Bypassing with economizers

With shell boilers, economizers will generally only be fitted to boilers using natural gas as the main fuel and then only on larger units. It would be unlikely that a reasonable economic case could be made for boilers of less than 4000kg/hF and A100°C evaporative capacity. The economizer will incorporate a flue gas bypass with isolating dampers to cover for periods when oil is used and for maintenance. The dampers require electric interlocks to the selected fuel. 

Research is being conducted on the direct synthesis of ethylene glycol from synthesis gas. In one process very high pressures of 5,000 psi with very expensive catalysts Rhx (CO) are being studied. An annual loss of rhodium catalyst of only 0.000001% must be realized before this process will compete economically. At least five other alternate syntheses of ethylene glycol that bypass toxic ethylene oxide are being researched. 

Normally, Roots pumps are not as economical as gas ballast pumps for continuous operation at pressures above 40 mbar. With very large pump sets, which work with very specialized gear ratios and are provided with bypass lines, however, the specific energy consumption is indeed more favorable. If Roots pumps are installed to pump vapors, as in the case of gas ballast pumps, a chart can be given that includes all possible cases (see Fig. 2.74). 

A more economic solution is to replace the valves with POSRVs with remote pressure sensing. Instead of having a pressure pick-up to the pilot at the inlet of the main valve, pressure can be measured on the system, bypassing the inlet piping and its pressure losses. This solution is also easily field convertible if the snap-action pilot has integral pressure sensing. A typical remote sense point could be a tee into a gauge tap. 

The two valves being adjusted by the temperature controller are implemented such that their stroking fully overlaps, with one valve closing as the other valve opens, as shown in Figure 3.8(C). With smaller pipe sizes, these two valves could economically be replaced with a three-way valve located at the start of the bypass line. It should not be located at the end of the bypass line where thermal stresses would exist from the two different temperature streams. The three-way valve has flow characteristics similar to... 

Economizer.—The economizer is a feed-water heater using boiler flue gases as the heating agent. It usually consists of a single unit for an entire row of boilers, with a gas bypass to the stack. The surface is made up of vertical cast-iron tubes, 4 in. in diameter and 10 or 12 ft. long, through which the water rises. The outsides of the tubes are cleaned of soot by power-operated scrapers. Economizers obstruct the draft and often necessitate the use of fans. 

The economic evaluation is based on a flue gas cleaning (FGD and fly ash) system to meet the revised 1979 NSPS for both particulate matter and SO2 for a new, 500-MW coal-fired boiler. The boiler burns a 3 5 sulfur eastern bituminous coal containing 15.1 ash and having a heating value of 11,700 Btu/lb. The boiler has a heat rate of 9,500 Btu/kWh. The FGD system is designed with one spare scrubber train, 50 emergency flue gas bypass around the FGD system, and an onsite landfill located one mile from the boiler. The operating schedule specifies a 165,000-hour, 30-year life for the boiler and 5,500 hours of first-year operation. 

Another degree of freedom is the flow rate of air bypassing the OTM section and directly burnt in the main combustor. An economic optimization used to determine the optimal air bypass fraction reflects the trade-off between lower power consumption and the capital costs of the boost compressor, recuperator (whose size falls with decreasing flow rate) and membrane (whose area, for a given O2 separation fraction, decreases with increasing inlet air flow rate and hence average difference in O2 partial pressure across the membrane). 

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