Cogeneration in food and beverage manufacturing

It seems only a few months ago that major industrials were concerned about electricity price rises and saw gas as an easy option. With the emergence of competition for new gas supplies from large export projects, expectations are for a seller’s market and we now see predictions of significant gas price rises over the next few years. In this article, pitt&sherry Principal Consultant Dr Steve Edwards discusses cogeneration as an alternative form of energy generation in food and beverage manufacturing.

Cogeneration is becoming an increasingly important option to consider when designing and delivering new plant in the food and beverage manufacturing industry, according to engineering consultancy pitt&sherry.

Whether it is designing a greenfields start-up or smaller plant expansion, there are vital energy questions in regards to economics and the environment for developers to consider. According to Dr Steve Edwards, with the current volatility in the energy sector it is attractive for developers to turn to options more within their sphere of operations.

“Cogeneration and trigeneration are in your control and becoming increasingly attractive from a long-term management perspective,” explained Edwards.

“However, it generally takes a more immediate ‘kick in the pants’ for decision-makers to move out of their old procurement habits.

“More important spurs in making the choice are the immediate economic and environmental gains.”

Typical centralised electric power plants produce lots of electricity, but also produce a lot of heat and expend a fair bit of energy getting rid of it. The heat is not used and makes the production energetically inefficient, and a high degree of electricity is lost along the transmission lines to the plant. Edwards explained that a centralised plant works economically because the energy supply is historically cheap and the operations are a long way along the learning curve.

“But what happens if use can be made of the heat?” questioned Edwards.

“Cogeneration basically takes the generation of electricity to the site and allows the heat to be used rather than expending energy getting rid of it.

“If it is done correctly this is heat that would have been normally paid for in the boilers.”

For dairy processing, trigeneration is particularly pertinent, Edwards continued, as it uses heat to drive refrigeration and is well placed in the cycle for efficiencies. According to Edwards, trigeneration using absorption chillers needs very low-grade heat and is able to ‘scavenge’ what is left over after heat has been used for steam and hot water production.

“This option is energetically efficient and good for the environment - using less energy from traditional fuels means less greenhouse gas emissions,” explained Edwards.

“But is it going to work for you economically? These plants are not cheap and they require additional skills and maintenance.”

Economic decision

Even if the capital cost of a plant is not a concern, the greater issue is that energy supply is not as cheap as that used at a centralised power plant.

According to Edwards, to make it all pay back in an appropriate time requires efficient design, efficient use and the right energy prices.

He believes the first element in that efficiency is likely making sure all of the energy that comes in is being used.

“A powder plant may have little need for refrigeration, other than in milk receival and storage, while a cheese plant can be expected to require a high level of temperature control and a high level of refrigeration usage,” said Edwards.

“Conversely, the powder plant will have a higher relative heat demand. If refrigeration is taken out of the picture, each may use about the same amount of electricity relative to milk inputs.” It is important to make sure the plant being considered matches your needs, added Edwards.

Fortunately, there is a large variety of plant sizes, from the engines and turbines generating the electricity through to heat recovery steam generators, heat exchangers and absorption chillers.

“How we match up these differing demands is actually a relatively simple, but highly detailed, process,” said Edwards.

“The overall aim, not surprisingly, is not to waste anything, including capital.”

Factors to consider

It is difficult to get economies of scale if plant is too small, Edwards explained, adding that there is a maintenance requirement at the least for these machines.

“Even if this is outsourced it will require a site presence that will need to be paid for, along with travel costs, regardless how brief the time on site,” Edwards said.

“There are installation costs that will have economies of scale and plant costs that have economies of scale.

“Up to a point, larger plants will do better economically. An unfortunate reality, but possibly a useful rule of thumb is that plant sizes under 1 MW will struggle.

“It’s not that you shouldn’t look at it, but it is one of the risk factors.”

Usage also needs to be relatively constant for the plant to pay for itself, according to Edwards.

There are also technical requirements around the way plants operate that mean they run better, cleaner and with less maintenance if they run consistently.

Where there is a peaking requirement there is a need to provide for that through various solutions.

Edwards regards the spark gap as the ‘biggy’ when it comes to assessing how to approach generation in a food or beverage facility.

“It’s actually not independent of these other factors since only in extreme circumstances will it kill a project,” said Edwards.

“But if your plans have survived the stages of sizing and peaks and troughs then it is frequently the killer. It is also the most external of the three factors and the easiest to model.”

The spark gap is the difference in energy pricing between the chemical energy source, such as natural gas, biomass or syngas, and the cost of electricity delivered to site. It effectively dictates the final profitability of any cogeneration or trigeneration operation.

The chemical energy is the same fuel used in boilers or steam generators on-site and the electrical supply is what is used when in operation.

Once a facility has made the most effective use of its resources, a spark gap is still required to make it all work.

Where does that leave us?

The environment wants cogeneration or trigeneration to be installed, but the economy may have a different view, explains Edwards.

In order, the main risk factors and limitations for a greenfields site are scale of operations, peak matching and energy pricing.

“If you have a new demand with a reasonable scale of need, low hourly variation and good access to, say, piped natural gas at reasonable rates, then it is something worth doing more detailed study on,” said Edwards.

“If you are not fortunate enough to have these three, there are likely to be technical solutions that help - but at a cost.

“It has the potential to improve cost-effectiveness, help you look good and help save the world for your children - all at the same time.”