Coal-derived fuel

R. P. AHen, R. A. Battista, and T. E. Ekstrom, "Characteristics of an Advanced Gas Turbiue with Coal Derived Fuel Gases," Ninth NnnualEPRJ Conference on Gasification Power Plants, Palo Alto, Calif., Oct. 1990. 

Conventional coal-tar fuels from retorting Typical coal-derived fuels with different levels of hydrogenation Synthetic crude oils, by hydrogenation ... 

The addition of H2O and CO2 to the fuel gas modifies the equilibrium gas composition so that the formation of CH4 is not favored. Carbon deposition can be reduced by increasing the partial pressure of H2O in the gas stream. The measurements (20) on 10 cm x 10 cm cells at 650°C using simulated gasified coal GF-1 (38% H2/56% CO/6% CO2) at 10 atm showed that only a small amount of CH4 is formed. At open circuit, 1.4 vol% CH4 (dry gas basis) was detected, and at fuel utilizations of 50 to 85%, 1.2 to 0.5% CH4 was measured. The experiments with a high CO fuel gas (GF-1) at 10 atmospheres and humidified at 163°C showed no indication of carbon deposition in a subscale MCFC. These studies indicated that CH4 formation and carbon deposition at the anodes in an MCFC operating on coal-derived fuels can be controlled, and under these conditions, the side reactions would have little influence on power plant efficiency. 

Table 6-6 Contaminants from Coal-Derived Fuel Gas and Their Potential Effect on...

The typical fuel gas composition and contaminants from an air-blown gasifier that enter the MCFC at 650°C after hot gas cleanup, and the tolerance level of MCFCs to these contaminants are listed in Table 6-7 (58,71,72). It is apparent from this example that a wide spectrum of contaminants is present in coal-derived fuel gas. The removal of these contaminants can add considerably to the efficiency. A review of various options for gas cleanup is presented by Anderson and Garrigan (70) and Jalan et al. (73). 

Products which were evaluated in JFTOT and thermal stability tests were distilled and stored in glass. Petroleum- or coal-derived fuels which have been distilled or stored in metal equipment sometimes fail these tests due to contamination by metals. 

It is anticipated that coal derived liquids boiling above about 350 F will be disposed of to the utility market. Table V summarizes the potential utility markets for various types of coal derived fuels which include solvent refined coal, heavy boiler fuels, distillate boiler fuels, turbine fuels and methanol. Speculative locations for these markets are indicated on Figure 2. 

Table IV gives the properties of the SRC-II fuel oil compared to a low-sulfur residual oil utilized in a recent combustion test. The SRC-II fuel oil is a distillate product with a nominal boiling range of 350-900°F, a viscosity of 40 Saybolt seconds at 100°F and a pour point below -20°F. Thus, it is readily pumpable at all temperatures normally encountered in transportation of the fuel oil. The fuel oil has a very low content of ash and sediment as well as a low Conradson carbon residue. These characteristics are favorable from the standpoint of particulate emissions during combustion. Tests of compatibility with typical petroleum fuel oils and on stability of the coal distillates over time have not revealed any unusual characteristics that would preclude utilization of these coal-derived fuels in conventional boiler applications.

Synthetic liquid fuels derived from coal and shale will differ in some characteristics from conventional fuels derived from petroleum. For example, liquid synfuels are expected to contain significantly higher levels of aromatic hydrocarbons, especially for coal-derived fuels, and higher levels of bound nitrogen. These differences can affect the combustion system accepting such fuels in important ways. In continuous combustors, i.e. gas turbines, the increased aromatics content of coal-derived fuels is expected to promote the formation of soot which, in turn, will increase radiation to the combustor liner, raise liner temperature, and possibly result in shortened service life. Deposit formation and the emission of smoke are other potential effects which are cause for concern. Higher nitrogen levels in synfuels are expected to show up as increased emissions of N0X (NO+NO2) An earlier paper presented results of an experimental study on the effect of aromatics and combustor... 

Test Results with Middle Distillate Coal-Derived Fuels,... 

Emissions at Baseload Conditions for Coal-Derived Fuel... 

Comparison of Primary Combustor Wall Heat Loading for Coal-Derived Fuels at Baseload... 

Combustion tests of fuel oil blends derived from the Exxon Donor Solvent (EDS) process were carried out in a laboratory 50 hp test boiler and a commercial 1425 hp boiler. All tests showed that coal derived fuel oils burn cleanly compared to petroleum fuels with low levels of smoke and particulates. Emissions of N0X were related to fuel nitrogen content for both the petroleum and coal-derived fuels. 

Tests in the commercial boiler showed no significant differences in PNA emissions from EDS fuel oil and petroleum regular sulfur fuel oil. It is speculated that increasing the boiler size decreases the surface-to-volume ratio which provides a smaller quench zone for partially pyrolyzed combustion products. Emissions of PNA from the commercial scale combustion of coal-derived fuels may not be a problem. 

This paper covers the combustion performance of EDS fuel oil blends, with primary emphasis on the emissions of polynuclear aromatics (PNA). Previous testing of EDS fuel oil blends 0, 2) and other coal derived fuel oil blends (2) has shown that emissions of particulate and smoke are lower with coal-derived fuel oils than from the combustion of petroleum-derived fuel oils. 

N0X emissions tend to be higher due to the higher fuel nitrogen levels of coal-derived fuel oils. However, it appears, based on small scale lab tests (2) and limited commercial tests (3), that staged combustion should allow N0X emissions standards for coal-derived fuel oils to be met. One environmental concern that had not been addressed in these tests is the emissions of PNA. This is a potential concern due to the highly aromatic nature of coal-derived fuel oils. 

The emission measurements during this testing included N0X, smoke, particulate and PNA. N0X was determined by a non-disper-sive infrared analyzer, and smoke by the Bacharach test. Both the particulates and PNA were sampled by a source assessment sampling system (SASS). The SASS system isokinetically samples a fraction of the stack gas and traps particulates in a series of cyclones, which classify the particulate by size. Final filtration is through a fiberglass filter mounted in an oven heated to 200°C to prevent condensation of acids. In this program, the cyclones were not used, since previous work (3) had shown the particulate from coal-derived fuel oils to be small, with an average diameter on the order of 0.4 /um. The PNA which is not deposited on the particulate is collected on XAD-2 resin after the gas has been cooled to 15-20°C. PNA analyses were carried out on a combined extract from the particulate, XAD-2 resin, other condensates in the system, and the solvent rinses used to clean the SASS system. 

Results of these tests are shown in Table IV. The average emissions values are shown for the coal-derived fuels. As previous testing has shown, the EDS fuel oils produced little particulate relative to the petroleum fuel, while the smoke values for the coal-derived materials were equal to or less than that of the RSFO. As with the laboratory testing, the N0X level was a function of the fuel nitrogen level, although N0X emissions from the commercial unit were lower than from the laboratory boiler. 

The major difference from the laboratory testing is the PNA emissions. First, there is no difference in PNA emissions among the four fuels tested. Secondly, the PNA emissions from the commercial unit are lower than those shown in Table II for the laboratory boiler. This appears true for both the petroleum and coal-derived fuels. 

These results suggest that PNA emissions may not be a problem from large scale industrial and utility combustion of EDS fuel oils. However, additional large scale testing will be required for confirmation, particularly since the effects of staged combustion which may be required for N0X control have also not been considered here. Tests are currently being planned by the Electric Power Research Institute to address these questions. It is expected that these tests will help to resolve the question of PNA emissions from coal-derived fuel oil combustion. 

Both results suggest that PNA emissions from commercial combustion of coal derived fuel oils may present no problem. This is presumably due to the lower surface to volume ratio in the commercial units relative to the laboratory combustors. However, due to the relatively limited knowledge of the mechanism of PNA emissions and the effects of combustion variables, additional large scale testing will be required to demonstrate that PNA emissions from commercial use of EDS fuel oil would not present an emissions problem. 

At the beginning of 1976, EPA funded a laboratory program to develop an improved denitrogenation catalyst for heavy coal-derived fuel oil. The scope of this program consists of three tasks. 

For denitrogenation of coal-derived fuel, Ni-Mo is more effective than Co-Mo, and Co-Mo is better than Ni-W. The initial catalyst deactivation is caused mainly by carbon deposition. Higher temperature operation resulting in higher carbon deposition on the catalyst gives poorer performances in terms of denitrogenation, desulfurization, deoxygenation, and 975°F+ conversion. 

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