Testing plant materials for biofuel potential: Mackay pilot plant

Testing plant materials for biofuel potential: Mackay pilot plant

The QUT Mackay Renewable Biocommodities Pilot Plant, hosted by the Mackay Sugar Racecourse Mill, tests how plant material can be turned into ethanol and other products.

Key points
  • The pilot plant converts plant material to ethanol, testing which physical and chemical conditions produce the most ethanol for the smallest cost.
  • The process is versatile. Many different crops and crop residues can be used, and products other than ethanol can be recovered or produced from the by-products.
  • Fibre crops such as sunn hemp and kenaf have been tested and could also have economic and agronomic benefits for cane growers.

 

 

 

What the pilot plant does

The Mackay Renewable Biocommodities Pilot Plant is a unique research facility which processes waste biomass to produce ethanol and other high value bio-products.It is located at Mackay Sugar’s Racecourse mill.

Pilot plant technologist Peter Albertson says, “People are looking for cost effective ways to produce ethanol from biomass. That is what we are here for: to do the research to try to minimise the cost to produce that.”

When the process is optimised and cheaper, more companies will be interested in producing ethanol from biomass.

Staff at the pilot plant are keen to engage with grower groups who have a biomass waste they are interested in testing. They can help growers understand the technology and the economics of various biofuel production processes.

Peter and Heng operate and manage the pilot plant.

The plant’s scale is an intermediate between laboratory and larger industrial demonstration or commercial application.

“You can’t take what’s done at laboratory scale straight to commercialisation because you never know what you are going to get. That’s why the pilot plant is here,” says Peter.

The processing at the pilot plant more closely approximates what happens at a commercial scale, although biomass is processed in batches of 20 kilograms instead of continuous or semi-continuous operations.

The plant can operate under many different chemical and technological conditions depending on which biomass is being tested.

“We can use any fibre here; the conditions just need to be modified and perfected for those particular plant material types,” says pilot plant manager Heng-Ho Wong.

Most companies choose to test pretreatment processes at the pilot plant—the first step of treating biomass with chemicals to make it easier to break down later by enzymes. However, the testing goes through all steps to ethanol production, such as enzyme digestion and fermentation.

Heng can control the plant’s operating conditions from the computer.

“We can take the material that we produce from the pretreatment to the next stage and follow it all the way to the ethanol production, or clients can take away the materials produced for their own use,” says Heng.

Most of the time, the process performs more efficiently and faster than in the laboratory (usually 15 minutes for pretreatment). Up to five tests can be run per day, with a total of 100 kilograms of biomass processed.

Biomass types

The pilot plant has used the following biomass sources to produce ethanol:

  • sugarcane bagasse
  • sweet sorghum
  • eucalypt wood chips
  • corn stover (post-harvest residues of stalk, leaf, husk and cob)
  • sunn hemp
  • Bana grass (cow cane)
  • elephant grass
  • banana fibre.

Bagasse is a light, dry material.

The main waste plant fibre processed is sugarcane bagasse from the mill, which is why the pilot plant is based at Mackay Sugar’s Racecourse Mill, the source of the fibre.

“There is no limitation; we can use any biomass in the pilot plant,” Peter says. It is just a case of testing and improving the conditions of processing to get the best yield of ethanol.

The bagasse is a waste product, just as eucalypt thinning from the pulpwood industry can be used for ethanol.

The critical factor, Peter says, is using a biomass which is cheap—particularly for conversion into ethanol.

For biomass such as sweet sorghum, growers can get grain from the crop, juice can be extracted from the stems, then the leftover fibre can be used to produce ethanol. In this case, the biomass used isn’t just ‘waste’.

The pilot plant has also produced biodiesel from vegetable oils (cotton seed and canola oil) and other products are in the pipeline.

Any plant material can be processed: it’s just a case of finding the ideal chemical and physical conditions.

The products

The main aim of the technology used in the pilot plant is to get all the sugars out for biofuel production—specifically, ethanol.

By breaking down the fibres to release their sugars, then fermenting them, ethanol is produced. Ethanol can be used as fuel for cars or for industrial or pharmaceutical applications.

The production of the ethanol from biomass is quite costly, so any additional products that can be produced offset the cost of ethanol production. Large processing plants that produce ethanol routinely have areas which recover other, high-value products.

Valuable byproducts that can be extracted include:

  • Lignin, the plant stem’s natural ‘waterproofing’ chemical. Lignin can make up 20 to 30% of the biomass. It can be turned into biodegradable wax, coatings and waxes, bio-plastics and more.
  • Other chemicals within the plant material that can be concentrated and processed after extraction.

The pilot plant uses waste fibres to produce ethanol, which has environmental advantages over using food sources such as sugar to make ethanol (as happens in Brazil) or corn (as happens in the USA).

“It is much better for future populations and the environment if we are using plant waste for the ethanol rather than taking away a possible food source,” Peter says.

Opportunities for Queensland growers and mills

At the moment, many different crops are being studied for their potential as sources of biomass for ethanol production, such as agave, sunn hemp, elephant grass and corn stover.

In the future, Peter believes that mills on the east coast may be able to have associated ethanol plants.

“There are many sources of biomass which could be grown in Queensland. In semi-dry areas you could have sweet sorghum produced, where the grain is used, the sugar is pressed out from the stem, and you have the fibre which could be converted to ethanol as well,” Peter says.

“In areas that are drier there is agave, which could also be used. Those areas could be determined by rainfall maps and how much water is available to grow these different plant species.”

The biomass would then be sent to areas with larger mills.

Location and infrastructure

The pilot plant is located at the sugar mill because of the ready access to biomass and, specifically, the associated infrastructure.

Sugarcane billets (pieces of stalk) are transported by small rail trams from all over the region to Racecourse Mill. After processing for sugar, some of the leftover bagasse is transferred to the pilot plant. Most bagasse is burned for cogeneration.

A critical part of biofuel production is keeping cost of sourcing biomass down, which is essentially what happens with bagasse at the pilot plant. “Being backed onto the mill site here, we get the fibre at very low cost,” Peter says.

If a larger scale commercialised biofuel plant was developed at the mill, more crops could be planted or used for this purpose. The cane network infrastructure can be used in the off-season as well.

Other facilities which may produce higher value byproducts could also be attached to a larger commercial plant. Peter says: “Two or three other factories backed onto the mill could then use that fibre or the waste streams to produce new products.”

Mills throughout Queensland could be used in similar ways to cater for use of the bagasse that is produced in other regions.

How the process works

At the moment in the pilot plant, biomass is processed in small batches.For bagasse, after the sugar is milled out it, the stalks come to the plant crushed up and washed.

The bagasse has virtually no sugar left in it—except within the cell walls that make up the fibre itself, and this is what the process is after.

Turning biomass to ethanol takes three main steps: treating the biomass with chemicals and steam to make it easier to break down, using enzymes to make sugar molecules into even smaller ones, then using yeast to ferment the sugar into ethanol.

1a. Pretreatment

This step involves treating the biomass with chemicals to make it easier to break down later.

20 kilograms of bagasse is put into a hopper which leads to a conveyor belt. The conveyor belt loads the biomass into the horizontal reactor (where the first step happens).

The reactor is made of a metal that doesn’t corrode even under very corrosive conditions.

The reactor is pre-heated to a temperature between 135 and 175 °C by steam injected into the chamber.

Then chemicals are pumped into the chamber, and a stirrer mixes the heated biomass and chemicals at a constant temperature. This usually lasts about 15 minutes.

The treated biomass is then pressed to separate liquid (hydrolysate) from the solid residue.  Depending on the conditions for the process, the material may then ‘washed’ to recover the initial chemicals or other products. Certain types of chemicals can pull different sugars out of the plant materials, such as lignin or hemicellulose.

1b. Steam explosion

After the biomass has been treated with chemicals, the material is pushed from the horizontal reactor into a vertical reactor. Steam is injected to heat it to a very high temperature.

When the material is very hot, the pressure within the vertical reactor is increased to a very high pressure (normally 20 bar) by injecting more steam into it.

The pressure is then released very quickly. When this happens, the water within the cells of the fibres expands rapidly, turning into water vapour and busting open the cells.

This shatters the cells to break the fibres down even further.

2. Enzyme digestion (hydrolysis)

This stage breaks larger sugar molecules into smaller molecules so that they are easier to ferment. The sugar molecules come from the plant fibres.

Enzymes, which are substances that speed up chemical reactions, are added in a mixing chamber to the pulp created by the pretreatment process.

The pulp starts out solid and lumpy, but within three hours it has begun to break down into liquid; more at six hours, and very little solid is left after 24 hours.

3. Fermentation

In this stage, yeasts are used to convert the sugars in the liquid to ethanol. The concentration of ethanol can be up to 10% after this process. It is then purified for use as biofuel in a distillation column.

The plant material starts out in the hopper.

The biomass is loaded into the horizontal reactor.

The horizontal reactor

This material is what comes out of the vertical reactor after the steam explosion.

After the steam explosion, the material is removed to be put into a mixing chamber for enzyme digestion.

Fibre crops can add value to sugar

Local canegrower Joe Muscat has provided sunn hemp and elephant grass to the pilot plant for testing. The plant operators organised the harvesting.Joe lives at Oakendon, about 35 km south-west of Mackay. Traditionally the Muscats would have about 40 hectares of land under non-sugar crops.They now have several hectares each of seed crops for sunn hemp and kenaf, plus a couple of hectares of soybeans.

Through a Nuffield Scholarship, the Sugar Research and Development Corporation (SRDC) supported Joe to research best practices in producing, manufacturing and marketing fibre crops.

Fibre plants can be converted to many products including ethanol fuel, building materials, car components, bio-plastics, paper, absorption materials and geo-textile matting. They are also beneficial rotation crops with sugar.

In 2002, when sugar prices were down, the Muscats were investigating diversifying and adding value to their sugar operation. Most growers in far north Queensland produce only sugar cane or rotate sugar cane with soybeans.

“We looked at which crops had potential that we could grow here in the central region, and fibre was one that stood out,” Joe says.

Since then, the Muscats have produced kenaf, industrial hemp and sunn hemp (Hibiscus cannabinus, Cannabis sativa L. subsp. sativa and Crotalaria juncea respectively).

“We are probably producing a little less sugar, but we are doing that more economically. So our inputs are reduced and our production costs are being maintained,” Joe says.

Grower Joe Muscat

Kenaf (Hibiscus cannabinus)

Energy from fibre crops

Joe Muscat sent 200 tonnes of kenaf to the pilot plant in 2012 and 70 tonnes of sunn hemp in 2013 to investigate their potential as a fuel for cogeneration at the mill.

“It went through the handling process in the mill quite well”, he says. “What it can produce in energy is up there with bagasse, so that is very encouraging.”

1 kg of kenaf or sunn hemp can produce about 18,000 kilojoules of energy. Bagasse produces similar amounts of energy.

Sunn hemp also has acceptable chlorine residues (which can corrode boilers), which is another important criterion for bioenergy.

Using sunn hemp as a legume rotation crop, Joe says, is “really the low-end value of what its potential is”, given the wide range of materials possible from the crop.

Sunn hemp (Crotalaria juncea)

Economics of growing fibre crops

As part of the research project funded by SRDC, Joe Muscat is developing a business case that lays out the economics around the supply of sunn hemp in a large quantity by other farmers.

The business case examines:

  • economics of the production system
  • transporting the product
  • flow-on benefits
  • producing the crop
  • handling the crop
  • equipment
  • local infrastructure.

“We need to do the business case to make sure that it stacks up,” Joe says.

Growers need to identify which markets they will pursue at a commercial level that makes it profitable, with the infrastructure to sustain the production system, Joe believes.

Bioenergy and biofuel projects are inherently long term, usually because of the need for large amounts of capital.

Fibre crops can be grown on fallow land, which is generally about 12% of the total cultivated area on a farm.

“Sugar is going to be the baseline of our business, because this region has got the infrastructure”, says Joe. “But the world market is a very volatile system. To secure the business here I believe we need to have a third of the income stream coming from other crops, and that’s what we are going to work towards.”

The fibre project started in 2011 through Mackay Fibre Producers and the Grower Group Innovation Program.

Growing fibre crops in north Queensland

The Muscats sow fibre plants in October. When the days are long, they can produce more than 80 tonnes wet weight per hectare of sunn hemp — up to about 20–22 tonnes of dry matter per hectare — in the relatively short time of 120–130 days.Joe has measured kenaf growing at 66 mm per 24 hours. “It is a rapid accumulator of biomass.”

Sunn hemp also has the ability to fix about 300 kg of nitrogen, which is valuable for the following cane crop.

“We see flow-on benefits in our cane crops after rotating with other crops. We have improved our soil structure and our production results,” Joe says.

Sunn hemp is a nematode-resistant crop, so it is a beneficial crop to rotate with when nematode counts are high. Sunn hemp can be successfully harvested with a forage or cane harvester.

Sunn hemp is particularly good to grow in dry areas — it doesn’t like ‘wet feet’, says Joe. However, “we know that this crop can grow in this region very well and we can produce biomass at world standards. We can replicate that here in this region.”

In terms of fertiliser, the Muscats add some nitrogen and potassium to the crop.

Two pests are an issue for sunn hemp seed crops: pod borers and tiger moths. Tiger moths lay eggs into the flowers. However, Mackay Fibre Producer members have learnt how to manage them with insecticides.

Sunn hemp is easily harvested with modified equipment.

Find out more

Maximising sugar mill revenue in Mackay

Contacts

Ian O’Hara, Mackay Renewable Biocommodities Pilot Plant, Centre for Tropical Crops and Biocommodities, Queensland University of Technology
Phone: 07 3138 1551, 0437 541 295
Email: i.ohara@qut.edu.au
Joe Muscat, Mackay Fibre Producers Group
Phone: 0429 377 162
Email: josephm@jcsenterprises.com.au