The low-down on rice straw for power-gen

By John Diecker, Lee Enterprises Consulting, Inc.

Special to The Digest

World rice production for the 2016/2017 crop year is estimated by the United States Department of Agriculture to total 481.5 million tons. The vast majority of this will come from Asia where, in addition to being a major food crop, its cultivation has strong ties to society and tradition. In tropical regions two, and in places three, crops per year are sometimes possible.

The straw-to-grain ratio of rice varies widely depending upon plant variety and location. Using a conservative figure of 1.0 implies that approximately 481.5 million tons of straw is produced annually. Some of this straw sees use as animal fodder or bedding but most of it is unused. In some areas, it is burned in the fields for disposal; creating local air quality problems in the process.

For discussion purposes, if we assume that 30% of rice straw produced is available for power generation, that the straw has an energy content of 14 MJ/kg and that power plants have an energy conversion efficiency of 35%, then straw alone should be able to generate over 200,000 GWh per year of electricity.

Of course, it’s never that simple….

Straw and Ash Characteristics

Potential users of rice straw are faced with the same seasonality, collection, transportation and storage issues that accompany many other types of agricultural waste. The low density of straw presents additional complications.

Traditional manual methods of harvesting rice result in the gathering of a large amount of straw as a matter of course. This straw though, being collected by hand, is not easily baled. Increasingly, mechanical combine harvesters are being used. This type of collection results in the straw being left in the field where it can be baled but the typically small sizes of rice fields in many areas are a can make this difficult. Nevertheless, mechanical harvesting can only increase in popularity throughout the region.

Rice straw contains a significant amount of chlorine and has a high alkalinity. Its ash content can be as much as 20% and that ash contains large amounts of silica and potassium.

Technologies

The technologies commercially available for converting rice straw to electricity are the same as those used for other types of agricultural waste.

Combustion. Boilers and steam turbines are commonly used for biomass power projects. The technology is well known and reliable. The characteristics of rice straw do present some challenges though. The straw alkalinity can cause accelerated corrosion in the boiler and the low melting point of the straw ash leads to slagging and fouling.

Where straw is concerned, combustion technology is mostly used for projects in the 10 to 20 MW range or even larger. This is possible in China and India where there are vast quantities of straw available. For various reasons, presumably economic, there do not seem to be many manufacturers of equipment for the smaller, 1 to 2 MW type projects typically encountered in other parts of Asia.

Gasification. For smaller projects, gasification would seem to be an appropriate solution. As with boilers, straw alkalinity and slagging are matters of concern for gasifiers. In addition, operating the gasifier at the relatively lower temperatures required to prevent agglomeration of the ash results in a tarry, low quality, producer gas.

In Southeast Asia, biomass gasification projects have been based around updraft gasifiers supplied from China and India that can be described as “basic” and lack sophisticated controls. The gas produced is not sufficiently cleaned prior to use, necessitating frequent engine maintenance. As a result, these projects have historically experienced very low availability to the extent that they become commercially unviable. Decommissioned and abandoned biomass gasification equipment litters the countryside.

Pyrolysis. Because slow pyrolysis produces a liquid fuel, it avoids some of the problems associated with thermal gasification. This liquid pyrolysis oil cannot be utilized directly in an internal combustion engine-driven generator set however. Additionally, the moisture level of the straw must be kept quite low for good pyrolysis. Options for using or altering the pyrolysis oil are available but it would seem that this technology may best be considered a type of fuel pretreatment as far as power generation from biomass is concerned.

Anaerobic Digestion. Biogas can power an internal combustion engine directly and this seems to be a viable small-scale option. The anaerobic digestion process requires that the straw be mixed with other substrates, typically some kind of animal manure, which tends to limit the size of the project.

Fuel Pretreatment

In other than some larger scale plants where bales of straw are fed directly to the boiler, rice straw requires some level of pretreatment prior to use. Normally this will involve chopping or grinding to reduce the particle size for more effective combustion or to assist in breaking down the lignin wall for anaerobic digestion. It frequently also involves drying and densification. Other processes, including the following, could further increase the quality of rice straw as a fuel.

Washing/Wet Storage. It has been demonstrated that leaving straw exposed to rain in the field before collection significantly reduces its alkalinity. The moisture content of the straw needs to be kept low though, for efficient combustion or gasification.

Wet storage of the biomass is another option for reducing the straw alkalinity. The leachate from the process must be collected for disposal or other use however. This technology is probably best used in conjunction with anaerobic digestion.

Torrefaction. Pretreating the rice straw by torrefaction makes the biomass a much easier fuel to densify and burn or gasify. It also allows for more efficient storage and increases the possibility for mixing with other types of biomass.

Finally

While the technical difficulties in utilizing rice straw as a fuel for electricity production are considerable, they are not insurmountable. As with most agricultural waste, the biggest obstacles to overcome involve the collection, transportation and storage of the biomass. Getting farmers interested in taking the time and effort to collect the straw is a challenge. Further, once the suppliers have a captive customer for the straw, the price of that straw will invariably increase. This is regardless of the terms of any fuel supply agreement.

The author would certainly appreciate hearing of others’ successes, failures and thoughts on this topic.

John Diecker is a Thailand-based electric power and renewable energy consultant with Lee Enterprises Consulting, the world’s premier bioeconomy consulting group with over 100 consultants worldwide. The group has expertise in many areas, including in the subject discussed in this report.