Producing biomass: Biomass types: Crop residue: Stubble and straw
Stubble and straw are by-products of harvest.

Stubble consists of chaff (the material in the head that is separated from the grain at harvest), leaves and stalks. It can be used as a feedstock for producing biofuel.

Straw, the dried stalks of cereal plants such as wheat, is preferred over leaves and chaff for bioenergy production.

Benefits

With many grain crops, over half the above-ground biomass is not harvested.

Some growers leave it on the ground as mulch to prevent wind and water erosion, reduce evaporation, maintain soil carbon and recycle nutrients.

Straw is nutritionally void, so it cannot be used for animal feed. All the nutrition is in the seeds, which have been harvested from the stalks.

In high-rainfall zones, heavy stubble can clog up machinery, and harbour pests such as mice slugs and weed seeds. So growers in these areas usually burn it.

By selling straw for bioenergy production, while keeping some in the paddock, you may be able to save on the costs of burning, mulching and harrowing.



An example of a straw feedstock project is described in Section 5.6 (page 76) of the report, A bio-energy roadmap for South Australia, which was prepared for the state government in August 2015.

The report also describes the technology options for producing energy from straw and outlines what European countries (Denmark, UK, Spain) and China are doing to produce energy from straw.



Biodiesel self-sufficiency is achievable for most grain farms, according to a 2008 CSIRO study, Bioenergy opportunities for grain growers, which covers energy efficiency, regional biodiesel self-sufficiency and biofuels production. However, the study concluded that at the time the economics were not favourable.

Evaluating the economics

The delivered cost of cereal straw biomass (with moisture content of 10–14%) is estimated to be within a range of $53 to $138 per tonne (see Chapter 11 of Bioenergy in Australia: Status and opportunities, published by Bioenergy Australia in 2012. This includes the cost of collecting, baling and delivering the biomass.

This analysis is for cereal crop straw grown in the 450–600 mm annual rainfall zone of broadacre agricultural regions of southern Australia.

Estimating yields is covered in Section 11.2.3.

The delivered cost of straw bales is described in Section 11.2.6.

For a full list of references, refer to Section 11.5.

If straw harvesting becomes a regular practice, to maintain soil fertility you will need to replace the nutrients removed from the field in the straw. The cost of replacing nutrients is described in Section 11.2.4.

Being aware of the status of nutrients in your soil is essential so that you can anticipate and address fertility problems. How much fertility are you removing when you export hay and stubble from your farm? [PDF 95 kb] is the subject of the DAFWA Farmnote no. 97/2001. It describes:
  • how to determine amounts of nutrients and lime removed in stubble
  • when to replace nutrients
  • how to estimate the cost of replacing nutrients
  • how to estimate the grain yield cost of not replacing nutrients.


Biomass resources, local issues, and opportunities in the WA Wheatbelt are discussed in this 2009 thesis: Biorefining in the wheatbelt: A scoping study for assessing the feasibility of new biomass projects in Western Australia’s wheatbelt region.

Some of the information is a little dated now but there is useful information in there. The author seeks to provide a framework for developing projects that deal more broadly with biomass resources, looking for technical and economic synergies, and grouping processes so as to maximize the value extracted from the resource to improve viability of projects.

Harvesting stubble and straw

The harvesting process

One-pass grain and stubble harvest systems have been developed which deliver grain and large square bales of straw and chaff as separate streams of product. Both one-pass and two-pass scenarios are described in Chapter 11 of Bioenergy in Australia: Status and opportunities, published by Bioenergy Australia in 2012.

How much stubble do you leave on the paddock?

CSIRO’s assessment of the availability of crop stubble as a potential biofuel resource states the following:
  • There is little published information about the implications of harvesting stubble.
  • Not all of the non-grain above-ground biomass of a crop will be available for collection.
  • Cereal stubble consists of chaff (the material in the head that is separated from the grain at harvest), leaf matter and straw.
  • Wheat stems (straw) are preferred over leaves and chaff for bioenergy production.
  • The most common method of estimating stubble production is from grain yield data and knowledge about the ratio of grain to total above-ground biomass (known as ‘harvest index’, or HI).
  • The amount of straw kept on the paddock will vary depending on soil type, topography and rainfall conditions. In southern areas, protection from wind erosion over the summer fallow is a priority and growers may keep about 1 t/ha of stubble. Where protection from intense rainfall is a priority, growers are likely to keep larger amounts of straw.
  • Removing significant amounts of biomass is likely to lead to soil carbon depletion.
  • Retaining loose chaff, leaf and stubble to protect the soil will help maintain soil carbon but little is known about how much stubble needs to be retained to maintain soil carbon in the long term.

Supplying stubble and straw

Supplying and delivering cereal crop residues and grass crops, including stubble and straw, is described in detail in Chapter 11 of Bioenergy in Australia: Status and opportunities, published by Bioenergy Australia in 2012.

Baling, cartage, storage and delivery to processor are covered in Section 11.2.5.



In Western Australia, about 10 million tonnes of cereal straw are produced every year, on average.

DAFWA is mapping potential biomass collection points in the WA wheatbelt, and this information can be made available to investors.

Contacts

David Hall

Energy Developments and Resources (EDR)

4 Glenneth Court, Bonny Hills NSW 2445,
Phone: +61 (0)2 6585 5368

david.hall@energydr.com.au
Giles Perryman – Refgas Australia

Refgas Australia

Dunsborough, WA. 6281,
Phone: 0447 393 363

refgas@askwm.com
Col Stucley

Enecon

Suite 5, 651 Canterbury Road, Surrey Hills, Victoria 3127
Phone: +61 (03) 9895 1250

cstucley@enecon.com.au
EDR Energy Development and Resources Pty Ltd.

Contact: David Hall

4 Glenneth Court, Bonny Hills, NSW 2445
Phone: +61 (0) 26585 5368 and +61 418206293

david.hall@energydr.com.au
Tom Vogan

Director, Energy Farmers Australia

Geraldton, Western Australia,
Phone: 0418 735 336

tom@energyfarmers.com.au
Ian Stanley

Rainbow Bee Eater, Kalannie, WA 6468
Phone: 08 9665 1024; 0428 910 351

mooredale@bigpond.com
Colin Peace

Jumbuk Consulting P/L

PO Box 2252, HAWTHORN, VIC 3122
Phone: 0413 835 793

colin@jumbukag.com.au
Euan Beaumont

Director, Energy Farmers Australia

Geraldton, Western Australia,
Phone: 0427 611 424

euan@energyfarmers.com.au