Ahead of His Time

The idea of wet biomass storage has yet to catch on in the cellulosic industry, despite long-term use in the pulp and paper industry.
By Holly Jessen | September 16, 2014

Jim Hettenhaus is a passionate advocate of wet biomass storage. A longtime member of the ethanol industry who helped plan the first Fuel Ethanol Workshop, he is the managing director of CEA Inc., a consulting business.

Hettenhaus has been talking about the advantages of wet biomass storage over dry bales for years now. It’s a sustainable, economic and reliable method with a favorable life cycle with the added benefit of being used successfully on sugarcane bagasse bound for pulp and paper mills, he says. Back in the late 1990s, he started work with others on a National Renewable Energy Laboratory-sponsored study of wet biomass storage, followed by doing work for the USDA in the 2000s. He’s spoken about the advantages of wet biomass storage at multiple conferences, including this year’s FEW. He has also had many conversations about it, some with developers actively working to build cellulosic ethanol plants.

So far, however, the major players in building cellulosic ethanol plants are utilizing dry bales for feedstock storage. Hettenhaus believes that’s because change is difficult to embrace as dry bales is the storage method understood in the agriculture industry. “It’s so easy to do what they are doing now,” he says.

In addition, nearly two decades ago, when he started investigating the idea of wet biomass storage, it was thought that the cellulosic industry would be established by the mid- to late-2000s. Now the reports commissioned by the USDA and NREL are many years old and contain dated information about transporting biomass via railroad, a proposition that obviously wouldn’t be viable today. Still, the concept of wet biomass storage is as relevant as ever. “I just think that I was too soon, too soon, too soon,” he says.

One of the big advantages of wet biomass over dry is protection from the risk of fire, he says, citing multiple instances of bale fires. In addition, wet biomass takes 10 times less storage space than dry biomass and results in less than 5 percent storage loss, compared to more than 10 percent in dry bale storage. It also allows biomass to be collected wet in the field, rather than relying on the uncertain schedule of field drying. For a reliable supply of biofuel feedstock, Hettenhaus doesn’t believe dry bales are the way to go, with the exception of Idaho and further west, where conditions are dry and irrigation is used.  Finally, it has a positive effect on processing cost due to reduced ash and soluables content, improved feedstock quality and increased feedstock yield.

From 2005 to 2008, a stover storage validation trial was conducted using a modified version of the Ritter method. About 700 dry tons of corn stover was piled 35 feet high in Imperial, Nebraska, and samples were analyzed and converted to ethanol at a Purdue University laboratory, using standard methods. Although there are many methods for wet biomass storage, Hettenhaus says, one is to wash and shred the biomass, pumping a slurry with 3 percent stover solids to a storage pad. As the liquid drains though the pile, it is recirculated until the pile reaches the desired height. Water circulation compresses the pile and removes dirt and soluable material that would have to be hauled away at the end of the process anyway. “I’m not a plant operator anymore but if I were, I’d jump up and down, I’d say, ‘Why aren’t you washing this? … You need to remove this before it gets into the plant,’” he says, adding that the residue removed contains nutrients, which can be returned to the fields as fertilizer.

Another big advantage was that the holocellulose (cellulose and hemicellulose) concentration increased from 59 to 68 percent. And, fermentation actually starts in the pile. “Wild organisms begin the fermentation of the small amount of soluble sugar in the pile, forming organic acids,” he says. “The acids lower the pH and the anaerobic conditions preserve the corn stover pile.”

The Ritter method works on corn stover, straw, grasses, kenaf, and bagasse. It’s not a new storage method. “Bagasse is the feedstock most widely stored in this manner to supply pulp mills in sugarcane growing areas (such as) Central and South America, South Africa, India and Thailand,” he says, adding that the method has been used for more than 50 years.

The NREL sponsored report, which was published in 2003, included information about the results of successful wet storage studies that were conducted using sugarcane bagasse. “Between 1920 and 1960, many skeptics expected the material to rot in a wet pile,” the report said. “When the method was a huge success, a number of studies were undertaken to better understand the wet storage process, where the material is completely saturated to its water-holding capability.”

Two particularly well-documented studies took place in Argentina and South Africa. In Argentina in 1969 and 1970, wet stored bagasse showed, to the researcher’s surprise, no measurable yield loss and the advantage of improved feedstock quality and consistency, better pulping results and a better final product quality. The South African study, which was conducted several years later, found that pulp produced from bagasse stored using the Ritter method was superior in physical strength, freeness and chemical consumption.

Hettenhaus would like to see the biofuel industry take a page from years of successful wet storage by pulp and paper mills. If wet biomass storage were utilized by the cellulosic ethanol industry, the biomass collection area could shrink to the local dry mill corn supply. In addition, harvesters could go to a one-pass harvest of corn stover. Finally, the way he looks at it, it’s a wasted step to dry biomass only to get it wet again during the fermentation process. “I think we are missing an opportunity there,” he says.

Author: Holly Jessen
Managing Editor, Ethanol Producer Magazine