WO1999052830A1

WO1999052830A1 - Treatment of sewage sludge by combustion with animal waste

Info

Publication number: WO1999052830A1
Application number: PCT/BE1999/000048
Authority: WO
Inventors: Peter Verboven
Original assignee: Seghers Better Technology Group
Priority date: 1998-04-10
Filing date: 1999-04-12
Publication date: 1999-10-21
Also published as: BE1011950A6; AU3324299A; WO1999052830A8

Abstract

The present invention relates to a method for treating mechanically dehydrated sewage sludge, whereby the sewage sludge is loaded into a fluid bed reactor and is subjected to a combustive evaporation at a temperature of between 850 and 1150 °C in the presence of meat and bone meal as an additional fuel.

Description

TREATMENT OF SEWAGE SLUDGE BY COMBUSTION WITH ANIMAL WASTE

The present invention relates to a method for treating mechanically dehydrated sewage sludge, whereby the sewage sludge is loaded into a reactor and subjected to a combustive evaporation in the presence of an additional fuel, in order to increase the temperature during the combustive evaporation. The present invention relates in particular to a method for treating sewage sludge which originates from a waste water treatment plant.

Mostly, mechanically dehydrated sewage sludge contains a significant amount of water, which may amount to approximately 80% by weight because of the high water binding capacity of the organic compounds contained in the sewage sludge. The water may be bound in the sewage sludge in different ways and may be present for example in the form of colloidal bound water, intracellular and/or capillary bound water. As a consequence of its high water content, the sewage sludge occupies a large volume when dumped and thus requires large storage volumes. This is undesired. Besides this, the sewage sludge may contain active pathogenic organisms and be biologically active: it can become mouldy and rotten and spread around an unpleasant odour. Methods exist in the art for the mechanical dehydration of sewage sludge so as to decrease its volume. For a substantially complete dehydration, the sewage sludge is loaded into a fluid bed reactor and subjected to a combustive evaporation at high temperature, which from a legal point of view should be above 850°C. Simultaneously with the dehydration, the organic compounds present in the sewage sludge are combusted. The flue gasses that are thereby released contain a.o. sulfur oxides, especially S0₂ and S0₃, and nitrogen oxides NO_x, which must be removed in a separate flue gas cleaning step in order to comply with the emission standards imposed by law. The heat released during the combustion of the organic compounds ensures that simultaneously the water is evaporated from the sewage sludge. To solve the problem of providing a temperature which is sufficiently high to evaporate approximately all the water contained in the sewage sludge, mostly an additional fuel with a high calorific value is added to the combustive evaporation step. Frequently used additional fuels are for example char coal (calorific value approximately 33MJ/kg), petroleum, biogas etc.

The additional fuels known from the art however present the disadvantage of involving an unwanted formation of sulfur oxides. As a consequence, the flue gasses leaving the known combustive evaporation process not only contain sulfur oxides which originate from the combustion of the sewage sludge, but also sulfur oxides generated in the combustion of the additional fuel. Because the maximum emission values of sulfur oxides are restricted by law, a more intensive flue gas cleaning is required which has an adverse effect on the flue gas cleaning costs.

It is the aim of the present invention to provide an improved method for the treatment of mechanically dehydrated sewage sludge, which allows to simultaneously combust the organic compounds present in the sewage sludge and evaporate approximately all the water remaining in the sewage sludge after the mechanical dehydration, without involving the formation the above described additional unwanted impurities.

This is achieved with the present invention in that as an additional fuel, meat and bone meal is used. Meat and bone meal is a product that can be obtained by rendering dead animals. Meat and bone meal has been found to have a high calorific value of approximately 17 MJ/kg so that a relatively small amount of meat and bone meal is capable of providing a significant increase of the temperature during the combustive evaporation. The meat and bone meal can be used as such and requires no specific pre- treatment.

Up to now, meat and bone meal was usually recycled and/or used as animal feed or in the preparation of gelatine. Since a couple of years however, adverse reserves exist towards the recycling of meat and bone meal in food. As a consequence thereof the reuse of meat and bone meal is decreasing and more and more meat and bone meal is nowadays burned as a waste product. With the present invention, again profitable use can be made of the meat and bone meal, while simultaneously decreasing the volume of the sewage sludge with approximately 90%.

Surprisingly it has been found that by using meat and bone meal as an additional fuel, the concentration of the sulfur oxides in the flue gasses leaving the combustive evaporation can be decreased with at least 10 % as compared to the method known from the art. It has namely been found that at least part of the sulfur oxides formed during the combustive evaporation of the sewage sludge, can be neutralised in situ. The inventor is of the opinion that this must be attributed to the fact that meat and bone meal contains an amount of lime, which probably originates from the animal bones. The lime appears to be capable of reacting with the sulfur oxides generated during the combustive evaporation, so that in that way an in situ neutralisation of at least part of the sulfur oxides in the reactor can be obtained. This has the advantage that the flue gasses released from the combustive evaporation of this invention have a lower content of sulfur oxides, and that a simpler, cheaper flue gas cleaning will suffice, which allows to save on the costs for flue gas cleaning.

Because the lime is contacted with the organic compounds contained in the sewage sludge already during the combustive evaporation, the contact time of the lime with the acid components of the flue gasses can be prolonged, so as to further increase the neutralisation degree of the sulfur oxides in the flue gasses.

Surprisingly it has also been found that when meat and bone meal is used as an additional fuel, not only a neutralisation of the sulfur oxides can be obtained, but also a reduction of the nitrogen oxides generated during the combustion of the organic compounds present in the sewage sludge. The flue gasses leaving the combustive evaporation process of mechanically dehydrated sewage sludge mostly contain an amount of nitrogen oxides, which need be reduced in a separate flue gas cleaning step. With the present invention, the nitrogen oxide neutralisation step can be often dispensed with, or at least the amount of nitrogen oxides in the flue gasses can be decreased with at least 20 % as compared to the method known from the art. The inventor is of the opinion that the ability of meat and bone meal of reducing nitrogen oxides must be attributed to the fact that meat and bone meal contains an amount of proteins, the amine groups of which appear to be capable of selectively reducing the nitrogen oxides formed during the combustion of the organic waste contained in the sewage sludge. The method of the present invention thus not only allows to reduce the concentration of sulfur oxides in the flue gasses leaving the combustive evaporation of the sewage sludge, but also to achieve a significant decrease of the nitrogen oxides content of the flue gasses, so that a simplified, cheaper flue gas cleaning unit will suffice.

The amount of meat and bone meal added to the combustive evaporation, can be varied within wide ranges. The amount of meat and bone meal added will mostly vary between 7 and 20 wt. % with respect to the amount of sewage sludge, preferably between 10 and 15 wt. %. The amount of meat and bone meal used will depend on a number of parameters such as for example the temperature at which the combustive evaporation is carried out, the amount of sewage sludge that needs be combusted and the composition of the sewage sludge. Care should be taken that the amount of meat and bone meal is sufficient to achieve the preferred temperature of for example 850-1150°C, preferably approximately 1050°C. Also the amount of sewage sludge supplied to the combustive evaporation reactor is chosen such that the temperature pertaining during the combustive evaporation, remains within those margins.

In the method of this invention, the meat and bone meal can be mixed with the sewage sludge prior to the combustive evaporation or can be fed to the reactor during the combustive evaporation.

The energy contained in the flue gasses released from the combustive evaporation can be further recycled in the production of steam. Flue gasses often have a rather high temperature (approximately 850°C or more) and need to be cooled down before they can be introduced in the flue gas cleaning unit. In the method of this invention, the flue gasses leaving the combustive evaporation reactor are supplied to a steam boiler, where the heat contained in the flue gasses is used for the formation of steam. In that way, efficient use can be made of the thermal energy contained in the flue gasses. In the method known from the art, the flue gasses are mostly at least partly neutralised before being conducted to the steam boiler, in order to decrease their sulfur oxide content and to avoid unwanted corrosion of the steam boiler. The method of this invention allows the flue gasses produced in the combustive evaporation to be used as such without having first to subject them to a cleaning step.

The combusitve evaporation of this invention is preferably carried out in a fluid bed reactor. Fluid bed reactors are generally known to the man skilled in the art. Thereby, mechanically dehydrated sewage sludge is loaded into a fluid bed of particles, preferably sand. Due to the intense contact and the optimal temperature of the fluidisation air of the bed, an almost immediate evaporation of the water contained in the sewage sludge and an optimal combustion of the organic compounds of the sewage sludge can be achieved.

This invention also relates to a method for the combustion of meat and bone meal in the presence of a combustion moderator, in order to avoid that the temperature of the fluid bed reactor would become too high. It has namely been found that during the stoichiometric combustion of meat and bone meal, the temperature of the reactor can raise to approximately 1500°C. The combustion of the meat and bone meal is preferably carried out in a fluid bed reactor, whereby as a combustion moderator, use is made of mechanically dehydrated sewage sludge, preferably sewage sludge originating from a mechanical water purification. Because this sewage sludge mostly contains approximately 80% by weight of water, the amount of air that must be fed to the reactor for cooling the reactor, can be limited, thus involving the advantage of allowing to avoid the formation of an additional amount of flue gasses. The amount of sewage sludge added mostly varies between 60 and 80 percent by weight with respect to the amount of meat and bone meal, for example 8 ton/h sewage sludge are added in the combustion of 6 ton/h of meat and bone meal. For the combustion of meat and bone meal, the conditions described above for the combustive evaporation of mechanically dehydrated sewage sludge are similarly applicable. The invention is further elucidated in the added figure and description of the figure. In figure 1 , a possible embodiment of the fluid bed reactor used in the method of this invention, is shown. The reactor 1 comprises a fluid bed of particles 2. The fluid bed 2 preferably contains particles of a chemically inert material, for example sand, silica, alumiumoxide and/or zirconium particles or mixtures thereof. Preferably, use is made of silica or alumina particles. The particles preferably have a mean particle size of between 20μm and 2 mm, more preferably between 100 and 1000 μm so as to allow a stable fluid bed to be obtained and to achieve an optimal heat transfer from the fluid bed to the sewage sludge. The heat transfer is namely related to the surface/volume ratio of the particles which should be as large as possible. Larger particles present the advantage of providing a good fluidisation of the bed, but however show poor heat transfer capabilities. With smaller particles, the heat transfer has been found to be more efficient, however the fluidisation of the bed has been found to be unsatisfactory and involving the formation sluggish flows.

The reactor 1 contains an inlet 6 for the fluidisation gas (mostly air) of the bed. The gas is preheated by conducting it at least partly along the reactor 1. At the bottom of the reactor 1 , a perforated plate 3 is provided, through which the gas is conducted into the reactor 1. The sewage sludge 5 is supplied to the reactor 1 through an inlet 4. In the device shown in figure 1 , the additional fuel, preferably meat and bone meal, is also supplied to the reactor 1 through the inlet 4. It is however also possible to introduce the additional fuel through a separate inlet. The amount of meat and bone meal supplied to the reactor is mostly adjustable, and will be adjusted such that the optimum temperature of 850- 1150 °C, preferably 1050°C, for the combustive evaporation is obtained. If so desired, the flue gasses originating from the combustive evaporation can be conducted to a flue gas cleaning device, through a steam boiler. ln the present invention, use is made of a fluid bed reactor. It is however also possible to perform the combustive evaporation in other types of industrial reactors.

Example.

8.3 tons of sewage sludge are supplied to a fluid bed reactor. The bed was fluidised using air with a flow rate of 1.7 m/s. Per hour, 1 ton of meat and bone meal was added as a combustion inhibitor. The temperature of the fluid bed was approximately 850°C.

Claims

CLAIMS.

1. A method for treating mechanically dehydrated sewage sludge, whereby the sewage sludge is loaded into a reactor and is subjected to a combustive evaporation in the presence of an additional fuel, characterised in that as the additional fuel, meat and bone meal is used.

2. A method as claimed in claim 1 , characterised in that the amount of meat and bone meal used varies from 7 to 20 percent by weight with respect to the amount of sewage sludge.

3. A method as claimed in claim 1 or 2, characterised in that the combustive evaporation is carried out in a fluid bed reactor.

4. A method as claimed in any one of claims 1 to 3, characterised in that the combustive evaporation is carried out at a temperature of between approximately 850 and 1150┬░C, preferably approximately 1050┬░C.

5. A method for combusting meat and bone meal in the presence of a combustion moderator, characterised in that sewage sludge is used as the combustion moderator.

6. A method as claimed in claim 5, characterised in that the combustion of the meat and bone meal is carried out in a fluid bed reactor.