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Show of the testing was therefore devoted to the relative amounts of S02 removal occurring in the ductwork and baghouse. All of the laboratory pilot tests were conducted with a single, commercially-available hydrated lime sorbent. Hence, no information was developed on the potential benefits of using other sorbent materials. Evaluation of alternative sorbents is planned for the field demonstration. The cost of the S02 sorbent is a major contributor to the overall cost of operating a SNRB system. The Ca/S stoichiometry, which is defined as the molar ratio of injected Ca(OHh to S02 in the flue gas required to achieve the S02 removal goal, is thus an important operating consideration. Tests were conducted over a wide range of stoichiometric conditions - Ca/S02 ranged from 1.0 to 4.0 - to determine the impact on S02 removal performance. It was found that above a stoichiometry of about 2.0, only slight improvements in S02 removal were obtained with increasing stoichiometry. The approach of recycling the spent sorbent to improve sorbent utilization was also investigated, but was found to be ineffective. The reaction of the Ca(OHh and/or its derivatives with CO2 in the flue gases - which competes with the S02 reaction - renders the sorbent ineffective for re-use. In order to evaluate S02 removal performance over a range of temperatures typical of utility boiler economizers, the sorbent injection temperature was varied from 800· - 1100·F, resulting in baghouse temperatures of 600· - 850·F. Over the range of temperatures tested, S02 removal was essentially independent of injection temperature. This indicates that the S02 removal performance of commercial SNRB systems will not be adversely affected by variations in economizer temperature within the tested range. However, while the majority of S02 removal occurs in the duct, additional removal in the baghouse - as the sorbent resides on the bags in the form of a filter cake - is required in order to meet the project goal of 70% S02 removal. The effect of baghouse operating temperature on this incremental bag house S02 removal is also illustrated in Figure 6. As the baghouse operating temperature increases from 730· to 840·F, the total S02 removal increases from 60% to 80%. The 70% sulfur removal goal is exceeded at baghouse operating temperatures above about 800·F. It is hypothesized that the incremental baghouse S02 removal is due to the fact that at the elevated baghouse temperatures the sorbent continues to dehydrate, thereby generating additional surface area In the operating range of 600· - IOOO·F, the rate of reaction of Ca(OHh with S02 is not significantly affected. However, S02 removal is also a strong function of the available surface area of the sorbent. If significant dehydration continues as the sorbent resides on the filter bags, fresh surface is generated as the water vapor finds its way out of the sorbent particles. This newly-created surface area then becomes available for subsequent reaction with S02. 7 NOx/SOx Removal Compatibility Another important factor addressed by the laboratory pilot tests is the compatibility of the SOx and NOx removal processes. After all, the SNRB process requires that both objectives be accomplished in the same piece of equipment. Figure 6 also illustrates the compatibility of these processes. As previously mentioned, baghouse temperatures in the range of 800· - 850·F are required to meet the NOx removal performance goal of 90% NOx removal. The figure illustrates that this same range of baghouse operating temperatures is required to meet the S02 removal goal of 70%. This important result means that the two processes can be combined into a single SNRB baghouse without sacrificing the performance of either system. PRELIMINARY ECONOMIC EVALUATION The laboratory pilot tests provided a mechanism for assessing the impacts of various design features and operating conditions on the overall performance of the SNRB system. This information was subsequently used to develop design specifications and testing plans for the 5 MW e SNRB demonstration facility. A preliminary economic assessment of SNRB was therefore conducted in conjunction with the pilot tests to provide complementary information on the cost impacts of the various potential design, performance, and operating specifications for a commercial SNRB system. The results of this study were subsequently used to prioritize the factors to be investigated in the 5 MW e facility and to determine what additional testing may be required. Figure 7 provides a pictorial summary of the major factors contributing to the overalllevelized cost (in $ / ton of SOx + NOx removed) of operating a 500 MW e SNRB system. The case depicted is for a calcium-based SNRB system applied to a new boiler. The various costs indicated comprise all of the incremental costs associated with the presence of the SNRB system relative to those of the "base case" boiler system without the SNRB system. For example, the capital cost component identified for the baghouse is actually the incremental cost of the SNRB baghouse relative to the base case particulate collector. Likewise, the cost indicated for waste disposal is the incremental cost of waste disposal for the SNRB system relative to the fly ash disposal costs of the base case. In this regard it should be pointed out that the base case boiler system did not include SOx or NOx removal systems. A detailed discussion of the economic analysis is beyond the scope of this paper. The purpose of presenting these results is to indicate the type of information used to assess the sensitivity of the overall cost of a SNRB system to the various cost factors. In particular, the figure clearly illustrates the major influence exerted by: 1), the cost of the ceramic filter bags and 2), the cost of the S02 sorbent. |