HRSG Simulation-Questions & Answers

What is HRSG Simulation?
HRSG simulation is a method of predicting the design or off-design performance of Water tube,natural or forced circulation HRSGs or Waste Heat Boilers. Without designing the HRSG in terms of tube size,fin configuration,surface areas etc, we can obtain a lot of valuable performance data about HRSGs. Detailed theory can be found in my book "Waste Heat Boiler Deskbook" or papers Simulation is more applicable to gas turbine HRSGs as there are several operating conditions possible;the configuration can be complex and there could be multiple levels of steam pressure.(unlike in a say packaged steam generator). However,simulation may also be applied to other waste heat recovery applications involving clean gas streams and convective type of boilers. Note that we do not compute surface areas or tube sizes,geometry etc in simulation.

How is simulation done? The performance of a heat transfer surface is arrived at by solving three sets of equations: a. gas side energy balance b.steam/water side energy balance c.duty transferred = USDT where U=overall heat transfer coefficient,S=surface area and DT=log-mean temperature difference. While designing the HRSG physically,we estimate U and S separately,while in simulation,the term (US) is used as a single entity. This is then corrected for the effects of gas flow,temperature,gas analysis while performing off-design analysis. Thus actual surface areas,tube sizes,goemetry etc are not necessary in simulation. Once all the three equations are solved (the NTU method is also used to arrive at the duty),the energy absorbed in each surface such as superheater,evaporator,economizer is determined. A complex iterative process then establishes the steam generation,gas-steam temperature profiles in off-design condition. In "design" mode,the pinch and approach points are asumed for each evaporator surface to arrive at the steam generation and gas/steam temperature profiles. There is only one "design" mode but several "off-design" modes,which is why simulation is helpful. Why Simulation? Can you tell me what the exit gas temperature and the steam generation is in a gas turbine HRSG generating steam at a given pressure and temperature? What happens if the steam pressure is higher,lower? For example,one may assume a 300 F exit gas temperature in a single pressure system,which may not be thermodynamically feasible if the steam pressure is above say 250 psig.The exit gas temperature increases as degree of superheat and pressure increase..see my papers. This is thermodynamic problem and has nothing to do with surface areas. So arbitrarily assuming a 300 F or so exit gas and doing an energy balance can lead to grossly wrong estimation of steam generation,power etc. While planning or developing cogeneration,combined cycle projects,the consultant or the plant engineer has to evaluate various gas turbines,various steam parameters and various operating pconditions and see which is optimum for the plant and what could happen to the entire system due to the HRSG behaviour at some points. Simulation saves a lot of time as some parameters and operating conditions can be easily firmed up and others eliminated; the few possible options for gas,steam conditions are then further studied. Almost all of the performance information is available at this stage and hence the specifications for the HRSG can be written more clearly. Then the HRSG supplier can be approached for quotes etc. There is no use approaching the HRSG supplier without performing such an analysis as system optimization studies,such as optimizing steam pressures or temperatures,whether multiple pressure are required etc is determined by simulation consideraing the entire plant. Most HRSG suppliers merely design the HRSG for the steam parameters consultants give them and tell how big it is and how much it costs and what the gas pressure drop is etc. What results are obtained through Simulation? Gas-steam temperature profiles,steam generation in unfired,fired modes,amount of supplementary fuel,gas analysis before,after the burner,fuel inputs with gas turbine exhaust,fresh air fan operation,ASME efficiency,whether economizer steams at certain loads,energy absorbed by each surface,whether the superheater is too small or too large etc.( by amount of spray water calculated in fired conditions).

How else can I use Simulation? a.You can select gas turbines or see what happens with a given machine compared to another..Assuming electrical power output wise,several machines are comparable,due to variations in exhaust gas parameters the HRSG performance may be different with different machines and may be more efficient with certain gas turbines (requiring say less fuel to generate a given amount of steam). Thus it is a valuable tool even for gas turbine suppliers who supply the entire cogeneration plant. b.Perform "what if" analysis in existing or new HRSGs. See what happens if feed water temperature changes,steam pressure varies,exhaust gas flow,analysis or temperature changes,fuel inputs required if fan replaces the gas turbine etc. Did you know that supplementary firing to generate additional steam in HRSGs is more efficient than generating the same amount in a say packaged boiler? The efficiency is nearly 100 % in HRSGs compared to say 90-93 % (LHV) in steam generators. Simulation gives you this information. To understand why and how,please see my books,papers . c.You can also check if the HRSG is designed properly or if meets guarantees(See my paper on applications of simulation). Often variations in gas conditions,load of gas turbine,ambient conditions,steam parameters are cited as reasons for not being able to verify HRSG performance..With Simulation,you can evaluate field data and check if the guaranteed data are valid or unreasonable..There is no need to exactly duplicate design data in operation to confirm if the HRSG is designed satisfactorily! How can I obtain the HRSG configuration I want? There are 6 basic modules-see the figure below. SH+EVAP+ECON, SH+EVAP, SH, EVAP+ECON, ECON, EVAP By combining them in any fashion and using the same module any number of times as required,complex HRSG configurations can be developed and simulated. Using the concept of common economizer and superheater,you can feed an evaporator using an economizer located several modules away..A superheater can be fed by evaporators from different locations etc..Steam can be exported or imported into superheaters.. Up to 10 modules or pressure levels can be handled. See examples shown in the figure below.(These examples are available in the HRSGS software). You can also download a FREE demo,see below,which solves a three pressure level HRSG. Can I simulate fired HRSGs? Yes,both unfired and fired. You can also study what happens when a fan is used to generate the same amount of steam. program automatically computes fuel(oil,gas) input required to generate desired quantity of steam. Can I use it for other type of boilers? In addition to gas turbine,diesel engine exhaust boilers,you can also use it for other clean gas applications such as fume incineration applications. The only limitations of the program are that the HRSG design be of convective type-most HRSGs are and that the gas stream be clean. Radiant designs can be simulated with slightly lesser accuracy.Fouling as in municipal solid waste applications cannot be simulated very accurately but one can get a feel for the performance.

Features of the program,requirements
Runs on Windows 3.1 and above. It has eight other built-in programs which can be run without exiting HRSGs. Steam properties,Deaeration steam requirements,Steam turbine expansion and power output,burner fuel calulations,dew point of acid vapors,boiler duty calculations,conversion of gas volume % to weight % and vice versa and unit conversions. These auxiliary programs aid HRSGS user to fine tune the performance. Metric and British Units

Note: Theory on pinch point,approach points,how to select them,temperature cross problems etc are addressed in my book Waste Heat Boiler Deskbook.

Problem:I am a consultant trying to arrive at the steam parameters for a gas turbine -HRSG system. I want to know how much steam I can generate at a.600 psig,750 F b.850 psig,800 F c.400 psig,600F..I also want to know the electrical power that can be generated using a steam turbine at above conditions..what is the exit gas temperature from HRSG in each of the above cases?.. solution:With HRSGS you can obtain answers to above questions in a few minutes. Obtain the design parameters at various steam conditions using say 20 F pinch and approach points. There is a program built into HRSGS to compute steam turbine power output based on inlet steam parameters and exit steam pressure. Use the results to optimize your steam parameters. Problem:I am a consultant developing a cogeneration project. I want to know how the HRSG behaves at 0 F,60 F and 100 F ambient conditions as the exhaust gas parameters are different. I also want to know how much fuel I need to generate the same amount of steam I generate at 60 F gas turbine opeation using only a forced draft fan..also the firing temperature,oxygen in exhaust after combustion. Also will my economizer steam at 70 % gas turbine load at 60 F ambient? solution: Simulate the design at 60 F and run off-design at other ambient conditions and part loads of gas turbine. Input air flow at 60 F and once the desired steam is inputted,program asks for fuel data and automatically arrives at firing temperature,fuel input,ASME efficiency to generate your desired steam -all within a few minutes..You do not have to suggest a firing temperature..Also the extent of steaming in the economizer is obtained at any off-design condition..At part loads the economizer could steam..You can increase the approach point in the design mode to solve this or try increasing blow down if steaming is within 1-2 %. Problem:I am a plant engineer. We bought a HRSG with performance guarantees based on 550,000 lb/h at 1000 F and 79,400 lb/h steam at 600 psig,700 F with 230 F feed water. The HRSG is operating at 500 psig ( due to plant requirements) and generates 68,700 lb/h at 690 F. Turbine exhaust temperature is 970 F. How can we ensure that the HRSG performance is reasonable? Solution:One way is to suggest different gas flows at 970 F and arrive at the operating steam conditions at 500 psig in the design mode.. Vary pinch/approach points till you establish these parameters (68,700 lb/h) in design mode. Then in off-design mode you input the guarantee performance gas conditions and steam pressure and see what steam flow/temperature/exit gas temperature you get..If the steam flow is very much off from guarantees or the exit gas temperature is much higher (allow for reasonable instrument errors) then you may look into this further and start asking questions..HRSGS is a tool for anyone involved in HRSGS design or operation.

Problem:I am a consultant looking at multi-pressure HRSG(600 psig,750 F and 150 psig sat steam). I want a particular quantity of HP steam and would like to maximize LP steam..How can I do this using HRSGS? solution:You can simulate different HRSG configurations. Try first HP superheater,followed by HP evaporator and HP economizer. Then LP evaporator,LP economizer. By playing with pinch/approach points establish in the design mode the desired HP steam and whatever LP steam. Then use another configuration in the design mode. HP superheater,HP evaporator followed by LP evaporator and a common economizer which feeds both HP and LP evaporators. Since the heat sink is much larger now,you will generate more LP steam with this arrangement.. Play with pinch/approach points till you get desired HP steam. You will have a much lower stack temeprature with this option.. HRSGS allows you to study different configurations. Problem:I am generating steam at 600 psig,600 F from an incinerator at 1700 F. I do not want the superheater to be first heating surface as it will be subject to high tube wall temperatures. I want to use a screen section (evaporator)followed by superheater,evaporator and economizer. How much steam do I generate and what is the exit gas temeperature? solution:Using the common superheater/economizer concepts,we can simulate this HRSG configuration easily. The HRSG has 4 modules: evaporator,superheater,evaporator nd economizer. use a high pinch point for the first evaporator say 600-700 F and a low pinch point for the next evaporator ,say 50-70 F. You get the design mode data. Then simulate the off-design at any other gas inlet conditions.. You can use HRSGS not only for gas turbine/diesel engine applications but also for other clean gas heat recovery applications. As discussed earlier,the HRSGS program uses six basic modules as shown below and complex HRSG configurations can be developed using these building blocks. The actual HRSGS program contains several examples of unfired/fired HRSGs as shown. These are not available in the Free Demo disk,which contains an example of a three pressure HRSG, which can be downloaded.