Process for the gasification of sewage sludge

A process for the gasification of sewage sludge or other carbon-containing waste materials in a gasifier (1) is described. A solid fuel and oxygen-containing gas are also fed into said gasifier. The residues formed during gasification collect at the bottom of the gasifier in the form of molten slag. Gasification takes place in a fluidized bed (9) formed above the slag bath and constituted by the dried sewage sludge or waste materials, the solid fuel, the oxygen-containing gas and the gasification gas. The gas produced in the gasifier can be used for power generation or as a reducing gas for iron ore. Sponge iron can simultaneously be melted in the gasifier and reduced to pig iron.

The invention relates to a process for the gasification of sewage sludge or 
other carbon-containing waste materials according to the preamble of claim 
1. 
The hitherto used measures for eliminating or using sewage sludge have 
comprised the dumping or burning thereof after drying, or the use thereof 
as a fertilizer in agriculture. The dumping of sewage sludge requires a 
large amount of space and involves additional high costs. Combustion is 
problematical in that in the case of incomplete combustion dioxins can 
form and the undesired heavy metals in the ash can partly be leached out 
with water. There are considerable objections to the use sewage sludge as 
a fertilizer due to the heavy metals contained therein and it is largely 
no longer allowed. 
Another use of sewage sludge is described in German Pat. No. 29 23 726, 
according to which the dried sewage sludge is used as a reducing agent for 
reducing iron oxides. The dried sewage sludge and dry iron oxide are mixed 
and optionally pelletized and then heated over a long period, there being 
a partial reduction of the iron oxide to sponge iron. However, this 
process is unsuitable for economic industrial use. In addition, it fails 
to solve the problem of the disposal of heavy metals. 
DE-OS No. 32 03 435 describes a process for producing gas and extracting 
metals in a molten bath reactor, particularly an iron bath reactor, in 
which on the one hand gas is produced in that a carbon-containing fuel and 
oxygen are introduced into a meltable bath material, particularly an iron 
bath of the melting bath reactor and on the other hand metal is extracted 
in that the metal carrier is fed into the molten bath. The gas produced is 
largely coal gas comprising CO and H.sub.2. The fuel introduced into the 
molten bath material has a small proportion of the metal to be extracted, 
which dissolves in the melt and is enriched therein and can be drawn off 
after reaching a given concentration. The fuel can in part consist of 
products with moderate proportions of combustible substance, such as 
sewage sludge and industrial waste and on the one hand it is possible to 
extract therefrom metals, apart from heat, and on the other hand heavy 
metals can be separated in a manner not prejudicial to the environment, so 
that pollution is correspondingly reduced. However, apart from heat 
production, the aim of this process is mainly the production of metal and 
not the elimination of sewage sludge. The sewage sludge proportion in the 
total fuel is necessarily very limited, because as a result of endothermic 
reactions it greatly cools the molten bath and in addition the volatile 
components contained therein leads to a considerable and undesirable bath 
movement. 
The problem of the present invention is to improve the known process for 
the gasification of sewage sludge or other carbon-containing waste 
materials in a gasifier, in that additionally a solid fuel and 
oxygen-containing gas are fed in, a slag bath being formed in the bottom 
region of the gasifier, so that a complete utilization of the combustion 
energy contained in the sewage sludge and waste materials and an 
elimination of the heavy metals so that disposal is problem-free, the 
sewage sludge or waste material proportion in the total fuel can be 
increased in such a way that an economically suitable sewage sludge and 
waste material elimination is possible. 
This problem is solved according to the invention by the feature of the 
characterising part of claim 1. Advantageous further developments of the 
inventive process can be gathered from the subclaims. 
The invention is characterized in that the gasification is performed in a 
fluidized bed above the slag bath and which comprises the sewage sludge or 
waste materials, the solid fuel and the oxygen-containing gas. The sewage 
sludge is completely reacted at high temperature and in the case of an 
adequate residence time, so that there is no longer any risk of dioxins or 
other environmentally prejudicial gases forming. The heavy metals in the 
sludge are combined into the slag. 
The process is preferably combined with the production of pig iron, 
additional sponge iron being melted in the gasifier and reduced to pig 
iron. The gas produced in the gasifier is advantageously used for reducing 
iron ore into sponge iron, which is subsequently melted in the gasifier. 
It has been found that sewage sludge with a residual moisture of 10% can be 
admixed in a proportion of 15% with the fuel, without impairing the 
reducing and melting process. When using low volatility coal as an 
additional fuel, theoretical results have shown that the sewage sludge 
proportion can be increased to 40%.

The invention is described in greater detail hereinafter relative to an 
embodiment illustrated in the drawing, which diagrammatically shows a 
sewage sludge gasification plant. 
Coal with a grain size range of 0 to 50 mm, is fed together with fluxes 
through an inlet 2 into the top of a fluidized bed gasifier 1. Petroleum 
coke could be used instead of coal. Using a line 3, sewage sludge is fed 
to a centrifuge 4, where it is partly dewatered. The still pumpable sewage 
sludge is then passed through a line 5 to a dryer 6, to which steam is 
supplied via a line 7 and with the aid thereof the sewage sludge is dried 
to a residual moisture content of 10%. The thus prepared sewage sludge is 
then conveyed pneumatically via a line 8 to the fluidized bed gasifier 1 
and is fed into the latter in the middle to upper region of a fluidized 
bed 9. A partial flow of the oxygen-containing gas required for 
gasification purposes in the fluidized bed is fed via a line 10 into the 
lower area of fluidized bed 9, where it reacts to CO.sub.2 or CO and then 
rises therein, so that the fluidized bed is maintained. A further partial 
flow of oxygen-containing gas passes via a line 11 into line 8 and is fed 
with the sewage sludge into the fluidized bed gasifier 1. The heat forming 
during the gasification of the coal and sewage sludge produces such a high 
temperature in fluidized bed 9, that no dioxins can form due to incomplete 
combustion of the sewage sludge. The temperature is also sufficiently high 
to permit the formation in the bottom of the fluidized bed gasifier of a 
molten slag bath 12 formed during the gasification of the coal and the 
sewage sludge. The slag, which also contains the heavy metals in the 
sewage sludge is tapped at appropriate time intervals by means of a line 
13. It is also possible to feed the sewage sludge together with the coal 
into the fluidized bed gasifier 1 through the inlet 2 at the top thereof. 
The oxygen-containing gas can also be heated prior to the supply via lines 
10 and 11 into fluidized bed gasifier 1, so that the sewage sludge 
proportion in the total fuel can be increased. If additional sponge iron 
is to be melted in the fluidized bed gasifier 1, then it is fed in over 
the fluidized bed 9, preferably in the top region of the gasifier, is then 
melted and reduced in bed 9 and collects as a molten iron bath at the 
bottom of gasifier 1. Due to its lower specific weight, the molten slag 
then collects on the iron bath. 
As a result of gasification, CO and optionally H.sub.2 -containing gas is 
produced in fluidized bed 9 and is passed via a line 14 from gasifier 1 to 
a cyclone 15. In the latter coarse entrained coal and sponge iron 
particles are removed from the gas, the separated solid particles being 
returned via a line 16 to the fluidized bed gasifier 1. The gas from which 
the coarse dust has been removed then passes to a waste heat system 17, in 
which the thermal energy contained in the gas is used for producing high 
pressure steam, which is passed via a line 18 to a power station 19. The 
gas cooled in this way is fed into a dust filter 20, where the very fine 
solid particles are separated and also gaseous contaminants, such as 
sulphur or chlorine are absorbed. Finally the gas is passed via a line 21 
to power station 19, where it is burned. From the combustion energy and 
the high pressure steam energy from line 18 low pressure steam, which is 
fed via line 7 to dryer 6, and electric power which is removed by a line 
22 are produced. 
If the plant is simultaneously used for the production of pig iron, then 
the gas produced in the fluidized bed gasifier can also be used as a 
reducing gas for the direct reduction of iron ore to sponge iron. The 
sponge iron formed is then fed into the fluidized bed gasifier, melted and 
reduced to pig iron.