Energy costs, boiler

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

Fig. 1. Effect of energy use on total cost where total cost is the sum of capital and energy costs for the lifetime of the plant, discounted to present value. Point D corresponds to the design point if the designer uses an energy price that is low by a factor of four in projected energy price. Effects on costs of (a) pressure drop in piping, (b) pressure drop in exchangers, (c) heat loss through insulation, (d) reflux use, and (e) energy recovery through waste-heat boiler...

Waste-Heat Boiler. In a waste-heat boder (Fig. 6), the approach AT sets both the amount of the unrecovered energy and the amount of heat-exchange surface. When terms are added for energy value, and surface cost, the optimum occurs when... 

Flue gas recirculation (FGR) is the rerouting of some of the flue gases back to the furnace. By using the flue gas from the economizer outlet, both the furnace air temperature and the furnace oxygen concentration can be reduced. However, in retrofits FGR can be very expensive. Flue gas recirculation is typically applied to oil- and gas-fired boilers and reduces NO, emissions by 20 to 50%. Modifications to the boiler in the form of ducting and an energy efficiency loss due to the power requirements of the recirculation fans can make the cost of this option higher. 

Coal competes primarily in the market for low-cost boiler fuels. Coal is also characterized by a relatively low energy content per unit of weight (at best two-thirds chat of residual oil). Consequently, low-cost... 

Provide metering facilities for each energy source for each major cost area (e.g. the boiler house or a particular production process) ... 

Flash steam and heat recovery systems provide a useful and simple means of recovering both heat energy and steam condensate that would otherwise be lost by BD. The capital cost for an average sized industrial boiler house is relatively low, and capital payback periods are usually within 12 to 24 months. 

Most waterside problems develop insidiously. Over time, scale and other types of deposit are gradually formed on internal heat transfer surfaces, which gradually raises the cost of providing heat energy. Some types of deposition can be very difficult and costly to remove. Corrosion wastes away the fabric of the plant (sometimes very quickly) and may produce an unexpected and untimely boiler plant shutdown, with a consequential loss of space heating, electricity, or process manufacturing capability. Likewise, fouling reduces the size of waterways and increases boiler operational problems. 

For almost all larger boiler plants, the relatively high cost of producing MU water and the heat energy inherent in condensate deems it necessary, wherever possible, to recover for reuse condensate to the greatest extent. 

Figure 23.41 illustrates the overall trade-offs as a function of the number of intermediate steam mains for the case of maximized energy recovery13. Two curves are shown in Figure 23.41. The upper curve corresponds with minimum fuel requirement and the lower corresponds with minimum utility cost. Both correspond with no condensing power generation from utility steam. Once the number of intermediate mains has been chosen, then the settings for those mains are chosen, either for the upper curve to correspond with the minimum fuel for the utility steam boilers or for the lower curve for minimum utility cost (fuel and power). As the number of intermediate steam mains is increased, the fuel required and the utility cost both decrease. However, the decrease in cost diminishes as the number of steam mains increases.13...

Biomass gasifiers have the potential to be up to twice as efficient as using conventional boilers to generate electricity. For even greater efficiency, heat from the gas turbine exhaust can be used to generate additional electricity with a steam cycle. These improvements in efficiency can make environmentally clean biomass energy available at costs more competitive with fossil fuels. 

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