| JP2002332805 | STEAM ENGINE, POWER SYSTEM, AND VEHICLE DRIVEN BY THE ENGINE |
| JP09310606 | POWER GENERATION SYSTEM USING WASTE |
| JP06117276 | POWER GENERATING PLANT |
| CLAIMS 1. A high efficiency waste-to-energy power plant designed to reduce corrosion in the boiler comprising: a) a boiler burning municipal solid waste (MSW) or other similar solid residues and producing high pressure steam (pressure between 60 and 100 bar and temperature less or equal to 4000C) and low pressure steam in a reheating section (pressure between 10 and 40 bar and temperature less or equal to 4000C) from feedwater, with a flue gas outlet in fluid communication with the first side of a low temperature ambient air preheater designed to resist low temperature corrosion due to acid condensation; b) an external high pressure steam superheater, having a first side steam inlet in fluid communication with the high pressure steam outlet of said MSW boiler, designed to superheat steam equal to or above 4000C and a first side superheated steam outlet with pressure between 60 and 100 bar; c) an external low pressure steam reheater, having a first side steam inlet in fluid communication with the low pressure steam outlet of said MSW boiler, designed to reheat steam above 4500C and a first side superheated steam outlet with pressure between 10 and 40 bar; d) a small electrical generator driven by an internal combustion machine (ICM), gas turbine or gas engine, that supplies approximately the plant self load, said ICM having the combustion gas outlet mixed with pure hot ambient air, from the high temperature air preheater second side outlet described in item T), such that this mixture is further heated in a duct burner consuming gas or liquid fuels and in fluid communication with the second side of the said external low pressure reheater and this hot gas mixture provides heat to reheat steam flowing through the first side of the said low pressure steam reheater in fluid communication with the low pressure steam reheater outlet of said MSW boiler; and e) said external low pressure steam reheater having a combustion gas mixture outlet in fluid communication with the second side inlet of said external high pressure superheater after being heated in a duct burner consuming gas or liquid fuels in order to provide heat to superheat steam flowing through the first side of said high pressure steam superheater and in fluid communication with the high pressure steam superheater oulet of said MSW boiler; and f) said external steam superheater having a combustion gas mixture outlet in fluid communication with the first side inlet of a high temperature air preheater whose second side inlet is in fluid communication with the second side outlet of said low temperature air preheater located after said MSW boiler; g) said high temperature air preheater second side gas outlet is in fluid communication with the second side gas inlet of a feedwater heater to said MSW boiler and whose second side outlet gas is mixed with part of air preheated in said low temperature air preheater and this mixture is used as combustion air to said MSW boiler. h) a high pressure steam turbine operating by means of high pressure superheated steam having an inlet in fluid communication with the first side outlet of said high pressure external steam superheater and a low pressure steam outlet in fluid communication with the low pressure first side inlet of the reheater section of said MSW boiler. i) a low pressure condensing steam turbine operating by means of low pressure reheated steam having an inlet in fluid communication with the first side outlet of said low pressure external steam reheater and said low pressure steam turbine having a turbine steam outlet ; j) a first electrical generator in communication with and powered by said high pressure steam turbine. k) a second electrical generator in communication with and powered by said low pressure steam turbine. 2. A high efficiency waste-to-energy power plant in accordance to claim 1 further comprising; a) a steam condenser having an inlet and a condensed steam outlet, said steam condenser inlet in fluid communication with the low pressure turbine steam outlet and said condensed steam outlet in fluid communication with said MSW boiler, wherein said condensed steam outlet provides feed water to said MSW boiler. 3. A high efficiency waste-to-energy power plant in accordance to claim 2 further comprising; a) a deaerator having a deaerator inlet and a deaerator outlet; b) said deaerator in fluid communication with said condensed steam outlet; and c) said deaerator outlet in fluid communication with said MSW boiler. 4. A high efficiency waste-to-energy power plant in accordance to claim 3 further comprising; a) a water preheater having a preheater water inlet and a preheater water outlet; b) said preheater water inlet in fluid communication with said condensed steam outlet; and c) said preheater water outlet in fluid communication with said deaerator. 5. A high efficiency waste-to-energy power plant in accordance to claim 4 with the following characteristics; a) the steam cycle is comprised of only one pressure (between 60 and 100 bar) such that there are no said reheaters, i.e., high pressure steam from said MSW boiler (at temperature less or equal to 4000C) is further superheated in said external superheater (at temperature higher than 4500C) and drives a high pressure condensing steam whose steam outlet in fluid communication with said steam condenser inlet. |
HIGH EFFICIENCY WASTE TO ENERGY POWER PLANTS COMBINING MUNICIPAL SOLID WASTE AND NATURAL GAS BACKGROUND OF THE INVENTION
The present invention relates to power plants generating electric energy burning municipal solid waste (MSW) as the main fuel and known worldwide as waste-to- energy (WTE) plants. Conventional WTE plants burn waste in specially designed grates and the hot flue gases generate steam in a boiler. Due to the very corrosive nature of these flue gases the steam temperature and pressure are limited to 400°C/40 bar resulting in low thermodynamic efficiencies, around 20%, for power generation. One way to overcome this difficulty is to combine a natural gas turbine with a waste incinerator in such a way that the low superheat steam produced in the MSW boiler is further heated using the hot exhaust "clean" gases from the gas turbine in one external superheater. This has been discussed in several patents in special U.S.Pat. No. 5.724.807, U.S.Pat. No. 4.882.903, U.S.Pat. No. 4.957.049, U.S.Pat. No. 4.852.344. and U.S.Pat. No. 5.072.675. Many WTE plants have been built using these concepts the most important one is the Zabalgarbi plant in the city of Bilbao, Spain. This power plant generates 100 MWe and the thermodynamic efficiency for the MSW part of the fuel is approximately 30% and for the natural gas around 50%. The problem with this concept is that most of the electric energy produced comes from the natural gas (the natural gas turbine is a GE LM6000 generating 46 MWe in open cycle) and only 25% or less is produced by the MSW. Although in some cases this can be a good solution from the energy point of view it is not from the environmental side since natural gas is fossil and contributes to global warming cancelling the benefits of landfill diverting. Also natural gas prices vary sometimes in a very unpredictable way and may not be economical to dispatch such plants, however WTE plants have to run 100% of the time which poses additional problems to the grid operator. The present invention reduces drastically the amount of natural gas, sometimes to less than 20%, needed to increase the efficiency of the MSW. So 80% of the net energy comes from MSW allowing the natural gas to be replaced by landfill gas or biogas from anaerobic digestors since these gases are not available in large amounts. The efficiency of the MSW can reach values of more than 33% and the natural gas efficiencies are higher than that for the gas turbine or engine if they were used in a conventional combined cycle without MSW. The proposed concept has other advantages such as being specially suited for high moisture MSW as well as for small incinerators using refractory walls. Nevertheless large waterwall boilers can employ the scheme with many advantages as will be seen next.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A system is disclosed for a power plant configuration combining turbine (engine)/generators, FIG. 1 , generally designated 10, burning natural gas, or other similar fuels such as biogas, landfill gas, diesel oil, with one or more steam turbine/generators 17 and 18, using steam produced in a MSW boiler, composed of an evaporator 4 and 22, with or without waterwalls, one superheater 6 and an optional reheater 5, one or more economizers 7 and 8, and a combustion air preheater 9. In addition to the boiler described above, the steam circuit comprises the following components: an optional back-pressure steam turbine 17, a condensing steam turbine 18, a steam condenser 19, a condensate pump 20, a deaerator 23, a feed water pump 21. The steam circuit also contains one optional external superheaters 3, an external reheater 2 and one or two duct burners 11 and 12 (optional) in the exhaust gas flow path of the gas turbine (engine). After the external superheater 3 there is a high temperature air preheater 13 to further heat up part of the air heated in the low temperature air preheater 9. This hotter air from 13 is mixed with the exhaust gas from the gas turbine (engine) 10 before the duct burner 11. After air preheater 13 the flue gases from the gas turbine (engine) may preheat the boiler feedwater in heat exchanger 25 (optional) and then are mixed with the remaining air preheated in 9 and this mixture is used as hot combustion air in the MSW boiler.
Corrosion is avoided by using one or more external superheaters 2 and 3 (optional) heated by the clean gas exhaust coming from the internal combustion machine (ICM) 10 mixed with preheated air at 9 and 13. This mixture is heated to temperatures between 600 0 C and 700 0 C, with duct firing 11 and 12 (optional) to adjust the steam superheating temperature, in the same way the existing natural gas combined cycle power plant do.
To increase the overall efficiency of the plant the amount of natural gas burnt in the duct burners 11 and 12 must be optimized, the steam cycle efficiency increased (higher pressure and temperature and reheating) and the stack losses minimized by lowering the waste boiler flue gas temperature and also decreasing the excess oxygen in the stack. This can be achieved using condensing heat exchangers (CHX- glass tubes, teflon tubes or teflon coated steel tubes) in 9 to preheat all the combustion air used in the plant from ambient temperature to say Tair2=140°C and in economizer 8 to preheat the feedwater. The stack 16 temperature can be as low as T9=70°C allowing not only the sensible heat recovery but also the latent heat from water condensing increasing the heat transferred from the waste combustion in furnace 14 to the steam. This, cooler and low 02, flue gas at T9 can be partially recirculated as secondary combustion air, after the Air Pollution Control System (APC) 15, to control the waste combustion temperature and to reduce NOx formation in the MSW furnace 14. We can also run the plant without the ICM 10, just increasing the pure airflow Y and natural gas duct firing 11 and 12 during maintenance periods. In this case the amount of energy produced by the natural gas approximately matches the plant parasite load and practically all the energy exported by the plant will come from the waste. Of course in this case the natural gas efficiency will be lower since it is limited by the steam cycle efficiency. However, such a plant without an ICM 10, can be a good solution if we build the plant close to an existing landfill and replace the natural gas with landfill gas since in general the amount of landfill gas is limited. This would be the best solution, from the environmental point of view, since all the power produced will come from waste including the plant self consumption. In some cases is better to use a gas engine/generator instead of a gas turbine. This is shown in FIG. 2. Gas engines differ from gas turbines with respect to their use in combined cycle applications in two ways: while in gas turbines almost all heat rejected goes to the exhaust flue gas in gas engines a substantial part of the heat losses occur in the water cooling the cylinders. Thus we can introduce an additional feedwater preheater 24, before or after optional heat exchanger 25, to capture the heat from the engine cooling system to increase the efficiency of the plant which at the same time reduces the need for a heat sink to cool the engine. The other way gas engines differ from gas turbines is that in engines the 02 content of the exhaust gases is much lower, between 7% and 11%, while in turbines this number varies between 14% and 16%. Thus duct firing for gas engines would in general requires additional fresh air but this is not true here since the air introduced from heat exchanger 13 brings the 02 level of the ICM 10 exhaust close, most of the time higher, to that of gas turbines. Then in contrast with natural gas combined cycle plants, where only gas turbines are used, we can employ either gas engines or turbines choosing the best solution for each particular case. This has special advantages for small machines, say below 2
MWe, where in general gas engines are much more efficient than gas turbines.
Also in the proposed scheme the combustion air for the MSW boiler is preheated to approximately 150°C and the 02 content is close to 18% this helps to reduce NOx formation and to vaporize the water in the MSW early in the combustion grate. This is particularly advantageous for high moisture waste that otherwise would require additional fuel to promote continuous combustion.
EXAMPLE
Consider a WTE plant burning 792 ton/day (33 ton/h) of MSW with LHV of 10.04 MJ/kg corresponding to E4= 92.12 MWth. Combining the MSW boiler with a GE gas turbine GE5 (5.5 MWe with efficiency of 30.7%) with the following steam cycle parameters:
High Pressure = 100 bar / 400 0 C
Low Pressure = 25 bar / 490 0 C Condenser Pressure = 0.03 bar
Steam Turbines lsentropic Efficiencies -> HP= 76.5% LP= 87.5%
Results:
Gross Energy = 41.258 MWe
Total NG consumption = 23.2643 MWth ( 17.915 MWth in GT and 5.3493 MWth in burner)
Energy from MSW = 30.064 MWe (73% of total) MSW Efficiency = 32.64%
Energy from NG = 11.193 MWe (27% of total) NG Efficiency = 48.11 %
Plant overall efficiency = 35.76%
According to General Electric the GE5 gas turbine in pure combine cycle has an efficiency of 44%. Thus the proposed scheme increases the efficiency of the waste as well as of the natural gas.
ADVANTAGES AND ORIGINALITY OF THE SYSTEM
The difference between this power plant configuration and other patented or existing configurations combining natural gas, or similar fuels, and MSW is that the waste fraction of the total fuel consumption is much higher, so that the waste contribution to the net energy exported by the plant can reach 80% or more. The ICM 10 is chosen not to match the large amount of steam produced in the waste boiler but just to provide the plant own power consumption. This does not increase capital cost since in general WTE plants have this machine as emergency power backup. This helps plant start up and shut down, specially in the load rejection case when a steam turbine trip follows a loss of external power. Also the use of gas engines instead of gas turbines represents an advantage for small ICM since they are more efficient at low power. This is particularly important in case biogas (landfill gas) replaces natural gas since engines can burn these fuels directly.
Since the combustion air preheating occurs naturally, at the same time it recovers low temperature energy in the CHX 9, high moisture waste can be processed more easily. Also the reduced amount of O2 in the combustion air to around 18% helps to decrease the NOx formation in the MSW boiler.
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