Low-Grade Heat Recovery in UK Food/Drink...

Low-Grade Heat Recovery in UK Food/Drink Industry

Richard Law

Speaker Background

• 2nd year PhD student at the School of Chemical Engineering and Advanced Materials

• Working on design of novel waste heat recovery systems

– Heat Pumps, organic Rankine cycles…and heat exchangers

• Developing software for the selection of the best available technology for waste heat recovery (on an individual heat source basis)

Highlights

• Introduction to waste heat recovery

– inc. the economic and environmental benefits

• Energy use in the sector

• Typical waste heat sources

• Potential uses of waste heat

• Waste heat recovery technology

Waste Heat Recovery

• Why?

– Climate Change Act (2008)

• Targets an 80% reduction in greenhouse gas emissions by 2050 (34% by 2020; based on 1990 levels)

• Based on current trends, we will NOT achieve these targets…

Waste Heat Recovery

Meeting carbon budgets - 3rd Progress Report to Parliament (2011). Department of Energy and Climate Change

Waste Heat Recovery

• Why?

– Processing industries account for 20-25% of greenhouse gas emissions

• Around a quarter of which from Food/drink sector

– Demand for industrial produce is unlikely drop (especially in food/drinks processing)

– Reduce industrial energy consumption (and greenhouse gas emissions) by increasing energy efficiency

…by recovering waste heat!

Waste Heat Recovery

• Why?

– Rising Cost of Key Utilities:

Department of Energy and Climate Change

Waste Heat Recovery

• 11.4TWh of recoverable waste heat emitted to environment each year via waste streams (5% of energy use)

– 2.8TWh from food industry (~7% of energy use)

Units of

waste energy per year

(kWh/year)

Cost per unit gas

(£/kWh)

Greenhouse gas emissions

per unit (kg/kWh)

Potential cost savings

per year (£m/year)

Potential emissions

savings per year (tCO2eq/year)

Total 11.4 x 109 0.0321 0.1836 £365.94m 2,093,040

F&D sector 2.8 x 109 0.0321 0.1836 £89.88m 514,080

Waste Heat Recovery

• Big incentive for waste heat recovery in the sector

– Potential cost savings of around £90m

– Potential CO2 savings of over 500,000 tonnes

…Waste heat recovery should not be ignored!

Scope of Study

• Low-grade waste heat recovery: – Streams of less than ≈260oC

– Often the most difficult to recover

• Food industry: – All food/beverage processing: dairies, breweries,

bakeries, industrial fryers etc (inc. animal food)

– Chosen as most processes occur at low temperatures, therefore many low-grade heat sources…

Sector Energy Use

• Sector consumes around 42TWh of energy each year

– Around 25% of the total for all processing industries

Low temperature (<300deg.C) processes

64%

Drying/separation 7%

Electric motors 7%

Refrigeration 7%

Other uses 15%

Electric motors and refrigeration systems also emit low-grade heat Drying/separation: •Includes distillation, evaporation tanks, spray dryers etc •Also mostly occurs at low-temperature (< ~200°C)

Low-temperature processes: •Dominates energy use •Includes common processes such as baking, frying, pasteurisation etc

Sector Energy Use

‘Other uses’ includes generic energy consumers such as lighting, space heating, and instrumentation. Unlikely to include a significant amount of high temperature (>300°C) processes

Sector Energy Use

• At least 85% of the sector energy use is consumed at temperatures of less than 300oC

• Fair assumption that most (if not all) of the waste heat available will be in the low-grade range

Sources of Low-Grade Heat

• Heat sources can be from both generic and sector-specific processes

• Generic unit operations include:

– Air compressors: Cooled to produce 60oC hot water, or 40oC air heat source

– Boiler: Flue is commonly vented at around 200oC despite the availability of economisers etc

– Spent cooling water, condensate return: up to 150oC (pressure dependant)

Sources of Low-Grade Heat

• Sector-Specific operations:

– Cooking of food: Fryers or Ovens

• Gas/Vapour heat source at 150-200oC

– Drying of food products: Spray or rotary dryers etc

• Air/Vapour heat source from exhaust at 90-160oC

– Evaporation & Distillation processes:

• Typically produce water vapour heat source at ~100oC

– Refrigeration:

• Water heat source at condenser at around 60oC

Potential Uses of Waste Heat

• Heat transfer between source and sink – Simplest solution

– Sector-specific or generic heat sinks (linked to energy usage)

• Often a surplus of waste heat (esp. low-grade)

• Other options should be explored: – Upgrade waste heat (using heat pump)

– Convert waste heat: • To electricity (using ORC etc)

• To refrigeration (using absorption chiller)

Heat Recovery Technology

• For heat transfer between source and sink:

– Direct re-use

• Very simple solution

• Not always suitable in food industry (contamination issues)

– Heat Exchangers:

• Many available, tailored for various types of fluids

• Can provide a very economical solution


The Rotating Regenerator (Heat Wheel)

Generic advantages: •Gas-Gas applications •High effectiveness (>90%) •Off-the-shelf purchase (lower cost) Food-industry specific advantages: •Can be designed to facilitate self cleaning (fouled streams, e.g. dryer exhaust) •Can recover latent heat (e.g. dryer exhaust, over/fryer exhaust) Has been demonstrated in heat recovery of dryer exhausts, to heat fresh air for space heating (CADDET, 1998)


The (liquid-liquid) Plate Heat Exchanger

•Can be used for almost all heat exchanger duties (exc. extreme pressure and temperature - unlikely in WHR) •May be joined by gaskets, brazed or welded depending on operating conditions •Constructed from a wide range of materials •Gasketted plate heat exchanger allows ease of access for cleaning (useful for fouled streams found in food industry) •Low approach temperature

Compact size eases retro-fit burden


Other Heat Exchangers:

• Shell and Tube: Very well known technology

– Wide-range of materials, inc. glass: useful for heavily fouled or greasy streams

• Scraped-Surface:

– For heat transfer involving complex rheologies

• Run-Around Coil:

– When cross-contamination cannot be tolerated


(Closed-Cycle) Heat Pumps

•Surplus of waste heat expected in many food processing plants due to many heat sources of < 100°C. May not be a matching heat sink

•Heat Pump may provide solution

•Temperature lifts in excess of 50°C recently reported for COP of greater than 3

•Heat pumps with condenser temperature greater than 150°C in development (current technology limited to 100oC)


(Closed-Cycle) Heat Pumps

•Attitude towards heat pumps poor in UK food industry: 36% engineers (Sinclair, 2001) consider heat pumps ‘risky’ or are ‘unsure’

•Evidence of heat pump utilisation should be presented to UK food industry engineers to help change opinion

•Modular, ‘off-the-shelf’, heat pump solutions may also help increase confidence

•For example, from recent Heat Pump summit…


Danish Technological Institute case study: Industrial cleaner

Heat source: Humid air leaving the cleaner Heat sink: Hot water input to the system COP: 4 Small scale: 25kW output per unit Energy consumption to unit cut by 50% Payback time: 1.5 to 3 years (only four month into demonstration) Saving 49 tonnes of CO2 per unit, per year Risky? Unsure?


Danish Technological Institute case study: Industrial cleaner


Mechanical Vapour Recompression (MVR)

•MVR used to compress vapour leaving an evaporative process, which may then be used to heat the evaporator contents

•Food/beverages industry runs a lot of evaporation and distillation processes (concentration of fruit juices, brewing, distilleries etc)

•COP in the region of 10 are commonly reported. This leads to small pay-back periods (as low as 2 years)

•Common in whiskey distilleries in Scotland. Large potential for expansion into other food/industry subsectors - brewing, soft drinks etc


Organic Rankine Cycle


Mechanical Vapour Recompression (MVR)

•MVR used to compress vapour leaving an evaporative process, which may then be used to heat the evaporator contents

•Food/beverages industry runs a lot of evaporation and distillation processes (concentration of fruit juices, brewing, distilleries etc)

•COP in the region of 10 are commonly reported. This leads to small pay-back periods (as low as 2 years)

•Common in whiskey distilleries in Scotland. Large potential for expansion into other food/industry subsectors - brewing, soft drinks etc



•DRD Power currently demonstrating a 200kW unit at a chemical site on Teesside

•Modular, skid-mounted unit - minimal retrofit (providing there is space): just pipe-in the heat source, wire-in the generator

•~100°C vapour heat source

•Demonstration scheme published by Carbon Trust

•Expected payback time quoted as ~3years

•More demonstration schemes and/or modular units will lead to increasing interest in ORC for waste heat recovery in UK

Selection of WHR technology Select method of WHR according to the simplest appropriate solution, and ultimately the payback time

Simplest, cheapest solution is direct re-use of the heat source into the heat sink

•Requires only pipe/duct work


Next level: Heat transfer via heat exchanger

•More expensive than direct re-use •More equipment (heat exchanger) required and larger installation cost


Next level: Heat Pump/ORC Solution •When a surplus of waste heat is present •Requires combination of heat exchangers, compressors/pumps etc •Complex, high capital cost solution


Final option: Secondary Enterprise/Over the Fence heat sink

•Requires significant research •Large capital to set up project •Not often considered in UK

Conclusions

• 2.8TWh of recoverable waste heat is emitted to the environment each year from the food/drinks sector

• Full recovery has the potential to save around £90m/year and 500,000 tCO2eq/year

• Various options for waste heat recovery: Heat exchangers, Heat Pumps etc

Interest?

• My supervisor, Prof. David Reay, and I have experience in feasibility and design studies for many types of waste heat recovery systems – Including heat recovery by heat exchanger, and heat

pump – Technical, economic and environmental assessment

• We may be available to carry out such work in the coming months

• For enquiries:

[email protected] or [email protected]

Thank You

• I’d like to thank:

– My PhD supervisors: Prof. Harvey and Prof. Reay

– Dr Barbara Sturm (for inviting me to talk)

– Everyone for listening

Low-Grade Heat Recovery in UK Food/Drink...

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