opportunities for low-grade heat recovery in the uk...

Opportunities for Low-Grade Heat

Recovery in the UK Food Processing

Industry

Richard Law*, Adam Harvey, David Reay

Sustainable Thermal Energy Management in the Process Industries International Conference

(SusTEM2011)

Author Background

1st year PhD Student at School of Chemical Engineering and

Advanced Materials, Newcastle University

Working on EPSRC funded OPTITHERM project: OPTImising

THermal Energy Recovery, utilisation and Management

Overall aim to produce an Expert System for the selection

of best available technology for the recovery of low-grade

industrial waste heat

Part of Process Intensification Group (PIG)

See http://pig.ncl.ac.uk for information

Introduction: Why recover waste heat?

Climate Change Act (2008)

Targets 80% reduction in greenhouse gas emissions by

2050, 34% by 2020

Won’t achieve the targets (based on current trends)

‘Carbon Taxes’ to be introduced in 2013


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


Processing Industries account for 20-25% of

greenhouse gas emissions

Demand for industrial produce unlikely to drop

Especially in food/drinks processing: people need to

eat!

Reduce energy consumption (and emissions) by

increasing overall plant efficiency

By recovering waste heat


Economic incentive:

“Fuel prices for manufacturing industry, in cash terms 1990-2009” from Quarterly Energy Prices (2010).

Department of Energy and Climate Change

Introduction: Available Waste Heat

It is estimated that 11.4TWh of recoverable waste heat

is emitted to environment per year via waste streams in

process industries

Around 5% of total energy use

2.8TWh from food/drinks processing

5-7% of total energy use

Enough energy to heat ~160,000 homes

Recovery of this heat would be significant in reducing

emissions and costs

Scope of study

Low-grade waste heat recovery:

Streams of < ~260°C*

Food Industry:

All food/beverages processing: e.g. meat and fish

production, dairy, brewing, bakery etc

Including production of animal food

Chosen because most processes occur at low

temperature. Therefore, many low-grade heat

sources

* Profiting from low-grade heat. (1994) Watt Committee

Low temperature (<300deg.C) processes

64%

Drying/separation 7%

Electric motors 7%

Refrigeration 7%

Other uses 15%

Energy use in the sector

Electric motors and refrigeration

systems also emit low-grade heat

‘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

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

At least 85% of the sector energy use is consumed at

temperatures of less than 300°C

Fair assumption that the vast majority (if not all) of

waste heat will be available in the low-grade range

Energy use in the sector

Consumes around 42TWh of energy each year

Around 25% of total for process industries

Sources of low-grade heat

Sources of low-grade heat from both generic and sector-

specific processes

Generic unit operations include:

Air compressors: Cooled to produce 60°C Water heat

source or 40°C Air heat source

Boiler: Flue is commonly vented at ~200°C despite

availability of economisers and air pre-heaters

Spent cooling water, condensate return: upto 100°C

Sources of low-grade heat

Sector specific operations:

Cooking of food: Fryers or ovens etc

Gas/Vapour heat source at 150-200°C

Drying food products: Spray or rotary dryers etc

Air/vapour heat source from exhaust at 110-160°C

Evaporation & Distillation processes

Typically produce water vapour heat source at ~100°C

Refrigeration

Water heat source at around 60°C

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: Via heat pump (open or closed

cycle)

Convert waste heat:

To electricity: ORC or thermoelectric unit

To refrigeration via absorption chillers

Heat recovery technology

For heat transfer between source and sink:

Direct re-use

Simple solution: requires only pipe work and

auxiliary equipment

Not always suitable in food industry: contamination

problems

Heat Exchangers

Many available, each has own merits

Full details of each type found in paper


The Rotating Regenerator (Heat Wheel):

Image from www.datacentreknowledge.com

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)


Image from www.alfalaval.com

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


(Closed-Cycle) Vapour Compression Heat Pump:

•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


(Closed-Cycle) Vapour Compression Heat Pump:

•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?


(Open Cycle) Mechanical Vapour Recompression:


(Open Cycle) Mechanical Vapour Recompression:

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 may lead to small

pay-back periods

Common in whiskey distilleries in Scotland. Large potential for expansion

into other food/industry subsectors - brewing, soft drinks etc


Organic Rankine Cycle:



ORC recovers low-grade heat to generate electricity

Waste heat source temperature as low as 85°C (in current operation in Europe) or

40°C (research stage)

Useful when surplus of low-grade waste heat is present: all plants require electricity!

Technology has yet to seriously take-off in UK

Problems: Low efficiency - only 5-18% efficient (electricity produced/heat recovered)

Demonstration schemes and modular units required to spark interest in this

technology…

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

Selection of WHR technology

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

2.8 TWh of recoverable waste heat is emitted to environment from

the food processing industry per annum of which at least 85% is of

low-grade

Various options for WHR: heat exchangers, heat pumps, ORC etc

WHR technology should be chosen according to the simplest

appropriate solution (and project economics)

Often a surplus of low-grade heat present: potential for heat pumps

and ORC

Demonstration schemes should be set-up to encourage the uptake

of ORC and HP projects (including MVR)

Development of further modular ORC and HP solutions would also

encourage uptake of such projects

Acknowledgements

Supervisors: Dr Adam Harvey - PIG group, School of Chemical

Engineering and Advanced Materials, Newcastle University

Prof. David Reay – David Reay and Associates (Visiting Prof. at Newcastle University)

EPSRC (Project no. EP/G061467/1)

Collaborating partners on OPTITHERM project: Brunel University

Northumbria University

A number of industrial partners

Thanks for listening,

Any Questions?

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