Potential of Process Energy Optimization (PEO) on a Soda Ash Production Plant

Transcription

1 54 Energy Engineering Vol. 107, No Potential of Process Energy Optimization (PEO) on a Soda Ash Production Plant T. de Lange and Prof. L.J. Grobler Energy Cybernetics Potchefstroom, South Africa ABSTRACT An energy audit was conducted on a soda ash plant in Botswana, and it was found that the performance of the plant could be improved. Further investigation revealed that the performance of the carbon dioxide (CO 2 ) production section was heavily influenced by control problems experienced at the two coal-firing boilers. The CO 2 production section consumes energy in the form of steam and electricity and utilizes a solvent mono-ethanol-amine (MEA) to extract CO 2 from boiler flue gasses. Recommendations were made to repair and re-commission the boiler control system and to install an energy optimization system. Annual savings that could be achieved by optimal control of steam from the boilers to the turbine, CO 2 plant, and processing plant (thereby controlling the interactions between these sections) amounted to BWP 2, 6 million. This constitutes 10% of the total annual coal bill. A total project life cycle of only 5 years was used, and the escalating savings due to inflation was disregarded. The IRR of the project was calculated to be 114%. This article covers the results of the energy audit. It also discusses the proposed solution and the savings potential that was identified regarding the CO 2 plant. INTRODUCTION Soda ash is a chemical used in the manufacturing of glass and detergents. It is also used in metallurgical applications and chemical

2 55 manufacturing. The soda ash plant consists of a few processes or sections. This article will only focus on the process and operation of the CO 2 production section. Description of the CO 2 Production Process CO 2 in flue gas from the two coal-fired boilers is extracted to be used in the carbonation of Sodium Carbonate (Na 2 CO 3 ). In this exothermic chemical reaction Sodium Bicarbonate (2NaHCO 3 ) is formed. Figure 1 shows a block diagram of the CO 2 plant. Flue gas from the boilers is scrubbed, using a soda ash-rich brine solution to remove the sulphur dioxide (SO 2 ) from the gas. SO 2 is removed from flue gas since it reacts with and contaminates the MEA solution used in the subsequent purification process. The scrubbed flue gas is then passed through a packed CO 2 absorption column, where it contacts the MEA solution. In Heat Exchanger 1 (HE1) the CO 2 -rich MEA solution is preheated before it is pumped to the top of the CO 2 stripping column. CO 2 is stripped by boiling the solution, using low pressure (LP) steam in the two kettle-type re-boilers. Gaseous CO 2 is compressed and pumped to the carbonation section. The main energy consumers of this section are the re-boilers that use LP steam to heat the MEA. Some electrical energy is used for pumping and various other purposes; however, this article focuses on the steam consumption of this section. Purpose of the Audit The purpose of the energy management (EM) and Process Energy Optimization (PEO ) study was to maximize production while minimizing production costs and resources. This had to be accomplished without violating process or equipment constraints. The audit revealed that plant performance could be improved and that the CO 2 production section was consuming large amounts of energy. CO 2 PRODUCTION SECTION PERFORMANCE AND OPERATION The first step in the evaluation of the CO 2 production section was the analysis of this section s performance over a period of three months.

3 56 Energy Engineering Vol. 107, No Figure 1. Block diagram of CO 2 production section

4 57 CO 2 Production Section Performance The performance of the CO 2 production section can be seen in Figure 2. Steam energy consumed at the re-boilers is shown as a function of CO 2 production. The trend line in Figure 2 indicates the average performance of the CO 2 plant. As expected, the CO 2 production increases with an increase in energy consumption. There is, however, a large scatter around the average performance line. The base load is high, indicating that, although no CO 2 is produced, the system consumes a lot of energy. It can therefore be concluded that process or energy improvement is possible. In Figure 2, Point A indicates that on one day 3,400 GJ of steam was used to produce 56,550 m 3 of CO 2. On another day only 2,350 GJ of steam energy was consumed (see Point B) to produce approximately the same amount of CO 2. This constitutes a 30% difference in steam consumption for production of the same amount of CO 2. Possible causes of the large scatter or variance in performance as well as the large base load were explored. From this it was found that the energy consumed by the CO 2 production section was a function of the following [1]: The amount of flue gas used to produce CO 2 and the percentage CO 2 it contained Percentage MEA in solution The rate at which the MEA solution was pumped through the CO 2 production section Heat exchanged at the re-boilers, depending on the amount of steam to the re-boilers Current Operation Methodology Some difficulty was experienced in controlling the coal-fired boilers and cutting back on steam production when the soda ash plant was not at full production. According to plant personnel, the CO 2 re-boilers were used as a heat sink at times when too much steam was produced. It was thought that the excess heat dumped at the re-boilers would ultimately be dissipated through the two heat exchangers (HE 1 and HE 2 in Figure 1). The excess heat would eventually heat up brine that was pumped back to the ponds, thereby improving evaporation rates. In addition to heat being dumped at the CO 2 re-boilers, another

5 58 Energy Engineering Vol. 107, No Figure 2. CO 2 production section performance

6 59 decision was made by plant personnel to operate the CO 2 plant at a lower percentage of MEA in solution. This was done to save on MEA costs. Effect of Current Operation Methodology on the CO 2 Plant When less MEA is used to extract CO 2 from flue gasses, the rate at which the rich MEA solution is pumped from the CO 2 absorption column to the CO 2 stripping column, and the rate at which the lean MEA solution is pumped from the CO 2 stripping column to the CO 2 absorption column should increase to produce the same amount of CO 2. These two rates should be equal, since both rates are controlled with the levels of the two columns. If the rates (or volume per unit of time) of pumping the solution around increases, the amount of energy used to boil the liquid increases. In Figure 3 the original MEA cost (1) and energy cost (2) lines are shown. The total cost line is line (3), and the optimum operation point is at A. With the current operating methodology the following will occur: Dumping heat will increase the energy cost; therefore, the total cost will rise (line 5). Optimum operation will now be at point B. Lower percentage MEA in solution will cause the actual operating point to move to point C. Therefore, when energy is being dumped the percentage of MEA in solution should be increased to operate at optimum operation levels (B), not decreased (C). The total cost will have increased with the shift (8) shown in Figure 3. Determining how much energy can be saved and the monetary value of the savings was the next step in our analysis. ENERGY SAVINGS OPPORTUNITY The cumulative sum of differences (CUSUM) analysis is a tool used by the audit team to monitor and track the performance of the CO 2 plant. Applying the CUSUM analysis revealed that periods of good operation and other periods of poor operation occurred, as can be seen from Figure 4.

7 60 Energy Engineering Vol. 107, No Figure 3. MEA and Energy cost vs. percentage MEA in solution

8 61 The dashed line in Figure 4 indicates periods of poor operation while the dotted line represents periods of good operation. The solid line is a period of average operation. Figure 4 shows a long period of poor operation, after which something changed and the operation improved for a few days. Operation alternated between modes of good, average, and poor throughout the measured period. Energy could have been saved if the plant were primarily operated in a mode of good operation, as had already happened some of the time. The amount of energy that could have been saved was calculated by subtracting the performance line during the good performance periods from the performance line during all periods. The savings resulting from changing average operation to good operation can be seen in Figure 5. Average performance of the CO 2 production section during all modes of operation is shown in the solid line. Figure 5 also shows the performance line during the good operation periods in the dotted line. Steam could be saved if the plant were operated at good operation levels most of the time. Savings were calculated in terms of the coal required to generate that amount of steam. Table 1 shows the coal bill for the calculated base year (BWP 25,734,143). The annual savings calculated on this base year coal cost was BWP 2,560,795. If a total project life cycle of only 5 years is used and escalating savings due to inflation is disregarded (a very conservative view), the IRR of the project is calculated to be 114% with a payback period of 1.15 years. Apart from these savings, dumping energy at the re-boilers affects the production process in various ways. These effects will be discussed in detail in the next section. EFFECTS OF DUMPING ENERGY AT THE REBOILERS Dumping of excess steam at the re-boilers directly affects the operation/performance of the CO 2 plant in the following manner: Temperature T 1 (shown in Figure 1) will increase if not all of the

9 62 Energy Engineering Vol. 107, No Figure 4. CUSUM Chart of CO 2 production section

10 Figure 5. CO 2 Production vs. Steam Energy 63

11 64 Energy Engineering Vol. 107, No Table 1. Estimated value of savings opportunity excess heat is dissipated through HE 1&2. This in turn will cause less CO 2 to be absorbed at the CO 2 absorption column and a drop in relative yield of CO 2. Therefore, it indirectly affects the downstream carbonation process. The temperature in the CO 2 stripping column will increase and a larger volume of gas will be boiled off, containing impurities. These impurities will be compressed along with the CO 2 and pumped to the carbonation section. The first step in solving these problems is the repair and re-commissioning of the boiler control system. This will enable plant operators to cut back on steam production when the need arises. Process Energy Optimization (PEO ) will enable operators to improve the operational efficiency of the CO 2 production section. The PEO solution can be deployed by installing a decision support system (DSS). The effect of a DSS on the performance scatter shown in Figure 2 will be to narrow the scatter around the average performance line. It will also enable operators to shift the performance from average performance to good operation levels. THE DECISION SUPPORT SYSTEM (DSS) In essence the DSS will firstly aim to assist the operators of the plant in anticipating the steam demand of the plant. This demand should be met in the most cost effective manner. The DSS will structure the demand as follows: Steam demand to the calcining section will be prioritized.

12 65 Secondary to the steam supply to the calcining section will be that of the turbine for generation purposes, and the supply to the MEA re-boilers for CO 2 production purposes. The remaining amount of steam will be made available to other steam users such as the MEA re-claimer and salt production section. As far as possible the existing infrastructure (i.e. sensors and equipment) will be used and the introduction of new equipment (if any) kept minimal. Some production-related problems and bottlenecks can be eliminated by the energy performance measurements and tracking system. This information must be made available to operators in real time to facilitate their meeting production targets. The system will function as a DSS; thus, human intervention will not be excluded from the control loop. This strategy will eliminate risk, and once the performance of the system has been verified the option to close the control loop can be considered. CONCLUSION The Energy Audit conducted on the soda ash plant showed that the performance of the CO 2 plant could be improved. Dumping energy as a result of limited boiler control had an adverse effect on the production process. It can be concluded that these problems are symptoms of control problems experienced at the boilers. Therefore, an integrated solution was proposed. The PEO solution aims to optimize and maintain the boilers, as well as the production section, at optimum levels. Annual energy savings for the whole project was calculated to be BWP 2, 6 million. This investment could be reclaimed in 1.15 years. References [1] Grobler, Prof. L.J., Bosman, I.E., Molefhi, B., De Lange, T. & Kaliswayo, L., Energy Audit Report, 2006.

13 66 Energy Engineering Vol. 107, No ABOUT THE AUTHORS Tanya de Lange holds a degree in chemical engineering from the North-West University, South Africa. She is an employee of the North- West University and Energy Cybernetics, where she focuses on the energy engineering field. She is a member of the Southern African Association for Energy Efficiency (SAEE). Tanya is a certified energy manager (CEM) as well as a certified measurement and verification professional (CMVP). L.J. Grobler is a professor at the School of Mechanical Engineering, North-West University, South Africa. He specializes in energy management and process energy optimization. He is certified by the Association of Energy Engineers of the USA as a certified energy manager (CEM) and as a certified measurement and verification professional (CMVP). Both can be reached at Energy Cybernetics, PostNet Suite 148, Private Bag X1277, Potchefstroom, 2520, South Africa.

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