Page 69 - Wastewater Solids Incineration Systems

38 Wastewater Solids Incineration Systems

Municipal WWTP residuals will vary widely because of origin and methods of
6
treatment; however, the normal range of F is approximately 9200 10 000 dscf/10 Btu.
d
Any analysis yielding F values outside this range should be questioned before using
d
the values in a heat and material balance. Running new samples or re-running the
analyses on the same sample is generally advisable. As mentioned earlier, even a cor-
rectly performed laboratory analysis can sometimes yield skewed results because of
interactions by some chemicals present, or because of the lack of homogeneity within a
particular sample that leads to the analysis of a nonrepresentative subsample.
When comparing heating values, it is always best to use a moisture ash-free
(MAF) basis to avoid skewed results from varying collection and wastewater treat-
ment processes. Using an MAF basis makes it easier to see how a particular feed cake
compares to others, because it minimizes differences arising from the location (i.e.,
different regions of the country) and dewatering techniques.

2.0 ACTUAL FURNACE OPERATING CONDITIONS

All combustion calculations in the previous sections have assumed complete com-
bustion. In actual practice, complete combustion is difficult to achieve, and furnaces
always operate with some excess air. The amount of excess air required to reduce CO
and total hydrocarbons (THC) to regulatory limits is dependent on furnace design
and the combined effects of the three “Ts” of combustion: time, temperature, and tur-
bulence. Based on a feed cake with specific values for total solids, volatile solids, and
heating value, the only way to minimize auxiliary fuel use is to reduce furnace
exhaust temperature and excess air to the lowest values that allow safe, stable opera-
tion while achieving regulatory compliance.
Simply making a heat and material balance for a given excess air value and theoret-
ical temperature of the products of combustion (TTPC) does not mean that the furnace
will be able to perform. Often in multiple-hearth furnace operation, excess air must be
increased to greater than that necessary to achieve good combustion of hydrocarbons
and CO. This excess air is necessary to prevent temperatures on the combustion hearth
in the middle of the furnace from getting too high. Some fluid bed furnaces are operated
with freeboard water sprays that are used to lower the exit temperature while auxiliary
fuel is added to the bed to keep it at the minimum combustion temperature. These prac-
tical considerations must be kept in mind when designing new systems or modifying
operating parameters of an existing operating system.