OCR Text |
Show NET SAVINGS(%) ~r---------------------------------------~ --No Preh •• t ----Air Preheat + 1000"F 40 30 20 10 °0~--------~5----------1~0--------~lL5~~----~W ~/FUEl COST RATIO IS I Ton ~ : S I MMBtu) Net Operating Cost Savings for Preheated Oxygen Enriched Combustion Figure 8 oxygen enrichment levels of 25%, 35%, and 100% respectively. As discussed before, the large relative increases for lower enrichment levels are due to the larger volumes of enriched air available for preheating. The net operating cost savings by preheated and oxygen enrichment also decrease linearly with the oxygen to fuel cost ratio, but at more gradual rates than the corresponding enrichment cases without preheating. At Y = 6.0 the net operating cost savings of all preheated cases become the same value of 37%. This point corresponds to the break even oxygen to fuel cost ratio for enriching the 10000 F preheated air with 10000 F oxygen. If the oxygen to fuel cost ratio is greater than 6, the net operating cost savings of preheated oxygen enriched combustion systems become smaller than that of the air preheat system without enrichment. Since the rate of return on investment for preheating of enriched air is lower than that for preheating of air as discussed previously, the overall economics favors preheating of air without enrichment. If the oxygen to fuel cost ratio is less than 6, then both enrichment and preheating can be economically viable. Al though the optimum selection of oxygen enrichment and/or preheating could not be made without an evaluation of the capital cost for each application, the net operating cost analysis presented in Figure 8 provides important guidelines in screening viable options. OXYGEN OR OXYGEN ENRICHED AIR SUPPLY SYSTEMS AND COSTS The cost of oxygen or oxygen enriched air depends on many factors including: (1) the air separation process, (2) cost of electric power, 159 (3) capacity utilization factor, (4) oxygen use pattern and (5) back up supply requirement. In order to properly identify the most cost effective method of oxygen supply these key parameters are examined in this section. The state of the art oxygen supply systems consists of cryogenic plants, liquid supply and vaporization systems, PSA and membrane systems. The size of these facilities range from less than one ton per day to over two thousand tons per day. The oxygen purities range from 99.9% oxygen with cryogenic processes to 28%-35% oxygen with membrane units. In addition to the installation of one of the above systems, oxygen may be supplied from excess capacity of an existing large cryogenic plant and transported to a use point by pipeline. This diversity in oxygen supply systems results in a wide range in oxygen economics depending on the specific application. A brief description of each air separation process and its characteristics is given below. The large cryogenic plants have historically been the workhorse of the industrial gas industry. This includes both gas and liquid producing facilities. The gas plants are facilities located adjacent to the use point, dedicated and designed specifically for the application. The liquid/gas producing facilities have the capability to produce liquefied product for shipment to remote customers. Cryogenic plants for the most part have been quite large. This is due to the economics of scale resulting from the complexity of this process. Through the use of a low temperature distillation process, high purity oxygen is achievable at high recoveries. For oxygen enrichment applications where low purity oxygen is acceptable, cryogenic facilities may be optimized at lower purities between 70-80% where unit power will be the lowest. However, purities of the existing plants typically range from 95 to 99.9% oxygen to serve the purity requirements of many applications. Recent trends have been toward decentralization of such large liquid producing facilities towards customer on-site plants. This type of system better serves the smaller customers, in that the cost of liquefication and distribution is eliminated. The power requirement for a modern large plants optimized for oxygen enrichment applications is expected to be in the range of 200-250 Kwh/ton of gaseous oxygen without product compression. To produce liquid oxygen about 700-800 Kwh/ton is required. Pressure Swing Adsorption (PSA) oxygen generation systems have been primarily employed on smaller applications such as waste water treatment and steel mini mills. These units have been in the 1-40 TPD size range with oxygen purities in the 80-95% range. The adsorption separation process is usually an ambient temperature cycle driven by the selective adsorption properties of a molecular sieve subjected to a pressure change. Most commercial systems use two to three adsorbent beds which go through cyclic steps of adsorption, desorption and purging. A single bed system is also available |