Concept for a Lignite-Fired Power Plant Based on the Optimised Oxyfuel Process with CO2 Recovery

Lignite-fired Oxyfuel Process with CO2 Separation

Concept for a Lignite-fired Power Plant Based on the Optimised Oxyfuel Process with CO2 Recovery

Kurzfassung cate trading for CO2, will soon be forcing any flue gas stream with up to 80 % CO2 concen- power plant operating company to take mea- tration, depending on the carbon content of sures in reducing its CO emissions. the fuel. Ein Braunkohlekraftwerkskonzept 2 nach dem optimierten Thereby, the dominating role of coal-based In order to limit the flame temperature during Oxyfuel-Prozess power generation on the German energy mar- combustion with pure oxygen, extensive flue mit CO2-Abscheidung ket cannot be neglected since a considerable gas recirculation into the combustion zone is number of modern and efficient coal-fired necessary. Thus, the combustion throughput power plants currently exist which will also is increased artificially, replacing air nitrogen Durch die zukünftigen gesetzlichen Regelun- gen zum Klimaschutz, insbesondere die ge- be needed in future for security of supply. by recycled flue gas which mainly consists of CO . Figure 1 shows a basic Oxyfuel plante Ausgabe und den Handel von Emis- The so called Oxyfuel process (O /CO com- 2 sionszertifikaten für CO , werden sich in ab- 2 2 process scheme with CO recovery and lig- 2 bustion process) with CO separation pro- 2 sehbarer Zeit die Betreiber von fossil befeuer- 2 nite as fuel. ten Kraftwerken gezwungen sehen, nach vides an opportunity to produce electricity wirtschaftlichen Lösungen zur Senkung ihrer from carbon-rich fuels by means of a rather The Oxyfuel process was first proposed in CO2-Emissionen zu suchen. simple process scheme and largely available 1981 [6] during the emerging discussion Allerdings darf die in Deutschland bestehende technology without emitting CO2 into the at- about CO2 as a greenhouse gas. Already at Dominanz von Braun- und Steinkohle bei der mosphere. As already proven by previous that time, experts regarded the process to as a Stromerzeugung sowie der entsprechende particularly attractive option to capture the Bestand an modernen und leistungsfähigen studies, it is possible both to retrofit existing Kraftwerken nicht vernachlässigt werden, da- coal-fired power plants and to apply the tech- CO2 produced by fossil-fuelled power plants. her sind auch CO2-Minderungsmaßnahmen für nology to new power plant designs. The product gas, consisting of almost 100 % den Primärenergieträger Kohle zu untersu- CO after appropriate drying and further In co-operation with Vattenfall Europe Gen- 2 chen. cleaning, was suggested to be used economi- eration, an Oxyfuel process layout for a Eine vielversprechende Möglichkeit hierfür cally in Enhanced Oil Recovery (EOR, injec- 920 MW lignite-fired power plant was devel- bietet der sogenannte Oxyfuel-Prozess mit tion of inert gases into depleted oil fields to CO2-Abscheidung. In Zusammenarbeit mit oped and optimised at the Dresden Universi- increase the yield). After reaching full ex- Vattenfall Europe Generation wurde am Lehr- ty of Technology. This work was mainly a stuhl für Kraftwerkstechnik der TU Dresden ein ploitation of such an oil field, it can be sealed part of an overall study comparing several entsprechendes Kraftwerkskonzept auf der and the CO2 is enclosed safely. There are also Basis von Rohbraunkohle erarbeitet und erste methods for lignite-based electricity genera- other ways of storing CO2 in a environmen- Optimierungen durchgeführt. tion with significantly decreased CO emis- 2 tally friendly way, for example Enhanced sions into the atmosphere. In dem vorliegendem Beitrag werden das erar- Coal Bed Methane Recovery (driving out beitete Konzept mit wesentlichen Optimie- rungsschritten sowie die Hauptergebnisse der In this article the optimised Oxyfuel plant methane from coal seams), storing it in bisherigen Untersuchungen vorgestellt. Hier- scheme with CO2 separation is presented to- aquifers, exploited salt deposit caverns or bei wird auch ein Ausblick auf zukünftige Ent- gether with selected optimisation approaches. similar geologic formations. wicklungsschwerpunkte gegeben. The main plant components and important One advantage of the Oxyfuel process is the design issues are described and an outlook is relatively simple plant layout using only a given to their future development. Introduction few additional components coupled with the ease of almost complete CO2 recovery. Be- Oxyfuel Process with CO2 Recovery sides CO2, the flue gas virtually consists of Near-future regulations for the German ener- Basic Description water vapour only, most part of which can be gy sector, as a key point introducing certifi- extracted by condensation. The Oxyfuel-Process, sometimes also re- It is principally possible to retrofit conven- AutoorsAuthors ferred to as O2/CO2 combustion, is charac- tional pulverised coal-fired steam generators terised by pure oxygen feeding instead of air with the Oxyfuel scheme. This, in fact, was Dipl.-Ing. S. Hellfritsch into the combustion chamber. Since air nitro- also the motivation for most of the theoretical Chair of Power Plant Technology, gen is excluded, it is aimed at producing a and experimental work performed so far in Dresden University of Technology, Dresden/Germany.

Professor Dr. techn. P. G. Gilli Flue gas H O,SO , Product Air seperation 2 X Chair of Power Plant Technology recirculation residual gases stream Dresden University of Technology unit Dresden/Germany. CO2 Oxygen Dipl.-Ing. N. Jentsch Steam Water and Project Manager Lignite residual gases Compression Dept. Technology / Basic Development generator 50 - 75 % removal Vattenfall Europe Generation, CO2 Berlin/Germany. Figure 1. Principle of a lignite-fired Oxyfuel process.

76 VGB PowerTech 8/2004 Lignite-fired Oxyfuel Process with CO2 Separation this area, whereas the potential for the devel- heat recovery by exhaust vapour compression — incoming air at 5 to 6 bar and about opment of entirely new Oxyfuel plant con- (WTA) is the currently most suitable solution 20 °C, cepts was discovered and evaluated later. for that problem, since the technology was — outgoing oxygen at 1.5 bar and 10 to already promoted to high maturity especially 15 °C. Most of these feasibility studies show by the German RWE. promising results for the Oxyfuel process in These figures are valid for oxygen produc- comparison to other technologies for CO2- An exhaust vapour compression to 4.2 bar is tion with a purity of 99.6 %. Requirement for free power production, especially in combi- carried out in three steps, whilst the corre- higher purities will affect the maximum sys- nation with carbon-rich fuels like coal. sponding condensate with a temperature of tem pressure (incoming air) and the invest- still 140 °C after the actual drying process is ment costs, especially for the distilling col- further utilised for raw coal preheating to ap- umn. Lignite-fired Oxyfuel Power Plant proximately 65 °C. The final water content of Concept with CO2 Separation the coal is 12 % by mass. The air entering into the cold box has to be free of particles and water vapour in order to In the proposed 920 MW Oxyfuel power prevent fouling or freezing. Therefore, the air At the Chair of Power Plant Engineering at plant concept, provision is made for two lig- separation unit uses two molecular sieves, the Dresden University of Technology, a nite drying units in parallel to ensure suffi- only one of which is in use at a time while novel innovative Oxyfuel concept with CO 2 cient availability and part load capability. It the other one is being regenerated. separation for a 920 MW lignite-fired power is further assumed that pre-pulverisation and plant was developed and simulated. The fuel particle break-up during the drying process A highly optimised, multi-stage turbo com- used in this model is dried Central-German result in particle sizes sufficiently small for pressor featuring a complex cooling system lignite (LHV raw: 11.7 MJ/kg; dried to 12 % pulverised coal combustion. This means that that naturally requires a large cooling water water content: 23.0 MJ/kg) and it is assumed no additional coal pulverisers are assumed flow but therefore provides minimum power that the produced CO2 has to be delivered for after the drying process. consumption usually carries out air compression. transportation at a supercritical pressure of Air Separation Unit 100 bar at ambient temperature. However, this is not the main objective for Cryogenic air separation units can be consi- By successively improving the internal waste dered very mature and are most cost-effec- successful integration into the power plant heat utilisation of the power process, an opti- tive for the production of large quantities of process. For the process in case, the air com- mum integration of all plant components oxygen from air. A further advantage can be pression for the air separation unit is carried could be obtained. Figure 2 shows the seen in the fact that the by-products, nitrogen out in two adiabatic compression stages with overall plant scheme with the main parame- and argon, can be obtained with a high purity intermediate cooling, so the waste heat can ters indicated. The most important plant com- at only low additional costs. be recovered in a temperature range suitable ponents will be explained in the following. for feedwater preheating which finally helps The principle of a cryogenic air separation is to save steam extracted from the turbine. Lignite Drying Unit low-temperature distillation. Main part of the Of course, the manufacturer has to agree to In general, a higher flue gas water content in- air separation unit is a so-called ‘Cold Box’, containing a highly optimised low-tempera- such an unusual objective and re-defining the creases the necessary efforts for cleaning ture heat exchanger, a distilling column with air compression as a substantial part of his CO and also increases the low-temperature 2 high- and low-pressure section and often a product, since air separation units are usually heat released during flue gas condensation. small expansion turbine. delivered as standardised packages only. Especially when using high-moisture coal in the Oxyfuel process, as in the present case, it Seen as a black box, which has to be con- To provide the amount of oxygen required is therefore advantageous to use an upstream structed and optimised as a whole by the for a lignite-fired Oxyfuel power plant with fuel drying unit. The atmospheric fluidised manufacturer, the following parameters can 920 MW gross power output, as it is referred bed lignite drying process with internal waste be assumed: to in this article, three of the currently largest

° Air CO2 98.2 % (100 bar; 25 C) 135 °C 62,5 °C HP pump 86 % CO2 96 % CO2 CO compression 3x Flue gas Particle filter 2 10 % H2O 2 % H2O Compressor cooler 1 160 °C 62 °C FGD 30 °C 5.75 bar TEG 74 % CO 2 Flue gas Air compression 2x 22 % H O Flue gas 2 cooler 2 condensation unit 30 bar 80 bar ° Cold box 200 C 160 °C

Gross power output: 920 MW Nitrogen, Oxygen Recirculation Net power output: 663 MW argon preheater preheater Fuel heat input: 1633 MW (raw lignite)

300 °C Live steam: 2313 t/h 340 °C 320 °C Live steam parameters: 600 °C/290 bar IP steam parameters: 620 °C/58 bar Raw lignite (w = 51 %) condenser pressure: 0.040 bar

Oxygen demand: 505 t/h fluidized bed Dry lignite mass flow: 279 t/h lignite drying CO separated: 552 t/h unit Oxygen 99,6 % 2

310 °C CO2 separation rate: 98 % Steam generator Plant net efficiency: 40.6 % (including air separation Feedwater tank and CO compression) Dry lignite (w = 12 %) 2

Figure 2. Process scheme of the optimised lignite-fired Oxyfuel power plant with CO2 separation.

VGB PowerTech 8/2004 77 Lignite-fired Oxyfuel Process with CO2 Separation air separation units in the world would have flame temperature is not to exceed 1800 °C, fired steam generator with dry ash removal to be operated in parallel. However, this does about 75 % of the flue gas flow at the steam (dry bottom) are necessary e.g. to prevent not necessarily mean a big and unacceptable generator exit have to be recycled to the fur- slagging. rise in operation costs compared to a conven- nace. One way to eliminate the recirculation, or at tional power plant. In fact, such on-site air However, is not correct to assume that the least to greatly reduce it, is a circulating separation units are very reliable and can be Oxyfuel process requires the feed of an enor- fluidised bed combustion system, which en- operated with marginal costs for service and mous additional gas volume flow to the fur- ables to introduce a comparatively large personnel especially at large scale. nace in comparison to conventional boilers. number of heat exchangers in the combustion An important point to be considered is the Instead, flue gas recirculation is merely a chamber (wing walls) or in the primary cycle part load capability of the cryogenic air sepa- matter of replacing the missing nitrogen from (fluid bed heat exchangers). Thus, more heat ration process, which is about 60 % for a air during the combustion process. This com- can be transferred without a temperature rise parison makes it obvious that the complex air standard package where the compressor is when reducing recirculation. feed system can be omitted and is replaced the limiting factor. Besides the overall power by approximately the same amount of recir- As circulating fluidised bed combustion al- plant availability, this would be another rea- culation ducts. Furthermore, the flue gas ready is considered a suitable alternative for son for designing an Oxyfuel power plant quantity after branching off the recirculation conventional power plants (France, Poland, with more than only one air separation unit. is significantly smaller than for conventional United States, Canada), there is a chance that Steam Generator combustion with air, as shown in Figure 3. it has advantages in comparison to pulverised coal-fired combustion in the case of O /CO Design and engineering of the steam genera- Another characteristic of a coal-fired Oxyfuel 2 2 combustion, also as the maximum size of tor are the most important steps in the devel- process is the inherent NOx reduction mecha- fluidised bed combustion units has been con- opment of a coal-fired Oxyfuel power plant. nism [4] which gives reason for the assump- siderably increased in recent years. A state-of-the-art lignite-fired power plant tion that no secondary measures for NOx re- has a pulverised coal fired single pass steam duction need to be taken (decrease in furnace Another possibility of eliminating or at least generator, which was also assumed for the height seems possible). Also the coal pul- reducing recirculation is the pulverised coal verisers and the corresponding flue gas ducts definition of the Oxyfuel plant concept. combustion with slag tap furnace, as here will not be necessary, if dried lignite is to be Other options are fluidised bed combustion higher flue gas temperatures can and must be used as fuel. with low combustion temperatures and the used. It usually exhibits higher NOx emis- possibility to remove more heat from the The flue gas recirculation as an essential com- sions but this is probably a smaller disadvan- combustion chamber, or a slag tap furnace ponent of an Oxyfuel boiler should consist of tage in the case of O2/CO2 combustion. multiple independent paths (e.g. 4 ϫ 25 %) for for which the recycled flue gas flow could be Boilers with slag tap furnace permit to size security and availability reasons. Besides, the greatly reduced. But whatever type of com- the steam generator smaller due to the higher dried lignite should be conveyed pneumatical- bustion, significant modifications compared temperature differences between flue gas and to conventional boilers have to be taken into ly to the burners by a part flow of the recycled flue gas. water/steam. So this alternative, which was account. in former times mainly used for hard coal, Most relevant will be the tightness of the The major part of the required oxygen can should be further investigated (e.g. concer- boiler, which is to prevent air infiltration and be mixed into the recirculation before enter- ning the increased slagging and fouling in the ing the furnace (about 30 % vol.), while a thereby dilution of the flue gas with nitrogen. convective heating surfaces which has to be small amount should be injected separately That consideration does not only apply to expected). at the burner ports in order to maintain flame the boiler, but also to the preliminary firing stability [3]. According to the authors´ information, lignite system (coal feed, burners) and all process combustion in slag tap boilers was not suc- Alternative Steam Generator Types stages in the flue gas path. cessful in the past because of too low com- Another key point of an Oxyfuel boiler is the The above-mentioned high quantities of flue bustion temperatures, therefore dried lignite flue gas recirculation system. If the adiabatic gas recirculation in case of a pulverised coal- had to be used in order to keep up the tem-

200 41.0 % 2.00 Central German lignite 180 40.8 % 1.75 excess oxygen for combustion 15 % 160 40.6 %

1.50 other (O2 Ar) 140 40.4 % N2 H O 120 40.2 % 1.25 2 CO2 100 40.0 %

1.00 80 39.8 %

preheating in MW 39.6 % 0.75 60 Net plant efficiency 40 39.4 %

0.50 Feedwater, oxygen- and recirculation 20 39.2 %

Flue gas composition in kg/kWh 0.25 0 39.0 % Oxygen: 150 °C Oxygen: 300 °C Oxygen: 300 °C Oxygen: 300 °C recirculation: recirculation: recirculation: recirculation: 0.00 170 °C 170 °C 245 °C 320 °C Combustion in air Oxyfuel Oxyfuel raw lignite raw lignite dry lignite (12 % water) Feedwater preheating Oxygen- und recirculation preheating Net plant efficiency

Figure 3. Quantity and composition of flue gas from different coal Figure 4. Efficiency enhancement by preheating oxygen and combustion concepts. recycled flue gases.

78 VGB PowerTech 8/2004 Lignite-fired Oxyfuel Process with CO2 Separation peratures. However, this disadvantage does CO2 Compression and 580 °C belongs to the 1000 MW BoA unit in not apply to O2/CO2 combustion. Final Removal of Water the Niederaußem power plant of the German RWE which was commissioned in 2002. Low-temperature Flue Gas After desulphurisation the CO2 is com- Economiser and Flue Gas pressed to 30 bar in three stages and dried by However, taking into consideration the rapid Condensation Unit means of an absorbent (usually a glycol, for material development during the last decade example TEG [2]). The drying process is fol- which promises even better steels in the com- The waste heat from flue gas cooling and lowed by a further compression stage to a ing years, it is by far not unrealistic to anti- condensation is recovered for feedwater pre- pressure of 80 bar. By cooling the supercriti- cipate live steam pressures of more than heating in economisers (above the dew point) cal CO its properties change into a liquid- 300 bar and temperatures of about 600 °C for and a flue gas condensation unit. 2 like state of high density. Gaseous impurities the near future. There are two preheaters installed, the larger (oxygen, argon) remain dissolved in the For modelling of the Oxyfuel plant in case, of which will reduce the flue gas temperature product stream and a high-pressure pump steam parameters of 290 bar/600 °C (live from about 200 °C to 150 °C prior to entering provides the required transportation pressure, steam) and 58 bar/620 °C (intermediate pres- the electrostatic precipitator, that means also which in this particular case is estimated to sure steam) were chosen, while the condenser before branching off the flue gas recircula- be 100 bar. pressure was set to 40 mbar assuming the use tion. of an optimised cooling tower. A smaller flue gas-heated economiser is lo- Optimisation of Energetics and Preheaters in Flue Gas Recirculati- cated in the flue gas flow after branching off Plant Installation Engineering on and Oxygen Feed Stream the recirculation; that cools the flue gases down to approximately 80°C. Most studies on Oxyfuel-based power plant Previous studies concerning lignite-fired concepts published so far definitely assume The subsequent flue gas condensation unit Oxyfuel power plants with CO2 separation, that the oxygen fed into the combustion has to be designed acid-proof and might be for example [2], do not consider all possible process is not to be preheated, obviously in compared with heat shifting systems of mod- measures for integral process optimisation. expectation of an increased flame tempera- ern conventional power plants. During the In the following it will be outlined how a net ture and thus the need for even more flue gas wet process of flue gas condensation, also the efficiency of 40.6 % for the plant concept recirculation. Further, the flue gas recycle bulk of SO and fine particles that passed the x proposed here could be obtained, whereas the stream is always of a comparably low tem- electrostatic precipitator are dissolved in the maximum net efficiency for comparable opti- perature that results from branching at a posi- condensate. tion in the flue gas path where prior particle mised lignite-fired Oxyfuel processes pub- removal was possible (usually below 200 °C, Flue Gas Desulphurisation Plant lished so far is about 34%. in the following referred to as cold recircula- (FGD) Use of Lignite Drying Unit tion). During the phase of concept definition for the Fuel drying is not only an appropriate instru- To our knowledge, only one study did so far Oxyfuel power plant it was assumed that SO x ment for increasing the efficiency of any con- consider a hot recirculation for a coal Oxy- emissions present in the flue gas are not rec- ventional power plant process, but has sever- fuel process at a temperature of about 340 °C ommended for sequestration together with al other advantages for the Oxyfuel process: [2]. However, no further comment was given the CO even independent from the type of 2 Due to the flue gas condensation, which is on realisation options of such a scheme, es- storage that is planned. Thus, a flue gas necessary for subsequent CO treatment, in pecially concerning thermal impact on the desulphurization unit has to be included 2 case of firing undried lignite a very large large recycle fans required. Nevertheless, in which is located downstream of flue gas con- amount of low-temperature heat would be set another publication [4] the definite advan- densation. free of which only a small fraction could be tages of a hot recirculation (‘heat recircula- The flue gases can e.g. be desulphurised in a utilised in an internal heat recovery process. tion’) are explained, which are improved conventional limestone scrubbing process. It A lignite drying unit, ideally with internal flame stability and emission behaviour. With- should be noted that due to the lower flue gas waste heat recovery by exhaust vapour com- out any doubt, some efforts will be necessary flow rate (after branching of the flue gas re- pression, diminishes the water content in the to remove most of the ash at about 350 °C in circulation) the concentration of SOx is high- flue gas in a very efficient way. As a result, order to protect the recirculation fans. er in the raw gas since the nominal mass flow the low-temperature heat released during flue Although these practical disadvantages re- of SOx originates from the coal. Hence, a bet- gas condensation can be utilised to a much sulting from a hot recirculation scheme do ter desulphurisation rate can be expected and higher percentage and thus energy losses are not apply in the same way for hot oxygen at the same time the FGD plant can become decreased, meaning also less required cool- feed to the boiler, safety concerns and the smaller. The electric power consumption of ing capacity. In this way the heat from flue earlier mentioned expectation of even higher the FGD unit was estimated 7.0 MW which gas vapour condensation can almost entirely flame temperatures will have been the rea- can be considered a rather conservative as- be recovered for internal use. sons for never considering oxygen preheat- sumption. Of course there still exists an underlying op- ing. Thus, in previous studies it seemed logi- However, also other desulphurisation options timisation problem, especially concerning the cal to feed cold O2 and cold recycled flue could be applicable that are possibly more optimum resulting water content of the lig- gases to the boiler, since obviously also the amount of recycled flue gas can be reduced suitable for an Oxyfuel concept with CO2 nite after drying. For the current studies, lig- separation. These could be for example the nite dried to 12 % water content by mass is thanks to the lower flame temperature. Wellmann-Lord process or even the separa- used, a moisture which can be obtained by In contrast to these assumptions it can be tion of SOx together with other gaseous im- existing fluidised bed drying technology. shown by both thermodynamics as well as purities in a distilling column. The two op- process simulations that the large amounts of State-of-the-art Steam Parameters tions mentioned are to be further investigated waste heat, mainly originating from flue gas at the chair of power plant technology at the The currently most up-to-date steam genera- cooling, are best utilised for boiler feed gas Dresden University of Technology. tor with live steam parameters of 275 bar/ preheating.

VGB PowerTech 8/2004 79 Lignite-fired Oxyfuel Process with CO2 Separation

For the plant concept proposed in this study, input to the boiler does not essentially affect suming cryogenic air separation unit, should Figure 4 shows the influence of different- the flue gas recycle flow required in order to definitely be carried out in a leak-proof heat ly biasing boiler feed gas preheating within keep the flame temperature constant. exchanger to eliminate losses. In contrast, the the internal heat recovery scheme on the net recirculation preheater could be constructed Based on these arguments, the Oxyfuel plant plant efficiency. For all cases shown there, concept proposed in this study comprises similarly to the well-known design of rotat- the flue gas temperature before entering the both an oxygen preheater (oxygen outlet ing air preheaters at conventional boilers flue gas condensation unit was the same. Ac- temperature 300 °C) and a 320 °C flue gas (Ljungström type). cording to the results, a boiler feed gas tem- recycle feed to the boiler. To avoid the prac- Efficient Waste Heat Recovery perature increased by 150 K, which in conse- tical disadvantages of a hot recirculation Scheme for Feedwater Preheating quence also leads to a higher steam extrac- scheme, the solution chosen here combines tion from the turbine for feed water preheat- the benefits of both hot and cold recircula- Waste heat recovery plays a dominant role ing, will raise the net plant efficiency by tion. This is achieved by the introduction for achieving an acceptable net efficiency of about 1.3 %. It is obvious but shall be men- of an additional counter current flow heat ex- the Oxyfuel power plant concept, since the tioned here that the use of air preheaters at changer which is called recirculation pre- electric own consumption is noticeably high- conventional power plants has a similar ef- heater. It is arranged in parallel to the oxygen er than that of a conventional power plant of fect and thus the same purpose. preheater, that means subsequent to the the same nominal size due to air separation The more flue gas waste heat is transferred economiser in the flue gas stream at a posi- unit and CO2 compression. On the other back to the boiler feed gases, the more ther- tion where one would find the air preheaters hand, with the Oxyfuel process a greater va- mal input is available for high-grade steam of a conventional boiler. riety of waste heat sources exist which can be beneficial if managed adequately. production. Thereby, at a constant fuel feed, Figure 5 shows the two main recirculation more live steam will be produced which has a schemes – hot and cold – in comparison to This means especially the compressor inter- large enthalpy gradient for power production the alternative using a recirculation pre- cooling of the air separation unit and of the in the turbine. heater. The recirculation preheater provides CO2 compression, the flue gas cooling and fi- the thermodynamic benefits of hot recircula- In the other case of using the flue gas waste nally the condensation of flue gas water tion which are explained above, while it is heat (below approximately 350 °C) for feed- vapour. The latter is almost obligatory for an still possible to use flue gas recycle fans in water preheating, the effect will be a de- Oxyfuel process with CO2 separation, since it the temperature range below 200 °C and not creased demand for extraction steam from provides the easiest measure to remove the to incorporate a separate hot gas particle re- the turbine. However, extraction steam has bulk of the water content in the CO2. moval. Instead, a single electrostatic precipi- already released most of its enthalpy and ca- tator is located in the flue gas stream at a The basic objective when designing the pability of power production (exergy). In ad- suitable temperature of about 160 °C prior to waste heat recovery scheme for the Oxyfuel dition, more steam now reaches the turbine branching off the recycle flow and warming plant concept was to combine all heat sources outlet and will have to be condensed, which it up again in the recirculation preheater. in a way that provides maximum waste heat increases the overall cooling losses of the utilisation and thus minimum cooling losses plant. Another argument for this special recircula- of the plant. Nevertheless, a second intention tion scheme is the fact that the heat exchang- was not to increase the complexity and num- For this reason the internal waste heat recov- ing flows – flue gas against oxygen and ber of components too far, such as branching ery scheme of the Oxyfuel plant should pri- recycle gas – are situated fairly close to each pipes or flue gas heated feed water pre- marily be directed to increasing the tempera- other anyway. ture of the boiler feed gases (oxygen and re- heaters. That means that system integration cycled flue gases). Furthermore, it could be Warming up of oxygen, which had to be pro- and cost reduction were significant factors proven by simulation that an increased heat duced before in the extensively power con- during process optimisation.

170 °C 170 °C 170 °C

Recirculation Recirculation fan fan Particle filter Particle filter Particle filter

Flue gas economiser Flue gas economiser Recirculation preheater

350 °CHot gas cyclone 350 °C 350 °C

Recirculation fan Steam generator Steam generator Steam generator

350 °C 170 °C 320 °C

Hot recirculation mode Cold recirculation mode With recirculation preheater

Figure 5. Comparison of different flue gas recirculation schemes.

80 VGB PowerTech 8/2004 Lignite-fired Oxyfuel Process with CO2 Separation

largest possible warming-up temperature compressor stage inlet temperature. 29 °C Condensate flow 100 % span. It was defined that compressor waste heat Flue gas condensation unit The last stage of feedwater preheating for the shall only be used for feedwater preheating, two of the stream portions with the lower since oxygen and recirculation preheating is 62.5 °C 13 % 62 % 25 % temperature consists of another flue gas-heat- already entirely performed using heat from ed heat exchanger located prior to the elec- flue gas cooling in the upper temperature trostatic precipitator. range. Another condition was that waste heat utilisation from the several compressors is to Compressor cooling Introducing the feedwater preheating scheme air separation unit be installed after the flue gas condensation

compressors explained above made it possible to utilise a 2 unit, which builds the basis of the feedwater large percentage of the waste heat generated

LT flue gas economiser LT preheating scheme with the lowest tempera- by the plant components, such as air separa- ture level. 135 °C tion, flue gas cooling and condensing as well Cooling of CO Flue gas economiser as CO2 compression. Further, it can be seen Following these conditions, it had to be de- from Figure 6 that obviously no low-pressure cided in which upper temperature range 159 °C feedwater preheaters are necessary up to a the compressor cooling should be installed. Feedwater vessel feedwater temperature of about 160°C. However, it turned out that the flue gas temperature is high enough to build the last stage ° Compressor Cooling Optimised 187 C to feedwater pump 100 % of feedwater preheating and hence the com- for Waste Heat Recovery Figure 6. Feedwater preheating by internal pressor cooling can provide the second stage waste heat recovery. The total electric power consumption of the (Figure 6). Anyway it is better to provide compressors of the air separation unit and low-temperature cooling media (correspond- Figure 6 shows the optimised feed water CO2 compression is about 200 MW, which is ing to the feedwater temperature in this case) preheating scheme. As can be seen from the more than one fifth of the gross power output to the compressors since this enables a small- flow diagram, the total feedwater stream is in of the plant and which is to be considered an er specific volume of the compression gas a first stage led through the flue gas conden- exergetic loss at first view. Hence, a primary resulting also in lower power consumption. sation unit for preheating. The heat released objective during concept definition and In this way the basics of the feedwater pre- during flue gas condensation is available ‘for process optimisation was to take as much ad- heating scheme (Figure 6) were fixed. free’ but can, at atmospheric pressure, only vantage as possible from the waste heat of The next question was how many stages be utilised in a low temperature range, which the compressors in order to best compensate (with intermediate cooling) the air compres- has a strict upper limit. for the losses. sion in the air separation unit and the CO2 After that first stage of feedwater preheating, Temperature range and quantity of the heat compression should comprise. Simple calcu- the feedwater stream is divided into three recovered depend on a number of factors. lations show that the power consumption of portions which are used for cooling the com- First of all there is the compressor outlet tem- compression and the compressor outlet tem- pressors of the air separation unit, of the CO2 perature before intermediate cooling, which peratures decline with an increasing number compression and in a smaller flue gas cooler for a constant overall pressure ratio depends of intercooled stages. However, it can also be (non-condensing) which is located behind the on the number of compression steps and the demonstrated that for a constant coolant flow electrostatic precipitator in the flue gas stage inlet temperature, which has a strong (for all intermediate coolers together) with a stream. This configuration is chosen accord- influence on the power consumption of the constant coolant inlet temperature the amount ing to the finding that compressor cooling compressor. Another important factor is the of recovered waste heat as a percentage of should be carried out with a minimum cool- amount of cooling media and its inlet temper- the compressor power consumption will de- ing medium inlet temperature but with ature, which, however, can also affect the crease (Figure 7). In addition, the temper-

250 70.0 %

C 200 69.0 % ° 70.0 % 40.64 %

150 68.0 % 68.0 % 40.63 %

100 67.0 % 66.0 % 40.62 % power consumption

50 66.0 % 40.61 % compressor outlet temperature in 64.0 % Heat recovery rate in % of compressor heat recovery rate net plant efficiency Compressor power consumption, waste heat in MW 0 65.0 % 62.0 % 40.60 % Single stage Two Three heat recovery rate of compressors compression stages stages heat recovery rate of compressors and flue gas condensation unit Compressor power Heat recovery rate net plant efficiency consumption Compressor stage 60.0 % 40.59 % Heat recovered outlet temp. 62 °C 62.5 °C63°C

Figure 7. Variation of the number of compression stages Figure 8. Variation of feedwater temperature after preheating (air separation unit). in flue gas condensation unit.

VGB PowerTech 8/2004 81 Lignite-fired Oxyfuel Process with CO2 Separation

ature range for heat recovery becomes small- A last optimisation approach was to decrease in figure 8 show the correct trend concerning er and thus the minimum required coolant the cooling media inlet temperature to the this design issue. flow increases. compressor coolers, which primarily aimed at reducing the power consumption. It should Summary These findings would suggest a one-step adi- be identified whether it is worth to loose abatic compression for maximum heat recov- some heat from flue gas condensation by set- ery in relation to the power consumption. On ting a lower feedwater outlet temperature This article presented an optimised lignite- the other hand, it can be seen from Figure 7 from the flue gas condensation unit, which fired power plant concept that is based on that from a certain point on the compressor means a lower cooling media inlet tempera- the Oxyfuel process with CO2 separation. stage outlet temperature is above 200 °C, ture to the compressor coolers and thus de- State-of-the-art technique was used for eval- which in practice leads to a higher electric creased power consumption. uating and optimising the concept, yet a net power consumption and a significant drop of However, simulations with slightly changed plant efficiency of 40.6 % could be achieved the isentropic compressor efficiency, even feed water exit temperatures from the flue at an estimated separation rate of 98 % for below the value of 85 % used for the simula- gas condensation unit have shown that no im- CO2. Process modelling included all plant tion. Besides, there also might be some re- provement can be expected from that step components, providing the CO2 at a pressure strictions by the manufacturers for the differ- concerning the net efficiency of the plant. of 100 bar ready for transportation. Some ent compressor types, so it was decided not optimisation examples and their results are Figure 8 indicates that with reducing the to exceed the 200 °C limit for the process presented. evaluation. cooling media inlet temperature, the heat recovery rate of the compressors alone im- The results of the study together with inter- As a result, the air separation unit for the proves, but at the same time less heat is re- nally conducted cost estimations prove that process in case uses a two-step compression covered in the flue gas condensation unit, the Oxyfuel process can be a promising op- with an overall pressure ration of 5.7. CO2 which has an even stronger influence on the tion for future ‘CO2-free’ power generation compression from 1 to 30 bar takes place in net plant efficiency. It was not possible to from coal. This is based on the assumption three stages. vary the feed water temperature in a wider that the price of electricity under market con- range, since water vapor started to condense It shall be mentioned that the cooling water ditions incorporating emission certificate in the intercoolers of the CO compressors for the compressors is not the feedwater it- 2 trading will be about 50 to 60 % above the which caused numerical problems in the self, but is provided by an intermediate cool- current level. simulation software. Nevertheless, the reults ing circuit. References

Verminderung der NEU [1] Hellfritsch, S.: Energie- und Massenbilanz für VERMINDERUNG DER Belagsbildung in Kraftwerken ein emissionsarmes Kraftwerk nach dem Oxy- BELAGS- UND fuel-Prozess. Diploma thesis No. 1637 (Chair KORROSIONSBILDUNG Brennstoffauswahl – Monitoring – IN KRAFTWERKEN Konstruktive Maßnahmen – Reinigung of Power Plant Technology, TU Dresden), Brennstoffauswahl – Monitoring – Konstruktive Maßnahmen – 2003. Reinigungssysteme 2004. 380 Seiten m. zahlr. Abb. u. Tab. E ISBN 3-934736-13-0. Kartoniert 38,- [2] Andersson, K., Johnsson, F., and Strömberg, Das Buch gibt anwendungsorientiert L.: Large Scale CO Capture - Applying the einen umfassenden Überblick über die 2 Concept of O /CO Combustion to Commer- verschiedenen Problemstellungen und 2 2 deren Beseitigung. cial Process Data. VGB PowerTech 10/2003, pp. 29 - 33. Instandsetzung von NEU Wärmeübertragern [3] Nozaki, T. et al: Analysis of the flame formed Schadensursachen – Schadenser- during oxidation of pulverised coal by an O2- kennung – Behebung – Vorbeugung CO2 mixture. Energy, Vol. 22, pp. 199 - 205, 2003. 398 Seiten m. zahlr. Abb. u. Tab. 1997. ISBN 3-934736-09-2. Kartoniert E 38,- Wärmeübertrager-Reinigungssysteme [4] Liu, H., and Okazaki, K.: Simultaneous easy PUBLICO PP Publications 2001. 498 Seiten m. zahlr. Abb. u. Tab. CO2 recovery and drastic reduction of SOx ISBN 3-934736-02-5. Kartoniert E 35,- and NOx in O2/CO2 coal combustion with heat recirculation. Fuel, Vol. 82, pp. 1427 - Fax-order senden an 0049(0)201 / 7988278 1436, 2003. Ja, wir bestellen Verminderung d. Belagsbildung [5] Douglas, M.A. et al: Oxy-Combustion Field (E 38,- / Expl. + Versand) Demonstration Project. Second Annual Con- Exemplar(e) Instandsetzung von WAT ference on Carbon Sequestration, Alexandria E (VA), May 5 - 8 2003. kostenloser Prospekt ( 38,- / Expl. + Versand) WAT-Reinigungssysteme [6] Horn, F.L., and Steinberg, M.: A Carbon Dio- (E 35,- / Expl. + Versand) xide Power Plant for Total Emission Control and Enhanced Oil Recovery, D.E.E. Brookha- ven National Laboratory, 1981. ᮀ Firma ALLO®-Pressure controllers/-Control valves Name/Vorname ALLOtherm®-Desuperheaters ALLOmatic®-Transmitters ® DIN EN ISO 9001 Adresse PLZ/Ort ALLO -Service after sale DGRL 97/23/EG new ALLO Mess- und Regeltechnik GmbH Wiesenstr. 51, Halle 38 · 40549 Düsseldorf · Fon +49-211-58 33 510 Ort/Datum Unterschrift address Fax +49-211-58 33 5111 · e-mail: [email protected] · www.allo.de

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