Simple solutions for reducing environmental impact and for energy savings in geothermal power plants

1
Simple solutions for reducing environmental
impact and for energy savings in geothermal power
plants

• Speaker Dr. Ing. E. Brunazzi
• University of Pisa, Italy
• in collaboration with
• DAE EFCE Working Party - Helsinki, Finland 5-6
  June 2003

Responsible of electricity generation from
renewable sources
2
Agenda

• Introduction
• HCl abatement systems and process optimisation
• NCG emission AMIS new technology for H2S and Hg
  removal
• Conclusions

3
Introduction

• Geothermal energy plays a major role among the
  so-called "renewable energy sources". Its
  contribution, in terms of energy supplied, is
  second to hydropower only.
• Uses
• For electricity generation at the beginning of
  2000, world-wide installed capacity of some 8,000
  MW and an annual output of around 50 TWh.
• Direct uses of the earth's heat, although not
  easily accountable amount to between 8,200 and
  9,000 MW with an annual output 29-34 TWh.
• Others (thermal treatment, recovery of
  chemicals)
• Leading countries USA, New Zealand, Italy,
  Island, Mexico, Filippines, Indonesia, Japan
• Many reservoirs are in Emerging Countries!

4
Perspectives
Introduction

• Geothermal power generation
• is already economically competitive with fossil
  fuels, at least in the geological settings
  characterizing most of the present installations,
  
• is exploited in both industrialised and
  developing countries. In some of the latter, its
  contribution to the national energy balance is
  substantial.
• The perspectives for the future development of
  geothermal energy
• appear promising.
• Moreover, they could greatly benefit (like all
  the other renewable sources) from the
  introduction of a pollution or carbon tax, as
  geothermal energy is among the most
  environmentally friendly sources of heat and
  electricity.
• Technologies under study
• Hot dry rock
• Injection of water in zones where natural
  reservoir do not exist

5
Geo Thermal
Introduction

• T 5000C in the Earth center
• Magma
• Hot water
• warm wells, geyser
• geothermal reservoir

6
Types of geothermal power plants
Introduction
A) Dry steam

• Worlds biggest plant The Geyser (140 Km north
  of San Francisco/CA) 750 MW 14 production
  units
• The first was Larderello in Tuscany/IT in 1904
  (Mr. Pietro Ginori Conti)

7
../Types of power plants
Introduction

• B) Flash steam power plant
• Water dominates
• T water 180-370C
• The majority of geothermal fields
• C) Binary cycle power plant
• Water at moderate T 120-180C

• Note for direct uses binary cycle - the
  secondary fluid is clean water
• 1st example Boise in Idaho (USA),
• the biggest plant Reykjavik (Island)

8
Milestones of power generation development in
Italy
Introduction

• In 40s Italy was producing 132 MW
• Until 60s shallow reservoirs explotation (below
  1000 m) in Larderello and Mt. Amiata areas.
• Starting from 70s launch of extensive programs
  of surface and deep exploration
• In 1/1/1998 Italian geothermal installed capacity
  was 668.5 MW, with 32 units in operation and 3
  units kept as a reserve in 1999 was 730 MW.
• Italy is the 4th country in the world
• Planned development
• Increase of the electrical capacity up to 770 MW
  and of the yearly generation up to 5.5 TWh by the
  year 2004.
• Increase of the thermal uses strictly related to
  the incentives (Post-Kyoto policies, etc.).
• Recovery of chemicals according to the results of
  the feasibility studies

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Introd/ Enel is owner of
Introduction

• Larderello A
• Valle Secolo
• Cornia
• San Martino
• Molinetto
• Travale-Radicondoli B
• Monte Amiata B
• Bagnore
• Piancastagnaio
• Bellavista
• Latera (Lazio) C

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Electricity generation
Introduction

• Geothermal Fluid H2O, CO2, H2S, Boron, trace
  elements (Hg, As), HCl
• Liquid effluents (Reinjection)
• Major concerns ?
• Landscape protection (visibility of well pads,
  steam pipelines and power plant)
• Noise
• Process corrosion (HCl)
• Environmental H2S smell
• Microseismicity and soil subsidence

• To maintain well pressure
• To avoid polluting superficial waters
• Reinjected water is heated by the Earth

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Scheme of a power plant
Introduction
H2S
HCl
Standard size of steam turbines (20, 40 and 60MWe
nameplate capacity)
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HCl abatement

• The presence of HCl in geothermal steam is
  reported throughout the world
• Superheated steam no problems
• Steam becomes saturated crossing the Wilson line
  and 15-20 ppmw of HCl can cause
• Generalised corrosion in transportation pipelines
  (2-6 mm/yr)
• Turbine failures (wet stages)
• Common practice steam washing with a cold
  alkaline solution.
• Various possibilities

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Abatement systems
HCl abatement

• Currently in use in Italian geothermal plant
• In line washing (annular flow regime)
• Low investment costs
• High pressure drop (dispersed flow) power loss
• Other possibility
• Conventional scrubbing equipment
• Enel installed one co-current washing column
  equipped with structured packings
• Investment costs corrosive fluids, T, P
  (valuable materials)
• Low pressure drop, hence lower power loss

14
In line scrubbing
HCl abatement
Equipment Spray nozzle, pipeline, internals
Equipment Cyclone and/or vane type demister

• The abatement depends on
• Injection nozzles
• Flow regime in the pipeline
• Possible internals (e.g. static mixer)

15
Separation of entrained liquid
HCl abatement

• Downstream steam/liquid separation
• Configuration currently in use
• Primary separator conventional cyclone
• Secondary separator Vane type demister (VTD)
  (dp10020-40 mm)
• Optimisation of the separation devices?
• Employment of novel axial flow separator under
  study
• More compact
• Vessel size reduction (investment cost)

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Axial flow separator
HCl abatement
Typically F-factor 16-19 m/s(kg/m3)0.5 dp100
7 10 microns Pressure drop 200 300 mmH2O
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Axial flow separator experimental separation
efficiency
HCl abatement
From Brunazzi, Paglianti and Talamelli, AIChE J.
2003
18
HCl abatement
High pressure test rig (P up to 20 bar)

• Spray-generation circuit
• ultrasonic nozzle
• liquid flowrate 130 l/h, 24 bar
• Carrier gas circuit
• fan 500 m3/h _at_ 20 bar (air)
• high efficiency separator in D1

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Analysis of in-line abatement the approach
HCl abatement

• Abatement of HCl with a NaOH solution absorption
  and chemical reaction (surface reaction regime)
• Surface exchange area A and mass transfer
  coefficient Kg
• The approach subdivision of complex washing
  systems into simple abatement units
• Spray
• Internals (e.g. static mixer)
• Pipeline
• Steam/liquid separator(s)
• Formulation of a theoretical abatement capability
  for each unit and computation of the contribution
  of each piece of equipment
• Implementation of models into a computer code
• Comparison of modelling results with field data
• Design of new systems
• Optimisation of existing ones

Paglianti et al. (Geothermics, 1996)
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Single units
HCl abatement

• The Spray
• The nozzle lt - gtDroplet dimension concentration
• Mass transfer coefficient KgD (motion of single
  drop)

• The Static Mixer
• A and KgM from literature on struct. Pack.

• The Pipeline
• Tipically, annular/dispersed flow regime is
  established in the steam pipeline
• Washing liquid splits between film and droplets
  (evaluation of the fraction of liquid entrained)
• d32 of droplets, KgP for film KgD for droplets

• The steam liquid separators
• VTD, Cyclone
• Necessary to avoid corrosion erosion phenomena
• Computation of droplet volumetric distribution
• Abatement efficiency, A - KgVTD, KgCY

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Schematic diagram of plant C scrubbing unit
HCl abatement
22
Comparison between field data and computed data
HCl abatement
23
Computed average abatement efficiency of the
units of the generic washing loop for a rough
estimate
HCl abatement

• For example
• Saturated steam with 30 ppm HCl
• Abatment system Spray, 50m long pipeline,
  cyclone

24 m2/m3
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Co-current washing in column
HCl abatement

• Low steam velocity lt 7 m/s
• low concentration of fine droplets
• higher d32
• Structured packings
• high exchange surface
• low pressure drop
• low liquid flowrate
• Further savings can result when recovering heat
  from the outflowing alkaline solution, also for
  the in-line abatement solution

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Energy savings
HCl abatement

• The washing process causes a net power loss
  linked to steam desuperheating, offset only
  partly by the partial evaporation of the wash
  liquid
• A simple solution?
• Heat recovery of the outflowing alkaline solution
• Plant modification heat exchanger (non
  corrosive, weakly alkaline solutions)
• Results?

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Enel field data
HCl abatement
Scheme A
3MW
20 t/h
P11bar
Power loss 70 Kw
DT10C
20 t/h
43 recovery! Power saving 1
P11bar
DT10C
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Optimisation of the washing process
HCl abatement
G 100t/h P 11 bar a T 236C HCl 20ppm NCG
7
Andreussi et al. 1995 (Proc. WGC 1995)
Maximum power saving achievable overall 2.6 (
41 recovery)
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NCG (non condensable gases) emissions

• NCG typical composition
• CO2 (95v), CH4 (1.5-2 v), H2 (0.5-1v), N2
  (lt0.5v), H2S (1-2v), Hg (some mg/Nm3), trace
  elements
• Ambient air concentration of these pollutants is
  well below current national laws or international
  standards
• However
• H2S odor nuisance can be perceived at very low
  concentrations
• Hg release should be minimised, given its
  mobility and capacity of bio-accumulation

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/NCG
H2S removal

• Hg removal processes are seldom used in
  geothermal operations
• H2S abatement is instead more widespread
• Italian geothermal fields are characterised by a
  very high NCG content (2-10wt), making the
  existing processes impractical or uneconomical
  and requiring the development of new technologies

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Simplified scheme of 20MW plant
H2S removal
CT
DCC
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Field test on a 8MW plant
H2S removal
Baldacci et al. (Geothermics, 2002)
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Primary emission treatmentAMIS a new
technology by Enel
H2S removal

• Why a new technology ?
• Drawbacks of the existing technologies
• high capital and om costs (royalties)
• need of proprietary reagents
• very expensive
• liquid reinjection?
• sulphur by products to dispose
• attended operation

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Main features of AMIS process
H2S removal

• Tailored for geothermal power plants
• Lower capital and om costs in comparison with
  commercial processes
• H2S and Mercury removal
• No chemicals required
• No solid sulphur by products
• liquid streams reinjected in the reservoir
• Unattended operation
• remote control

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Geothermal power plant (20 MW)
H2S removal
130 t/h
40 t/h
35
Geothermal power plant with AMIS plant
H2S removal
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Gas cooler and Hg removal
H2S removal
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Catalytic oxidation H2S-SO2
H2S removal
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SO2 absorption
H2S removal
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AMIS process effluents
H2S removal

• ATMOSPHERIC EMISSION
• RESIDUAL H2S and SO2

• WASTE DISPOSAL
• SPENT OXYDATION CATALYST
• SPENT Hg ADSORBER

Every 5-6 years
Once-twice a year
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NCG 16/ BG3 AMIS unitThe first full scale
demonstration
H2S removal

• 15000 kg/h geothermal off gas to be treated
  (mainly CO2), including 120 kg/h H2S
• Target 80 overall sulphur removal
• Hg concentration at ground level after 30 years
  is lt1/100 (without AMIS) and lt1/1000 (with
  AMIS) of that naturally present.

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Conclusions

• Plant problems solved
• HCl is no longer the cause of severe corrosion
  damage in Italian Geothermal Plants
• Pollutant emissions, H2S and Hg solved
• Optimisation phase
• energy savings
• design of more efficient power plants
• geothermal plants not only mechanical but also
  chemical plants
• absorption, adsorption, reaction
• because of the nature of the process fluids
• Sites of turistical interest landscape
  protection
• noise
• mitigation of visual impact of geothermal
  installations
• Collaboration between Enel University

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Thank You !!!