Patent Publication Number: US-2013247458-A1

Title: Fuel system and process for its production for environmental protective energetic use of urban sewage sludge

Description:
The present invention relates to a fuel system and to a process for the environmental protective energetic use of urban sewage sludge. 
     Fuels generally serve as an energy carrier in the production of heat or electric current. From prior art, a number of different fuels are known among which the so-called fossil fuels predominate. 
     Hard coal, brown coal, lignite, turf, natural gas and petroleum are part of the fossil fuels. 
     Fossil fuels have been exploited already since the 18 th  and 19 th  centuries, and they were considered as the basis for the Industrial Revolution. Particularly during the past 40 years, the worldwide energy demand and hence the consumption of fossil fuels have increased to such an extent that the production of energy from fossil fuels has caused environmental problems. 
     Fossil fuels are generally based on organic carbon compounds which release energy in the form of heat during the oxidative conversion with oxygen, as this takes place during combustion. Carbon dioxide and water is generated as a byproduct of this oxidative conversion. 
     Since carbon dioxide which is released during the combustion of fossil fuels originates from carbon compounds that have been stored over millions of years, this massive combustion results in the enrichment of the earth&#39;s atmosphere with carbon dioxide. 
     On the other hand, this carbon dioxide is frequently referred to as a so-called “greenhouse gas” which could contribute to disturbing the ecological balance on the earth. Carbon dioxide in the atmosphere is suspected to reduce the radiation of heat energy from the earth to the universe just as the glass roof of a greenhouse while the incidence of the sun&#39;s radiation on the earth is reduced only little. This effect is suspected to lead to global warming. 
     For controlling the emission of CO 2  to the atmosphere, climate-protection goals have been fixed by the European Union in consideration of the Kyoto Protocol, and in this connection there have even been introduced so-called Emission Certificates. Since 2005, the EU membership states are obliged by the EU Emissions Trading Directive to hand in a National Allocation Plan each time at the beginning of an emissions trading period. This plan fixes an amount of greenhouse gases each bigger emitter of a country is allowed to emit within a particular period. Article 9 of these Directives provides the examination and approval of this Allocation Plan by the EU Commission on the basis of 12 criteria. This concerns above all the compatibility of a country&#39;s own goals within the scope of the Kyoto Protocol, equal treatment of enterprises and the observance of the EU competitive law. If a company&#39;s emission exceeds the allowance that has been allocated to it, the company has to buy additional emission rights from another company. This can be done for instance at the Energy Exchange EXXA. On the other hand, if a company emits less than the allowance that has been allocated to this company, it may sell excess amounts of emission to other companies. However, for in fact reducing the fraction of CO 2  in the atmosphere, the allowed emissions are reduced step by step. 
     Major emitters of CO 2  are branches in industry and economy having a high energy demand. These are for instance power plants, petroleum refineries, coking plants, iron and steel works, the cement industry, glass industry, lime industry, brick industry, insulation material industry, ceramic industry, and cellulose and paper industry. 
     One way for avoiding the accumulation of CO 2  in the atmosphere is the use of the so-called regenerative energy. In general, these are wind power, water power, solar power, and the use of biomasses as a fuel or for the production of bio gas. However, if biomasses are used as an energy carrier, the problem exists that these biomasses have a clearly lower energy content compared to fossil fuels. On the other hand, biomasses have the advantage that they are extracted as an energy carrier from the current carbon cycle. This means, that on a scale of Earth history, carbon dioxide which is released as a result of the oxidative conversion of biomasses was generated only a short time ago and is also directly extracted again from the carbon cycle by the regrowing plants, if the biomasses are simultaneously planted again. Thus, a carbon dioxide balance is achieved and the accumulation of carbon dioxide in the atmosphere is avoided. 
     However, due to their clearly lower energy content, fuels based on biomasses as known from prior art cannot be used up to present with sufficient efficiency in the big industry. Moreover, the use of biomasses as a fuel requires fuel technologies which are different from those employed in the combustion of fossil fuels such as black coal or lignite for instance. This means, that the release of a defined amount of energy would require the combustion of a clearly higher amount of biomasses than of fossil fuels on one side and that the use of biomasses as a fuel on an industrial scale would require an expensive modification of already installed fueling systems on the other side. 
     Apart from their energy content, biomasses which are used as a fuel are different from fossil fuels also with regard to further properties such as ash content, volatile matters, hydrogen content and water content. But all these factors play an important part in the industrial use of fuels in dependence of the respective application, so that frequently it is not possible to exchange fossil fuels for biomasses as a fuel in the field of industrial applications. 
     U.S. Pat. No. 5,797,972 disclose a method for the production of pellets or briquettes from sewage sludge solids. Mechanically stable pellets or briquettes result from combining a major portion of sewage sludge solids with lesser amount of lime and binder materials suitable for imparting stability to the product and pressing or extruding the combined components into desired shapes. Coal may be included in the pellet or briquette composition to improve fuel value. 
     US 2004/0232085 A1 disclose a method for dewatering sewage sludge by using sludge-coal-oil co-agglomeration (“SOCA”) which comprises the steps of physically, chemically or biologically conditioning sludge to impart hydrophobicity and lipophilicity to the sludge (conditioning step), supplying oil and coal to the conditioned sludge with stirring to form sludge-coal-oil agglomerates (agglomerating step), enlarging the particle diameter of sludge-coal-oil agglomerates (size enlargement step), and remaining the enlarged sludge-coal-oil agglomerates over a sieve to selectively separate them from hydrophilic materials dispersed in tailing water(screening step). 
     However, mixing of the sewage sludge with e.g. coal as already performed in the prior art is based on the flue value of the resulting product, only. Other combustion properties of the resulting product, like homogeneity of the combustion and exchangeability of the product with conventional fuels have not been considered, yet. 
     The invention is therefore based on the object of providing a fuel which is ecologically more favorable on one side and which can be unlimitedly exploited for industrial applications on the other side. It is also an object of the present invention to provide a process for the production of such a fuel. 
     Concerning the fuel, this object is achieved by a fuel system according to claim  1 . Embodiments of the inventive fuel system are considered in the dependent claims and the embodiments as disclosed in the following description. 
     The present teachings may solve one or more of the above-mentioned problems and/or achieve one or more of the above-mentioned desirable features. Other features and/or advantages may become apparent from the description which follows. 
     At least some of the objects and advantages of the present teachings may be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. It should be understood that the invention, in its broadest sense, could be practiced without having one or more features of these exemplary aspects and embodiments. 
     The present teachings contemplate fuel systems and processes for the production of fuel systems, wherein the content of fossil carbon is reduced compared to that of fossil fuels, while the fuel-technological properties are the same. 
     The fuel systems and processes for the production of fuel systems in accordance with the present teachings consider a mixture of various fossil regular fuels and urban sewage sludge as biogenic carbon donors. As those of ordinary skill in the art would understand, as used herein, the term “biogenic carbon donor” generally refers to biomasses. 
     Accordingly, a fuels system is provided which is characterized in that it consists of a mixture of at least two different fossil regular fuels and urban sewage sludge as a biogenic carbon donor, wherein the amount of the urban sewage sludge is at least 10% with respect to the total weight. 
     According to the invention, urban sewage sludge is used as a biogenic carbon donor. In the meaning of the invention urban sewage sludge refers to the residual, semi-solid material left from urban wastewater or sewage treatment processes. 
     In a preferred embodiment according to the invention, the urban sewage sludge used as biogenic carbon donor is the product anaerobic digestion of raw sludge, for example in so called Imhoff tanks, also referred to as biosolids. 
     In another preferred embodiment according to the invention, the sewage sludge is raw sludge coming from the sedimentation tank or settling tank of urban sewage plants. 
     Fossil regular fuels which are preferably used in the fuel system according to the invention are brown coal, black coal, and anthracite. Examples of such usable coals are hard coal, fat coal, gas coal, long-flame coal, bituminous coal, pre-dried black lignite or pre-dried dull brown coal. 
     In a preferred embodiment of the inventive fuel system, the first fossil regular fuel has a Vitrinite reflection Rm of &gt;2.0, whereat the second regular fuel has a Vitrinite reflection Rm between of 0.4 to 2.0. 
     The Vitrinite reflection Rm gives information about the maturity and the calorification of the fossil regular fuel used. Furthermore, the Vitrinite reflection is associated with the combustion behavior of the deployed fossil regular fuel so that an optimization of the combustion behavior of the fuel system is possible by choosing fossil regular fuels having a Vitrinite reflection parameter within the specified range. Thereby, the combustion behavior of the inventive fuel system can be adapted to the combustion behavior of pure fossil fuels, like they are typically used in e.g. power plants, whereat as combustion behavior in particular the fuel value, the calorific value, as well as the ash residue should be understood. This enables the use of the inventive fuel system in existing firing systems without further plant specific modification. Hence, the inventive fuel system enables an ecological optimization of the firing systems without the need to make plant specific modifications. 
     According to a further embodiment of the inventive fuel system, at least three fossil regular fuels are used, whereat one having a Vitrinite reflection Rm &gt;3.0, a second having a Vitrinite reflection Rm within the range of &gt;2.0 and 3.0, and the third has a Vitrinite reflection Rm within the range of 0.4 and 2.0. 
     This further embodiment of the inventive fuel system enables in particular an adaption of the hardgrove index, so that the fuel system can be adapted with respect to this parameter to the plant specific conditions, like e.g. coal mills, too. 
     In a further embodiment of the inventive fuel system, the fuel system comprises beside the fossil fuel and the biogenic carbon carrier a refining product of the group consisting of coke, petrol coke, lignite coke, and charcoal. 
     According to another preferred embodiment of the invention, beneath the petrographical determination of the Vitrinite reflection also the composition of the maceral, including the composition of the sub-maceral is taken into consideration for calculating an optimized mixture of sewage sludge with a fossil fuel. For doing so, inert and reactive macerales are put into an optimized relation by mixing. As a result of these mixtures, the sedimentation of the activated sewage sludge is optimized. Activated sludge is a process for treating sewage treating using air and a biological floc composed of bacteria and protozoans. The fossil fuel mixture, e.g. coal mixture, which is optimized with respect to its maceral and sub-maceral composition, can advantageously be used as to improve the sedimentation of the sewage flocs in such processes in sewage treatment plants. 
     According to another embodiment, as reactive maceral a mixture of Vitrinite, Exinite, Resinite, and ⅓ Semifusinite, whereas as inert maceral a mixture of ⅔ Semifusinite, Micrinite, Fusinite, and mineral material is used. 
     Vitrinite macerals are derived from the cell wall material (woody tissue) of plants, which are chemically composed of the polymers, cellulose and lignin. 
     Exinite (also known as Liptinite) macerals are considered to be produced from decayed leaf matter, spores, pollen and algal matter. Resins and plant waxes can also be part of Exinite macerals. Exinite macerals tend to retain their original plant form, i.e., they resemble plant fossils. These are hydrogen rich and have the highest caorific values of all coal macerals. 
     Resinite macerals are ubiquitous, though minor, components in coals below medium-volatile bituminous rank. They are usually absent in coals of higher rank. 
     Fusinite is seen in most coals and has a charcoal-like structure. Fusinite is always the highest reflecting maceral present and is distinguished by cell-texture. It is commonly broken into small shards and fragments. 
     Semifusinite has the cell texture and general features of Fusinite except that it is of lower reflectance. In fact, Semifusinite has the largest range of reflectance of any of the various coal macerals going from the upper end of the Pseudovitrinite range to Fusinite. Semifusinite is also the most abundant of the inertinite macerals. 
     Micrinite occurs as very fine granular particles of high reflectance. It is commonly associated with the Liptinite macerals and sometimes gives the appearance of actually replacing the Liptinite. 
     According to a particular preferred embodiment, the mixture of inert and reactive macerals is based on the Composition Balance Index in waste (CB6) similar calculated to the Composition Balance Index (CBI) according to N. Schapiro et al, in AIME Proceedings, Blast Furnace, Coke Oven, and Raw Materials Conference 1961, Vol. 20, pg. 89ff, which is hereby incorporated by reference. Accordingly 
     
       
         
           
             
               CBI 
               W 
             
             = 
             
               
                 Total 
                  
                 
                     
                 
                  
                 inerts 
                  
                 
                     
                 
                  
                 in 
                  
                 
                     
                 
                  
                 the 
                  
                 
                     
                 
                  
                 fossil 
                  
                 
                     
                 
                  
                 fuel 
               
               
                 Optimum 
                  
                 
                     
                 
                  
                 inerts 
                  
                 
                     
                 
                  
                 in 
                  
                 
                     
                 
                  
                 the 
                  
                 
                     
                 
                  
                 fossil 
                  
                 
                     
                 
                  
                 fuel 
               
             
           
         
       
     
     In another preferred embodiment, the Composition Balance Index (CB6) of the fossil regular fuel used in the inventive fuel system is in the range of about between ≧2 and ≦5, preferably between ≧2.5 and ≦4.5. Surprisingly it was found that by choosing the CBl W  within that range, an optimized agglomeration of the fossil fuel with the sewage sludge flocs in the activated sludge can be achieved. These improved agglomeration leads to a stringer binding of the sludge flocs to the fossil fuel, which in turn on one hand reduces the time needed for sedimentation, on the other hand results in a fuel system having a significantly improved combustion behavior due to an optimized value of volatile compounds. 
     In general, there are three different procedures to produce the inventive fuel system:
         Procedure 1: The urban sewage sludge is putrefied, drained and dried and then mixed with the at least two different fossil regular fuels;   Procedure 2: The urban sewage sludge is mixed with at least two different fossil regular fuels and then putrefied, drained and dried;   Procedure 3: The urban sewage sludge is mixed with one of the at least two different fossil regular fuels, putrefied, drained, dried, and then mixed with at least one further fossil regular fuel.       

     The fuel system according to the invention provides a fuel which meets the requirements of fossil fuels concerning its fuel-technological properties while showing a clearly lower emission of CO 2  from fossil carbon carriers, based on the releasable energy content. 
     Thus, the fuels system shows an effective content of fossil carbon which is reduced by at least 11% compared to fossil fuels, referred to the calorific value, with the percentage of fossil fuels being put in a relation to the calorific value for calculating the effective content of fossil carbon. 
     The content of the fossil carbon in the inventive fuel system can be determined, for example by one of the radio carbon method, also referred to as  14 C-method, and the method of selective dissolution. 
     The  14 C-method is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 ( 14 C) to determine the age of carbonaceous materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years “Befor Present” (BP), “Present” being defined as 1950. The year 1950 was chosen because it was the year in which calibration curves for radiocarbon dating were first established. 1950 also predates large scale atmospheric testing of nuclear weapons, which altered the global ratio of carbon-14 to carbon-12, the naturally most frequently occurring carbon isotope. 
     When plants fix atmospheric carbon dioxide (CO 2 ) into organic material during photosynthesis they incorporate a quantity of  14 C that approximately matches the level of this isotope in the atmosphere. After plants die or they are consumed by other organisms for example, by animals, the  14 C fraction of this organic material declines at a fixed exponential rate due to the radioactive decay of  14 C. Comparing the remaining  14 C fraction of a sample to that expected from atmospheric  14 C allows the age of the sample to be estimated. Since coal, like other fossil fuels, is the product of carbonization of plants in former geological eras, the  14 C-content of fossil and biogenic carbon sources significantly differs, thereby allowing to discern between both. 
     When using the method of selective dissolution the probe to be examined on its biogenic carbon content is subsequently treated with sulfuric acid and hydrogen peroxide. While the biogenic carbon is soluble in the at least one of the mentioned solvents, fossil carbon is not. As a result the probe is depleted from biogenic carbon. Accordingly by determination and comparison of the total carbon content of the fossil fuel prior and after depletion, the amount of biogenic carbon can be determined. The determination of the total carbon content may be performed, e.g. by determination of the ash content. Also this method allows to discern between carbon coming from fossil and biogenic carbon sources. 
     Concerning the process, the object of the invention is achieved by a process for producing a fuel system of the above-described kind, comprising the steps of:
         1. selecting a first fossil regular fuel having a low content of volatile matters, and a Vitrinite reflection Rm &gt;2.0, a second fossil regular fuel having a medium content of volatile matters, and a Vitrinite reflection Rm between 0.4 and 2.0;   2. mixing the first fossil regular fuel with the second fossil regular fuel;   3. mixing the mixture obtained in step 2 with the sewage sludge;
 
wherein in Step 3 at least 10% by weight, with respect to the total mass of the fuel system, of the sewage sludge is admixed to the mixture obtained in step 2; or
   1. selecting a first fossil regular fuel having a low content of volatile matters, and a Vitrinite reflection Rm &gt;2.0, a second fossil regular fuel having a medium content of volatile matters, and a Vitrinite reflection Rm between 0.4 and 2.0;   2. mixing one of the selected fossil regular fuel with the sewage sludge;   3. mixing the mixture obtained in step 2 with the other selected fossil regular fuel,
 
wherein in Step 2 at least 10% by weight, with respect to the total mass of the fuel system, of the sewage sludge is admixed with one of the selected fossil regular fuel.
       

     The selected fossil regular fuel can be admixed to the sewage sludge prior or past putrefying, draining, and drying of the sewage sludge. This means, the fossil regular fuel according to the inventive method may be added to the sewage sludge, for example in the settling tank, sedimentation tank, or the vessel the anaerobic digestion takes place (e.g. an Imhoff tank). When admixing at least one of the selected fossil regular fuel to the sewage sludge prior to putrefying, draining, and drying the sludge, the regular fossil fuel beneficial can act as a filtration additive improving the draining of the sludge. 
     According to another embodiment of the inventive process, the fossil regular fuels are selected to provide a CBl W  of the fossil regular fuel mixture within a range of between ≧2 and ≦5, preferably between ≦2.5 and ≧4.5. 
     The mixture obtained by the inventive method may be mixed further with regular fuels. Suitable regular fuels are, for example coke, petrol coke, lignite coke, and charcoal. 
     The fuel system according to the invention which has been obtained by a process according to the invention is particularly suitable for power plant fuelling in electric power and heat production, for paper production, for the production of glass and mineral melts, and the cement production. 
     The fuel system according to the invention and the process for its production are described in the following by way of examples which are not in any way limiting to the invention. 
     In the following table 1, examples of the main characteristics of different fossil fuels and biogenic carbon carriers are shown. 
     EXAMPLE 1 
       
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Mixture 
                 Mixture 
                 Mixture 
                 Mixture 
               
               
                   
                   
                   
                   
                 A/B/C in % 
                 A/B/C in % 
                 A/B/C in % 
                 A/B/C in % 
               
               
                   
                   
                   
                 urban 
                 by weight 
                 by weight 
                 by weight 
                 by weight 
               
               
                   
                 Coal A 
                 Coal B 
                 sludge C 
                 33/33/33 
                 40/30/30 
                 50/20/30 
                 70/20/10 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Rm Vitrinite refection 
                 2.8 
                 1.2 
                   
                   
                   
                   
                   
               
               
                 parameters (raw, ar) 
               
               
                 water % 
                 5.00 
                 9.01 
                 25.91 
                 13.31 
                 12.48 
                 12.08 
                 7.89 
               
               
                 ash % 
                 7.60 
                 20.50 
                 27.03 
                 18.38 
                 17.30 
                 16.01 
                 12.12 
               
               
                 volatile matters % 
                 5.70 
                 21.26 
                 43.47 
                 23.48 
                 21.70 
                 20.14 
                 12.59 
               
               
                 sulfur % 
                 0.80 
                 0.93 
                 0.12 
                 0.62 
                 0.64 
                 0.62 
                 0.76 
               
               
                 hydrogen % 
                 2.48 
                 3.05 
                 3.31 
                 2.95 
                 2.90 
                 2.84 
                 2.68 
               
               
                 carbon total % 
                 78.85 
                 57.85 
                 23.92 
                 53.54 
                 56.07 
                 58.17 
                 69.16 
               
               
                 ncv J/g 
                 29.708 
                 21.833 
                 5.922 
                 19.154 
                 20.210 
                 20.997 
                 25.754 
               
               
                 ncv cal/g 
                 7.096 
                 5.215 
                 1.414 
                 4.575 
                 4.827 
                 5.015 
                 6.151 
               
               
                 Cfoss % 
                 78.85 
                 57.85 
                 4.48 
                 47.71 
                 50.24 
                 52.34 
                 67.21 
               
               
                 Cbiogen % 
                 0.00 
                 0.00 
                 19.44 
                 5.83 
                 5.83 
                 5.83 
                 1.94 
               
               
                   
               
               
                 ar = as received, 
               
               
                 ncv = net calorific value 
               
            
           
         
       
     
     A mixture auf coal A and coal B was mixed with urban sewage sludge dry in the ratios mentioned in table 1 according to procedure 1(i.e. the urban sewage sludge is putrefied, drained and dried and then mixed with the at least two different fossil regular fuels) in a mixing drum, until a homogeneous mixture was obtained. In the analytical examination the obtained mixture showed a water content, an ash content, a volatile material fraction, a sulfur content, and a total carbon content as mentioned in table 1. The biogenic carbon concentration was in the range of between 1.94 and 5.83% by weight of the total mass. Accordingly, the fuels system shows an effective content of fossil carbon which is reduced by at least 1.94% to about 6% absolute compared to fossil fuels, referred to the calorific value, with the percentage of fossil fuels being put in a relation to the calorific value for calculating the effective content of fossil carbon. The obtained mixture showed a ncv Hu of 4785 cal/g. 
     The obtained mixture showed an excellent combustion behavior and could be combusted in a stoker-fired furnace previously operated with fossil fuels, without any modification of the installation. 
     EXAMPLE 2 
       
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Mixture 
                 Mixture 
                 Mixture 
                 Mixture 
               
               
                   
                   
                   
                   
                 A/B/C in % 
                 A/B/C in % 
                 A/B/C in % 
                 A/B/C in % 
               
               
                   
                   
                   
                 urban 
                 by weight 
                 by weight 
                 by weight 
                 by weight 
               
               
                   
                 Coal A 
                 Coal B 
                 sludge C 
                 33/33/33 
                 40/30/30 
                 50/20/30 
                 70/20/10 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Rm Vitrinite refection 
                 2.8 
                 1.2 
                   
                   
                   
                   
                   
               
               
                 parameters (raw, ar) 
               
               
                 water % 
                 5.00 
                 9.01 
                 10.00 
                 8.00 
                 7.70 
                 7.30 
                 6.30 
               
               
                 ash % 
                 7.60 
                 20.50 
                 23.74 
                 17.28 
                 16.31 
                 15.02 
                 11.79 
               
               
                 volatile matters % 
                 5.70 
                 21.26 
                 52.81 
                 26.59 
                 24.50 
                 22.95 
                 13.52 
               
               
                 sulfur % 
                 0.80 
                 0.93 
                 0.15 
                 0.63 
                 0.64 
                 0.63 
                 0.76 
               
               
                 hydrogen % 
                 2.48 
                 3.05 
                 4.02 
                 3.18 
                 3.11 
                 3.06 
                 2.75 
               
               
                 carbon total % 
                 78.85 
                 57.85 
                 38.16 
                 58.29 
                 60.34 
                 62.44 
                 70.58 
               
               
                 ncv J/g 
                 29.708 
                 21.833 
                 9.992 
                 20.511 
                 21.431 
                 22.218 
                 26.161 
               
               
                 ncv cal/g 
                 7.096 
                 5.215 
                 2.387 
                 4.899 
                 5.119 
                 5.307 
                 6.249 
               
               
                 Cfoss % 
                 78.85 
                 57.85 
                 20.60 
                 53.02 
                 55.08 
                 57.18 
                 68.83 
               
               
                 Cbiogen % 
                 0.00 
                 0.00 
                 17.56 
                 5.27 
                 5.27 
                 5.27 
                 1.76 
               
               
                   
               
               
                 ar = as received, 
               
               
                 ncv = net calorific value, 
               
            
           
         
       
     
     A mixture auf coal A and coal B was mixed with urban sewage sludge dry in the ratios mentioned in table 2 according to procedure 2 (i.e. the urban sewage sludge is mixed with at least two different fossil regular fuels and then putrefied, drained and dried) in a mud basin, until a homogeneous mixture was obtained. The mixture takes place in an urban sewage plant. In the mud basin the sewage sludge is mixed with a mixture of coal A and coal B by simply adding the coal mixture to the mud basin and using the basins agitator to obtain a homogeneous mixture. Afterwards the mixture is drained by reducing of water. The admixing of the coal to the raw sewage sludge improves the necessary drainage of the sewage sludge. 
     In the analytical examination the obtained mixture showed a water content, an ash content, a volatile material fraction, a sulfur content, and a total carbon content as mentioned in table 2. The biogenic carbon concentration was in the range of between 1.76 and 5.27% by weight of the total mass. Accordingly, the fuels system shows an effective content of fossil carbon which is reduced by at least 1.76 to about 6% absolute compared to fossil fuels, referred to the calorific value, with the percentage of fossil fuels being put in a relation to the calorific value for calculating the effective content of fossil carbon. The obtained mixture showed a ncv Hu of 4522 cal/g. 
     The obtained mixture showed an excellent combustion behavior and could be combusted in a stoker-fired furnace previously operated with fossil fuels, without any modification of the installation. 
     Accordingly, the present invention contemplate fuel systems and processes for the production of fuel systems, wherein the content of fossil carbon is reduced compared to that of fossil fuels, while the fuel-technological properties are the same. 
    
    
     
       The present invention can be understood from the following detailed description either alone or together with the accompanying drawings. The drawings are included to provide a further understanding of the present teachings, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments of the present teachings and together with the description serve to explain the principles and operation. 
         FIG. 1  is a process flow diagram illustrating the production of a fuel system in accordance with the present teachings; 
         FIG. 2  is a flow diagram illustrating a first procedure for producing a fuel system in accordance with the present teachings; 
         FIG. 3  is a flow diagram illustrating a second procedure for producing a fuel system; and 
         FIG. 4  is a flow diagram illustrating a third procedure for producing a fuel system. 
     
    
    
     As illustrated in  FIG. 1 , the fuel systems and processes for the production of fuel systems in accordance with the present teachings consider a mixture of various fossil regular fuels and urban sewage, wherein the urban sewage sludge act as biogenic carbon donors. As those of ordinary skill in the art would understand, the mean random Vitrinite reflectance Rm gives information about the maturity and the calorification of a fossil regular fuel and the macerals comprised. The Rm is further associated, for example, with the combustion behavior of a fossil regular fuel. Accordingly, the combustion behavior of the fuel system may be optimized and adapted to the combustion behavior of pure fossil fuels (e.g., typically used in power plants) by choosing fossil regular fuels having a mean random Vitrinite reflectance parameter within a specified range, especially with respect to the maceral composition. In other words, the fuel technological properties of the fuel system (e.g., the fuel value, the calorific value, and the ash residue) may be adapted to the fuel technological properties of pure fossil fuels. This may, for example, enable fuel systems of the present teachings to be used in existing combustion systems without further plant-specific modification as shown in  FIG. 1 . 
     The fuel systems of the present invention may be produced using various processes. In various exemplary embodiments, for example, using a first procedure (i.e., Procedure 1) as shown in  FIG. 2 , urban sewage sludge can be putrefied, drained and dried and then mixed with at least two fossil regular fuels. 
     In various additional exemplary embodiments, using a second procedure (i.e., Procedure 2) as shown in  FIG. 3 , urban sewage sludge can be mixed with at least two fossil regular fuels and then putrefied, drained and dried. 
     In various further exemplary embodiments, using a third procedure (i.e., Procedure 3) as shown in  FIG. 4 , urban sewage sludge can be mixed with a first fossil regular fuel, putrefied, drained and dried, and then mixed with at least a second fossil regular fuel. 
     In various exemplary embodiments of the present invention, the fuel systems provide a fuel which meets the requirements of fossil fuels (i.e., requirements concerning fossil fuel fuel-technological properties), while showing substantially lower emissions of CO 2  from fossil carbon carriers, based on the releasable energy content. In various embodiments, for example, the biogenic carbon carrier fraction in the fuel system is at least about 10% by weight, referred to as the total mass. In various embodiments of the present invention, for example, the fuel systems and processes for the production of fuel systems consider a mixture of at least two differing fossil regular fuels and urban sewage sludge as a biogenic carbon donor, wherein the amount of the urban sewage sludge is at least about 10% with respect to a total weight of the mixture. 
     In various exemplary embodiments, for example, a first fossil regular fuel may have a mean random Vitrinite reflectance Rm of greater than about 2.0, and a second fossil regular fuel may have a mean random Vitrinite reflectance Rm ranging from about 0.4 to about 2.0.