Efficiency combined cycle systems

Computer simulations have demonstrated that a combined cycle system with MCFC stacks networked in series is significantly more efficient than an identical system with MCFC stacks configured in parallel. 

A similar opportunity exists for the public utility industry in the potential of LBG and MBG. The reduced environmental impact of a coal gasification plant which produces a perfectly clean fuel equivalent to natural gas, compared to direct combustion of coal may allow increased use of coal in areas where increased pollutant emission is barred. As these PSD areas increase in number, the advantages of coal gasification become more apparent. The potential of more efficient combined cycle generation systems which can be used with coal-derived gases is an added factor for implementing coal gasification. 

In contrast, gasification systems are far more efficient, reaching 60 percent, versus only 20-40 percent for some direct-fired plants.31 These types of systems heat the biomass in an environment where the solid biomass breaks down into a flammable gas, such as methane.This gas can then be used in more efficient combined-cycle power-generation systems... 

This combination of gas and steam turbines accounts for the name combined cycle. Gasification combined-cycle systems are among the cleanest and most efficient of the emerging clean coal technologies. Sulfur species, nitrogen species, and particulate matter are removed before the fuel is burned in the gas turbine. Thus, there is a much lower volume of gas to be treated than in a postcombustion scrubber. 

While the overall combined cycle is more efficient than conventional pulverized coal plants, energy losses do occur in the transformation of coal into a gaseous fuel, mostly due to the heat input needed for gasification. As a result of the added gasification process needed for combined-cycle systems using coal, the IGCC process is less efficient than the same combined power generation cycle run on a fuel that does not require pretreatment such as natural gas or fuel oils. 

Proposals for the propulsion of liquefied natural gas ships have called for combined cycle systems much like those used for electric power generation. Such systems are much more efficient than conventional steam systems. 

The fact that thermodynamically, some of the heat generated in the electrochemical reaction can be reinvested into the chemical fuel conversion explains that SOFC have the potential to reach a net electrical system efficiency above 60% with natural gas fuel [14]. In a combined cycle system, where the exhaust gas from the high-temperature fuel cell is used to drive a gas turbine, the overall system electrical efficiency may even reach over 70% [15]. The efficiencies of gas turbines and diesel engines decrease at part load, whereas the efficiency of the fuel-cell-based systems will be almost independent of part load up to very high turndown ratios [14]. 

Gas turbine-based power plants, particularly natural gas-fired cogeneration and combined-cycle faciUties, have proven to be highly rehable, efficient, and environmentally attractive. Advances in machine design, more efficient plant integration, and optimistic forecasts for the availabiUty of affordable natural gas worldwide have boosted the appeal of these systems for both base-load and peaking service. 

The capital investment required for gasification-based power systems is 1400 to 1600 /kW (1994 US dollars) and is projected to become less than 1200 /kW in the year 2000 because of the higher efficiency associated with gas-turbine combined-cycles currently being designed by turbine vendors. 

Dearation can be either vacuum or over pressure dearation. Most systems use vacuum dearation because all the feedwater heating can be done in the feedwater tank and there is no need for additional heat exchangers. The heating steam in the vacuum dearation process is a lower quality steam thus leaving the steam in the steam cycle for expansion work through the steam turbine. This increases the output of the steam turbine and therefore the efficiency of the combined cycle. In the case of the overpressure dearation, the gases can be exhausted directly to the atmosphere independently of the condenser evacuation system. 

---