HYDROGEN SULPHIDE REMOVAL FROM GEOTHERMAL POWER STATION COOLING - - PowerPoint PPT Presentation

TUBULAR BIOREACTOR FOR HYDROGEN SULPHIDE REMOVAL FROM GEOTHERMAL POWER STATION COOLING WATER

Rob Fullerton – Beca Ltd, New Zealand

Taupo – North Island NZ

Taupo Wairakei

Wairakei Geothermal Power Station

• Wairakei – one of

earliest geothermal power stations in the world

• commissioned 1958 –

1963, installed capacity 192MW

• current capacity 157MW

Cooling Water System

• ~17 m3/s Waikato River water for direct condenser cooling

A Station B Station CW in CW out

CW = Cooling water

Waikato River Steam Turbines Direct contact condensers Geothermal steam 0.2m3/s 17m3/s 17.2m3/s Water pumps 157MW power Waikato River

Cooling water + condensate

Cooling Water System

The Challenge

• H2S in the geothermal steam condenses into the cooling water

and is discharged back to the river – aquatic impacts

• H2S concentration in discharge about 1000mg/m3 (~1mg/L)
• Discharge Permit: H2S discharge to river

─ Prior 2012 : No consented limit - 10,000 kg/week ─ By 20 August 2012: 2,800 kg/week (72% reduction) ─ By 20 August 2016: 630 kg/week (94% reduction)

• Requires sulphide reduction to ~50 mg/m3

Waikato River Steam Turbines Direct contact condensers Geothermal steam containing H2S 0.2m3/s 17m3/s 17.2m3/s 1000ppb H2S To be reduced to 50ppb by 2012 Cooling water 157MW power Waikato River

Cooling water + condensate

The Challenge

• Need for strategy to control sulphide recognised
• Thinking started in 2000
• SOB naturally occurring – esp. in geothermal areas
• Biological sulphide oxidation - conversion of H2S to sulphate
• SOB biofilm observed on existing outfall structure
• Cooling water from power station 30 - 35oC, pH ~6,

dissolved CO2

• Could SOB be used to achieve 95% sulphide removal?

Sulphide Oxidising Bacteria - A Solution?

Initial Pilot Trials – 2000 - 2005

Algae overgrowth

Sheets – channels - Pipes

The Eureka Moment

SOBs seem to grow best in fast moving flow

Pilot Testing – Initial pipe trials

Sulphide sampling

• Biofilm established on inside of pipe – good H2S removal
• High velocity prevents excessive biomass build-up
• Lack of light prevents algal growth
• Pipe bioreactor proved in concept

Pilot Testing 2010 100mm dia. x 200m /150mm dia. X 400m

SOB Biofilm

Beginning section 100m End section 200m

100mm dia. x 200m results

150mm dia. x 400m results

𝑒𝑇 𝑒𝑢 = −𝑤𝑛 𝑇 (𝐿𝑡 + 𝑇)

dS/dt = rate of substrate change vm = maximum substrate utilisation rate ks = half saturation constant

𝑒𝑇 𝑒𝑢 = 𝜈𝑛 𝐶 𝑍 𝑇 (𝐿𝑡 + 𝑇)

S = substrate concentration µm = maximum specific growth rate Ks = half-saturation constant B = biomass concentration Y = biomass yield

B >> S

• Sulphide concentration is low
• biofilm quasi-steady state with constant

thickness, viz. growth = detachment

Sulphide Removal Model

• Calculate removal rate for each pipe segment (gH2S/m2/d)
• Fit to non-linear least squares Monod model

vm = 13.85 gH2S/m2/d Ks = 235mg/m3

Monod curve fitting

Design Curve

Minimum pipe length ~160m +25% safety factor = 200m

Power station cooling water Inlet sulphide = 1000ppb 100mm dia. x 200m Velocity 0.8m/s Flow = 6.7L/s

Sulphide

• xidising

bacteria biofilm forms on pipe inside surface Outlet sulphide < 50ppb

Pipe bioreactor - process concept

100mm

Full scale pipe bioreactor concept plant

Flow = 17m3/s ~2000 pipes x 200m in parallel

390km of pipe !! 200 x pipes per layer 10 layers

Waikato River Steam Turbines Direct contact condensers Geothermal steam + H2S 17m3/s cooling water 17.2m3/s 13m3/s 50ppb H2S Waikato River 157MW power

Proposed sulphide treatment system 2012

Pipe bioreactor

4.2m3/s 1000ppb H2S 2,800kg/week 17.2m3/s

Waikato River Steam Turbines Direct contact condensers Geothermal steam + H2S 13m3/s 13.2m3/s 1000ppb H2S Cooling water <80ppb H2S Waikato River 120MW power

Proposed sulphide treatment system 2016

Pipe bioreactor

630kg/week

Full Scale design Challenges

• Design began January 2011
• Never been done before – collaboration between contractor

and designer

• Physical constraints – high groundwater + hot (60oC)
• Power station can’t stop – build off-line
• Hydraulics – available head 1.5m – require pumping
• Energy efficiency – design low head system
• How to install 380km of 100mm dia. HDPE pipe

Innovative “over – under” syphon

• Bioreactor pipe configuration

― 1890 pipes ― 5 banks of 378 pipes ― 42 pipes x 9 layers ― Magnetic flow meter each bank 100m

Bioreactor Layout

Site Overview December 2011

Construction of pipe field

• PE Pipe Design

─ Extruded on site ─ 100m lengths ─ No joints ─ 2.5mm wall ─ Embedded into weak cement matrix – “soilcrete” ─ 378km in 5 months

Construction of pipe field

Construction of pipe field

Soilcrete embedment made from local pumice sand

Construction of pipe field

Construction of pipe field

Return chamber – end pipe field

Pump inlet channel

Bioreactor outlet channel

Bypass weir

Bioreactor Performance

Permit Compliance

Full scale performance

• H2S concentration meeting target of >80% (~50mg/m3)
• Power station meeting sulphide mass emission limit of

2,800 kg H2S/week

• Optimisation of operating conditions to minimise power

consumption

• Current performance gives confidence to meet 2016

discharge limit of 630 kg H2S/week

The world’s largest tubular bioreactor