Waste Gasification and Energy Efficiency

Transcription

1 Sardinia 2011 S. Margherita di Pula, 3-7 October 2011 IWWG Workshop on Thermal Treatment Waste Gasification and Energy Efficiency prof. Stefano Consonni Dipartimento di Energia - Politecnico di Milano

2 Combustion 2 Primay Energy source (fossil fuel, biomass, waste) COMBUSTION Combustion Products CO 2 H 2 O N 2 O 2 SO2 Oxidizer (air, O 2 ) HEAT Very flexible: can use basically any type of feedstock All primary energy in the feedstock is converted into heat No further use of combustion products Can generate a number of pollutants / hazardous species

3 Gasification 3 Primary Energy source (coal, biomass, waste, etc. ) CO H 2 GASIFICATION Syngas CO 2 H 2 O H 2 S COS N 2 Gasifing / Oxidizing agent (air, O 2, steam, CO 2 ) HEAT NH 3 Very sensible to characteristics of feedstock Only a small fraction of primary energy in the feedstock is converted into heat Gasification products are a valuable feedstock Syngas is highly toxic, explosive and contaminated with pollutants --> syngas treatment is crucial

4 From pirolysis to combustion 4 Source: F. Lamers & R. van Kessel

5 Gasification is NOT a new technology! 5

6 6 Gasification capacity and planned growth Source: NETL, US DOE - Gasification data base

7 7 Cumulative capacity and planned growth Source: NETL, US DOE - Gasification data base

8 8 Gasification capacity and planned growth Source: NETL, US DOE - Gasification data base

9 The three basic technologies: 1) fixed bed 9 Feedstock Feedstock Counter-current: feedstock downward air upward syngas upward Co-current: feedstock downward air downward syngas downward (ejected upward)

10 The three basic technologies: 2) fluidized bed 10

11 The three basic technologies: 3) entrained flow 11 slurry feed dry feed

12 E-Gas coal gasifier (Conoco-Phillips) 12

13 13 Integrated Gasification Combined Cycles (IGCC) Large scale ( MWel) Combined Cycle PRESSURIZED Gasification Island

14 Means to recover heat from syngas 14 reactants GASIFIER A reactants GASIFIER cooled raw syngas C T~1400 C RADIATIVE COOLER cooled raw syngas CONVECTIVE HIGH TEMPERATURE COOLER T~1400 C water/ reactants WATER PARTIAL QUENCH T~900 C CONVECTIVE HIGH TEMPERATURE COOLER T~900 C COLD GAS RECIRCULATION BLOWER reactants cooled raw syngas D reactants GASIFIER B GASIFIER CONVECTIVE HIGH TEMPERATURE COOLER T~1400 C water WATER QUENCH cooled raw syngas T~1400 C GAS QUENCH T~900 C

15 Some crucial features of coal gasification 15 Gasification at high pressure to ease the integration with high-efficiency Combined Cycles and maximize energy efficiency Sophisticated syngas clean-up treatment to warrant the utilization of gas turbines (or the use of clean syngas for chemicals / fuels production) Highly integrated plant to maximize recovery of thermal energy from syngas Very large scale to increase performances and reduce costs Mainly due to the poor quality of the feedstock, these features cannot be realized with waste gasification --> Waste gasification plants are substantially different from those fed with fossil fuels

16 Pros and cons of waste gasification 16 Motivations shared with gasification of fossil fuels: generation of high-quality energy carrier (syngas) adoption of internal combustion engines syngas clean-up ahead of combustion possible production of highly-valued chemicals possible production of liquid fuels / hydrogen Additional motivations: reducing conditions limit generation of dioxins/furans (although emissions depend on processes downstream) production of inert solid residues gas-phase combustion easier to control and operate Problems: feedstock size --> mechanical pre-treatment sensitivity to feedstock properties --> reliability, operability (very) low heating value of feedstock syngas clean-up very difficult and costly small scale --> poor performance, high cost (especially O 2 )

17 17 Energy perspective of waste gasification - 1 A significant fraction of the energy input to the gasifier is needed to bring the output flows to the temperature needed to operate the gasifier. For a feedstock with low LHV like waste, the fraction that goes into thermal energy can be as high as 40% of the input LHV The recovery of the thermal energy in the syngas is particularly crucial BUT it's made difficult, often impossible, by the poor quality of the raw syngas The thermal energy of the syngas can be recovered with much lower efficiency than its chemical energy To reduce the fraction of the energy input that goes into heat it is generally imperative to use O 2 as oxidant (rather than air) - which increases costs and requires high-value (mechanical) energy

18 18 Energy perspective of waste gasification - 2 For the same input power, the mass flow of a low LHV feedstock like waste is higher --> higher auxiliary loads, larger volumes, higher costs The thermal energy in ashes is difficult to recover --> the high ash content of MSW causes significant losses Syngas is most efficiently utilized in gas turbine-based systems (Combined Cycles). However, gas turbines must be fuelled at pressure, which requires: pressurized gasification --> extreme challenge syngas compression --> large power consumption Strong scale effects --> at the size typical of waste-toenergy plants, gross energy conversion efficiency is low, often VERY low. Even more so for the net energy conversion efficiency

19 Example: Themoselect process 19 Opportunities to use or convert thermal energy of syngas are COMPLETELY LOST

20 Example: Themoselect process 20 Energy in syngas is 50-60% of energy in feedstock Energy in syngas and in ashes cannot be utilized

21 A crucial feature of waste gasification 21 Cleaning the syngas is so difficult and costly Recovery thermal energy from syngas is so difficult/costly Pressurizing the gasification island is so challenging Scale is typically so small that rather than producing power by high-efficiency systems like Combined Cycles or combustion engines waste gasification plants typically adopt the same power system of conventional combustion plants: steam cycle by doing so however, the efficiency achievable by waste gasification is seriously hampered

22 Basic waste gasification concepts 22 FEEDSTOCK (coal, biomass, MSW, RDF, etc.) COMBUSTION EXTERNALLY FIRED CYCLE POWER (HEAT) (HEAT) GASIFICATION Syngas TWO-STEP OXIDATION (HEAT) SYNGAS INTERNALLY OXIDANT CLEAN-UP FIRED CYCLE POWER (air, O 2, steam) CHEMICALS "FULL" SYNGAS EXPLOIT- ATION WATER-GAS SHIFT PURIFICATION SYNTHESIS PROCESS HYDROGEN LIQUID FUELS (Diesel, DME, Methanol, etc.)

23 Nippon Steel Direct Melting System 23

24 Ebara 24

25 Energos 25 SECONDARY AIR GILLOTINE (FUEL THICKNESS ON GRATE) RECIRCULATED FLUE GAS O 2 = 7% t = 900 C to 1000 C C WID COMP FLUE GAS FEED PLUNGER SYNGAS λ= 0.5 H 2 = 5% CH 4 = 4% CO = 14% t 900 C DUPLEX (TRANSPORT MECHANISM) OIL COOLED GRATE

26 Hitachi Zosen: stoker 26

27 Kobelco 27

28 Mitsui 28

29 Techtrade 29

30 Waste gasification plants 30 In summary, nearly all waste gasification technologies proposed are "two-step oxidation" processes coupled with the same power system of conventional combustion plants: externally-fired steam cycle At most, waste gasification plants can reach energy performances close to those of combustion plants Actual performances are likely to be much lower, due to: High consumption for auxiliaries Loss of (part of) thermal energy in the syngas Higher thermal losses due to less favourable dimension / shape of components operating at high temperature

31 Summary of Gasification vs Combustion potential advantage / benefit of gasification vs. combustion a) Syngas is easier to handle, meter and control than MSW b) homogenous, gas-phase combustion of syngas is easier and can be better controlled Reducing conditions give: (i) better solid residues (metals) (ii) less dioxins, furans and NO X Syngas can be used, after proper treatment, in highly efficient internally-fired cycles (gas turbines, combined cycles, Otto engines) related drawbacks / issues that hinder the benefit of gasification a) syngas is highly toxic and explosive --> safety issues b) two-step conversion (gasification + syngas combustion/conversion) --> complexity, cost, reliability issues if syngas is eventually combusted, dioxins, furans and NO X are still an issue a) syngas treatment is complex and costly b) due to losses in gasification and syngas clean-up, energy conversion efficiency is low c) at the small scale relevant to waste treatment, efficiency of internally-fired cycles is low technological issue environmental issue energy issue

32 Waste gasification vs waste combustion potential advantage / benefit of gasification vs. combustion syngas can be used, after proper treatment, to generate high-quality fuels or chemicals gasification at high pressure enhances the opportunities to increase energy conversion efficiency and reduce costs related drawbacks / issues that hinder the benefit of gasification a) required syngas treatment very demanding and costly b) at the small scale typical of waste treatment plants, fuel/chemicals synthesis is extremely expensive pressurized waste gasification poses formidable challenges and has not been attempted by any technology developer economic issue technological issue no free lunch...

33 The MatER Research Center 33

34 Conclusions 34 Thank you for your attention!