Reducing carbon emissions in the food processing industry

How can the food processing industry meet the challenge of reducing its carbon emissions? Here we look at three of the technologies already available on the market that claim significant fuel and carbon emission reductions.

Increasing efficiency to reduce carbon throughput

What is efficiency? It is defined as the percentage of total energy input to a machine or equipment that is consumed in useful work and not discarded as, for example, useless heat. This means that any reduction in discarded heat directly increases plant efficiency.

That’s why the first step for enhancing energy efficiency should be the broad implementation of waste heat recovery systems (WHRS), also known as cogeneration and combined cycle. Some units already operate in Australia behind large heat sources, such as power plant gas turbines; however, very few smaller industrial applications are currently utilising this waste heat.

A second way to reduce carbon throughput is to increase the boiler/heater parameters via the installation of additional heating surfaces for preheating combustion air or fuel.

A third way of reducing carbon emissions is fuel diversification. Some fuels inherit higher carbon emissions than others and with oil prices starting to rise again and a carbon trading scheme looming, previously unattractive fuels like biomass and process by-products are becoming economically and environmentally viable alternatives.

Waste heat recovery

Currently, many engines, turbines and other heat sources operate in simple cycles and the heat they generate is discarded. Depending on the thermodynamic data, the electric efficiency of a simple cycle turbine/engine is only between 30-40% whereas combined cycles achieve efficiencies of up to 80% as most of the heat in the exhaust gas is captured and used.

Even the use of a small amount of the heat discarded makes a surprisingly high difference to the overall efficiency of a plant.

Suitable heat sources

In principle, all exhaust gas sources can accommodate a WHRS but obviously there are economic and technical constraints. Economically, the value of the heat recovered must amortise the cost of the capital invested over a reasonable period. Technically, the heat transfer rate and the exhaust gas composition have to be considered.

However, it should be highlighted that high-, medium- and even low-temperature heat sources are suitable. The potential of high and medium temperatures is clear as the heat transfer rate is high enough to design a compact heat recovery unit.

The problem with low-temperature heat sources is that the smaller the temperature difference between the working mediums, the larger the heating surface necessary. Larger heating surfaces directly affect the investment and the small amount of money involved in the generation of low-temperature heat stretches the amortisation period. Further oil price rises and a CPRS will change the economic attractiveness in the near future.

Focusing on high- to medium-temperature heat sources, the following heat sources, across almost all sizes, are suitable for retrofitting a WHRS:

• Diesel/gas engines
• Gas turbines
• Process heaters
• Dryers

What to do with the recovered heat

The hot exhaust gases can be used to preheat water and evaporate it or to produce electricity via a steam turbine. Superheated steam can be forwarded to a steam turbine or engine where the steam expands to generate mechanical power.

The following equipment can be connected to the steam turbine:

• A generator for electricity generation; or
• A mechanical drive for operating pumps, fans etc.

Possible applications include:

• Generating high- or low-pressure process steam;
• Heating process water, thermal oil, process fluids or chemicals;
• Preheating fuel/combustion air; and
• Generating cold for cooling processes.

What does waste heat recovery offer?

The implementation of additional equipment always needs a justification before setting up a project. In case of WHRS, the justification comes down to the fuel-efficiency increase and the following advantages:

• ‘On-hand’ energy can be recuperated for generating electricity, mechanical power, process steam, hot water, hot thermal oil, hot or cold air.
• The ‘free’ energy is available when needed as the exhaust gas flow often correlates with the power need.
• Adaptation to load fluctuations and quick start-up are possible.
• Most of the equipment is available ‘off the shelf’ with only the waste heat recovery boiler/heater needing to be customised to accommodate the flue gas composition.
• Retrofitting of existing systems is possible.
• High automation is possible and many industrial boilers operate 24/7 without supervision.
• Future carbon taxes may be reduced according to the efficiency increase.

Optimising boiler parameters

Increasing the boiler performance basically means the installation of additional heating surfaces for preheating fuel and/or combustion air and increasing the steam parameters, especially steam temperature and pressure.

Another way to reduce operational costs is to optimise the heating surface arrangement to reduce fouling and dust build-up. Depending on the flue gas composition fouling can be a serious issue which causes a considerable percentage of the maintenance costs

Feed water preheating

Old steam generators were designed to minimise investment and operational cost consequences were not seriously considered so many boilers/heaters don’t have economiser heating surfaces to preheat the feed water. In most cases, retrofitting an economiser is possible and this can increase boiler performance by up to 10%.

Fuel/combustion air preheating

Different ways of preheating fuel and combustion air are possible. One is by installing additional heating surfaces at the cold-end of the boiler/heater for using the ‘low’ temperatures. This is a typical approach for consumable preheating of up to 250 °C.

The second way is installing the heat exchanger surfaces directly behind the heat source for using the high temperatures. This is beneficial for rising consumable temperatures up to 500 °C.

Usually high-temperature preheating of combustion air is conducted with a recuperator. In a gas turbine application a recuperator captures waste heat in the exhaust stream to preheat the compressor discharge air before it enters the combustion chamber. However, recuperators can be installed behind all heat sources including oil, gas and coal firing.

Figure 2: Compact version of a recuperator/steam generator unit. The system operates behind a gas turbine with a flue gas flow of 47.3 kg/s at 497 °C. Passing through the recuperator the exhaust temperature decreases to 399 °C whereby the compressor discharge air is increased from 338 to 450 °C. In total, an additional 5.1 MW is added to the combustion air before entering the turbine combustion chamber. The flue gas temperature at the recuperator exit is still high enough to operate a steam generator. The exhaust enters the boiler at a temperature of 398 °C and the temperature further downstream allows the generation of 19.2 t/h superheated steam at a temperature of 200 °C and 10.5 bar. This is an additional energy output of 12.7 MW. In total, 17.8 MW are provided for enhancing process efficiency.

Where to now?

Several technical solutions are available to enhance the energy efficiency in the food processing industry and the good news is that there is no risk regarding the operational reliability of small- to medium-scale waste heat recovery systems, recuperators and multi-fuel boilers/heaters. The technologies have already been implemented by many industrial companies throughout the world.

The economic advantages from fuel savings and carbon reductions are obvious but simultaneously the food processing industry can deliver environmental advantages simply by using fossil fuels as efficiently as possible.

* The ERK Eckrohrkessel GmbH (www.eckrohrkessel.com) offers licences to manufacture Eckrohr boilers worldwide as well as engineering services concerning industrial boilers/heaters and thermal energy equipment for more than 30 years. Eckrohrkessel has already designed more than 5700 industrial boilers and is a global leader concerning exotic fuels and applications.

** Gasco Pty Ltd (www.gasco.net.au), designs and manufactures a wide range of heat transfer and combustion systems. Gasco has supplied over 200 API 12K water bath heaters mainly for heating high-pressure gas and crude oil. Next to water bath heaters, Gasco supplies fired heaters, thermal oxidisers, flares and hot oil heating systems to over 18 countries.

Waste heat recovery boiler behind a Caterpillar G3520 gas engine

Commissioned in Chile in 2004, this relatively small unit demonstrates what is already being realised in countries with higher energy prices.

The exhaust gas flow of 9471 m³/h (3.35 kg/s) enters the boiler at 469.0 °C and leaves the system after cooling down to 167.3 °C. This temperature downstream allows the constant generation of 1.6 t/h saturated steam (saturation temperature 191.6 °C) at 13 bar.

In this project the task was to produce low-pressure process steam but high-pressure superheated steam or hot water would also be possible. The energy can directly supply an assigned process or feed into the existing steam/hot water net on the site. Also, the generation of electricity is possible, which is particularly interesting in remote areas. The 1040 kWh boiler capacity would allow the electricity generation of approximately 250 kW.

Figure 1

Generating the 1040 kWh with fuel oil would require 102 L/h or 100 m³/h natural gas. This in turn would cause ~260 kg CO₂/h from oil firing or ~145 kg CO₂/h from natural gas firing. This may not sound a lot but assuming an annual operation time of 8000 hours the carbon reduction potential is 2100 Mg from oil firing and 1150 Mg from natural gas firing. At expected carbon costs of AU$20/Mg this equates to avoidable annual costs of AU$42,000 using oil and AU$23,000 using natural gas. Fuel savings come on top of the carbon emission savings but are not specified here as fuel costs vary across Australia.