation expressed as a percent reduction in COD is useful when comparing wet oxidations of the same kinds of materials, in the case of said U.S. Patent digested sewage sludge. However, this method of expression is ambiguous and inaccurate when comparing different kinds of feed materials used for wet oxidation. For example, in the wet oxidation of a dilute industrial waste with COD of 10 gel a 55% oxidation would result in the removal of only 5.5 g/l of COD.Wet oxidizing a strong pulping waste liquor with a COD of 180 g/l so that 55% of the COD is removed would result in the removal of 99 g/l of COD. Digested sludge can be characterized as a material which has a COD from a minimum of about 30 g/l to a maximum of about 100 g/l. Thus said U.S.

Patent's limitation of less than 55% COD removal can also be expressed as a COD removal of less than about 55 g/l. This figure can then be compared to wet oxidation in general, not limited to application to digested sewage sludge.

The following examples will further illustrate the invention.

Example 1 A mixture of 60 tons of coal and 40 tons of sewage sludge solids is prepared as a slurry. The coal has a heating value of 10,000 BTU per pound and the sewage sludge solids 6,000 BTU per pound. The slurry contains 760 tons of water and 147.4 tons of COD resulting in a COD of 180 g/l for the slurry. The slurry is subjected to a wet oxidation such that 150 g/l of COD is removed.

About 530 tons of air are supplied in order to accomplish this wet oxidation. There remains 30 g/l of COD in the effluent from the wet oxidation unit. 20 Tons of ash is removed from the wet oxidation unit along with about 20 tons of water.

The remaining slurry, which is directed to the biogasifier, has a COD of about 33 g/l. In the biogasifier 80% of this COD is removed by anaerobic digestion. 70% of the COD is converted to gaseous fuel. Approximately 75% of the heat of wet oxidation is recovered as useful energy so that the entire system will recover 1,050 million BTU's from the wet oxidation unit and 196 million BTU's as fuel value in the gaseous fuel from the gasifier.

In operating the process of this invention when at least a portion of the organic material supplied is biomass then it is possible to recover and recirculate inorganic nutrients such as nitrogen, phosphorous, potassium, and trace minerals, present in the effluent from the biogasifier, which are necessary for the growth of biomass and which are removed from the biomass generating system in the biomass itself. It is a feature of this invention that these materials can be at least partially recycled to the biomass generating system.

Example 2 Approximately 86,000 Ibs. of organic material is mixed with 344,000 Ibs. of water to form a slurry of 20% solids with a chemical oxygen demand of about 300 g""l. This slurry is subjected to wet oxidation and biogasification as in Example 1. In the wet oxidation step about 120 g/l of COD is oxidized resulting in production of useful energy of 480 million BTU's, assuming a 75% efficiency.

The effluent from the wet oxidation unit is a 12% slurry with a COD of about 180 g/l. This slurry is diluted to a COD of about 60 g/l in order to be more suitable for the biogasification step. In the biogasifier 80% of the COD is removed, with 75% of the COD converted to gaseous fuel. The effluent from the biogasifier is directed to a settler where the supernatant is recycledin order to dilute the feed to the biogasifier. Excess supernatant can also be recycled to the wet oxidation reactor. The settled solids are recycled to the wet oxidation unit.

Claims

1. A process for producing energy which comprises mixing organic material with water to form a slurry, subjecting said slurry to wet oxidation so as to oxidize more than about 55 grams per liter of the chemical oxygen demand of said slurry, separating the wet oxidized slurry into a solid ash and supernatent liquid phase, subjecting said supernatent liquid phase to biogasification, separating the biogasified material into an effluent and a solid residue, and recovering useful thermal or mechanical energy from the wet oxidation system and fuel gas, as a source of energy, from the biogasification step.

2. A process according to claim 1, wherein at least a portion of the effluent and/or solid residue from the biogasification step is recycled to the wet oxidation step.

3. A process according to claim 1 or 2, in which at least a portion of the organic material is biomass containing microorganisms and where at least a portion of the effluent from the biogasification step is recycled to the original slurry of organic material in water to promote generation of biomass.

4. A process according to any one of the preceding claims, in which at least a portion of the effluent from the biogasification step is recycled to the oxidized supernatent liquid phase prior to biogasification.

5. A process for producing energy substantially as herein described with reference to the Example.