Combustible Offgases


Figure 27. Basic elements of high temperature processes.

The primary combustible elements in sludge and in most available supplemental fuels are fixed carbon, hydrogen, and sulfur. Because free sulfur is rarely present in sewage sludge to any significant extent and because sulfur is being limited in fuels, the contributions of sulfur to the combustion reaction can be neglected in calculations without compromising accuracy. Similarly, the oxidation of metals contributes little to the heat balance and can be ignored. Solids with a high fraction of combustible material (for example, grease and scum) have high fuel values. Those which contain a large fraction of inert materials (for example, grit or chemical precipitates) have low fuel values. Chemical precipitates may also exert appreciable heat demands when undergoing high-temperature decomposition. This further reduces their effective fuel value. Table 5 provides a summary of typical chemical reactions that take place during combustion, along with heating values of the reactions.

Table 5. Chemical Reactions Occurring During Combustion,


High Heat Value of Reaction

c + o2 co2

-14,100 Btu/Lbs of C

c + Vz01 CO

-4,000 Btu/Lbs of C

CO + Vi02 C02

-4,400 Btu/Lbs of CO

H2 + 1/2 02 -v H2O

-61,100 Btu/Lbs of H2

CH4 + 202 -V C02 + 2H20

-23,900 Btu/Lbs of CH4

2H2S + 302 2S02 + 2H20

-7,100 Btu/Lbs of H2S

C + H20 (gas) CO + H2

-4,700 Btu/Lbs ofC

Sludge combustibles -> C02 + H20

-10,000 Btu/Lbs of combustibles

The following are experimental methods from which sludge heating value may be estimated or computed:

• Ultimate analysis-an analysis to determine the amounts of basic feed constituents. These constituents are moisture, oxygen, carbon, hydro- gen, sulfur, nitrogen, and ash. In addition, it is typical to determine chloride and other elements that may contribute to air emissions or ash- disposal problems. Once the ultimate analysis has been completed, Dulong's formula can be used to estimate the heating value of the sludge, Dulong's formula is:

Btu/Lbs = 14.544C + 62,208(H2 - 02/8) + 4,050S (30)

where C, H2, 02, and S represent the weight fraction of each element determined by ultimate analysis. This formula does not take into account endothermic chemical reactions that occur with chemically conditioned or physical-chemical sludge. The ultimate analysis is used principally for developing the material balance, from which a heat balance can be made.

• Proximate analysis - a relatively low-cost analysis in which moisture content, volatile combustible matter, fixed carbon, and ash are determined. The fuel value of the sludge is calculated as the weighted average of the fuel values of its individual components.

• Calorimetry - this is a direct method in which heating value is determined experimentally with a bomb calorimeter. Approximately 1 gram of material is burned in a sealed, submerged container. The heat of combustion is determined by noting the temperature rise of the water bath. Several samples must be taken and then composited to obtain a representative 1-gram sample. Several tests should be run, and the results must be interpreted by an experienced analyst. New bomb calorimeters can use samples up to 25 grams and this type of unit should be used where possible.

The preceding tests give approximate fuel values for sludge and allow the designer to proceed with calculations which simulate operations of an incinerator. If a unique sludge will be processed, or unusual operating conditions will be used, pilot testing is advised. Many manufacturers have test furnaces especially suited for pilot testing.

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