Combined heat and power generation

This is particularly important when considering the effect that combined heat and power generation (cogeneration) has on utility waste. 

Combined heat and power cogeneration). Combined heat and power generation can have a very significant effect on the generation of utility waste. However, great care must be taken to assess the effects on the correct basis. 

For most chemical plants, process steam is used at pressures of 1.825 MN/m" (250 psig), saturated or lower. When combined heat and power generation is economically justified, the steam may be generated at about 5.96 MN/m" (850 psig) appropriately superheated and used to drive back-pressure steam turbines passing out process steam at the required pressure level. 

More complex utility options are encountered when combined heat and power generation (or cogeneration) is exploited. Here the heat rejected by a heat engine such as a steam turbine, gas turbine or diesel engine is used as hot utility. 

The potential of stationary fuel cells for distributed generation depends on feed-in tariff policies and electricity and gas prices, as well as on market competition from gas engines and small turbines. SOFCs and MCFCs, mostly fuelled by natural gas, are likely to play an important role for combined heat and power generation in buildings. 

For the stationary generation of heat and power the PEMFC is also in development. Fuel cell systems for combined heat and power generation mostly run on natural gas, and sometimes on biogas. Reformate is fed to the anode in these stationary systems. Only for backup power systems, which are designed for only a limited operating time, is pure hydrogen often used as fuel for the anode. 

Significant energy conservation is achieved by the well-established method of combined heat and power generation (cogeneration). The heat is usually in the form of intermediate or low-pressure steam, and the power as direct mechanical drives or as electricity generated with turbo-altemators. The choice of systems is usually between back-pressure steam turbines, or gas turbines with waste-heat boilers for the process steam. The amount of power generated is usually determined by the demand for heat. 

The electrolyte in the phosphoric acid fuel cells (PAFCs), which operate at 200 °C, is phosphoric acid and the electrodes are carbon black or graphite plates in which Pt particles are dispersed. The PAFCs are generally fed by natural gas, and CO has to be removed (only a 1 % concentration is accepted) to avoid Pt poisoning. PAFCs are mainly used for combined heat and power generation (up to 200 kW) in... 

Fuel cells, especially PEMFCs, can be used for various applications ranging from portable power supply for use in consumer electronic devices to stationary deployment for combined heat and power generation. Another potential application is transportation, in which fuel cell systems are developed for the propulsion of cars. The performance, operating conditions, costs, and durability requirements differ depending on the application. Transportation applications demand stringent requirements on fuel cell systems. Only the durability requirement in the transportation field is not as rigorous as the stationary application, although cyclic durability is necessary. 

Concept for a Decentralised Combined Heat and Power Generation Unit for Biomass Gasification... 

CONCEPT FOR COMBINED HEAT AND POWER GENERATION HEATSVPPLYOF THE GASIFIER... 

Vdmamo Demonstration Plant, A demonstration plant for biofuel-fired combined heat and power generation based on pressurised gasification. Published by Skogs Boktryckeri AB, 1998, Treileborg, Sweden. 

In the case of stationary systems, where 80 to 90 % of the heat is also extracted from the fuel cell system as valuable energy (and not considered waste) the total (electric and heat) efficiency of the combined heat and power generation can achieve >80%. 

Today most electric power is produced in large centralized instaUations (> 250 MW) and about 5 0 % of the primary energy is converted into heat. Heat, however, is much more diSicult to distribute than electricity. Therefore in the majority of instaUations heat is treated as waste or only a smaU fraction is used. Heat from low-power, decentralized power stations can be better utilized. But the combustion technologies have much lower electric eSiciencies at lower power (i.e. below 100kW), and therefore fuel ceUs in distributed combined heat and power generation (CHP) are an attractive alternative. The electric efficiency of fuel ceU systems is sufficiently high (>30%) even at low power and also the heat can be utUized, so that the total efficiency can reach values >80 % [29, 30]. 

Polymer Electrolyte membrane Fuel Cells (PEFC) are used to power uninterruptible power supplies, combined heat and power generation systems, vehicles for materials handling as well as electric vehicles, busses and light duty road vehicles. This contribution gives a short introduction into the working principles of PEFC as well as the materials and components used. 

Braun, R.J., Klein, S.A., and Reindl, D.T. (2006) Evaluation of system configurations for solid oxide fuel cell-based micro-combined heat and power generators in residential applications. J. Power Sources, 158, 1290-1305. 

In parallel with these development, work has been carried out on phosphoric acid electrolyte fuel cells which operate at 200 C and are able to burn hydrogen containing CO2 impurity, using air rather than pure oxygen as the oxidant. They find application as combined heat and power generation in 40 kW sizes, while much larger units (4.8 MW) have been installed in Manhattan and in Tokyo for peak load generation in the electricity supply system[4]. 

J.-Fr. Hake et al. [92] compared the conventional generation of heat and electricity to that of small-scale combined heat and power generation by high-temperature fuel cells, and the results of which are shown in Fig. 13.16. The small-scale CHP devices studied were used to provide electricity, space heat, and warm water to both residential and commercial buildings. The conventional generation of electricity is much less efficient than that of small-scale CHP devices due to... 

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