Wood-Fired Steam Boiler for Mushroom Substrate Sterilization

Marc R Keith, Malo, Washington, USA

Here's a few pictures, more or less in chronological order, demonstrating the construction of our new steam generator. Details follow over the next few pages.

Click any picture to view full size-click again to return to nest.

This page demonstrates and explains the construction and operation of the new woody biomass fired steam generator, used to sterilize mushroom substrates at Mountain Mushroom Farm, a Mom-and-Pop USDA organic-certified facility. Mountain Mushroom Farm was designed and built from day one as a scale-model test bed for woody biomass conversion directly to high-quality food for human consumption, as well as renewable, clean energy.

Prior to building this wood fired steam boiler, Marc and Vivian Keith of Malo, Washington in Ferry County, owners of Mountain Mushroom Farm, had already developed techniques for converting the woody biomass which is being removed as part of forest fire fuels reduction efforts in Ferry County into gourmet mushrooms at a Biological Efficiency(hereafter referred to as BE) of 100%. This means for every 100 pounds of woody, conifer-based biomass delivered to the farm, they produce 100 pounds of food for human consumption. No trees are cut down for growing these mushrooms. The sawdust and bark, hog fuel, etc., being used are all part of the forest fire fuels reduction program, already in progress.

For well over 1000 years, Shiitake and other gourmet edible mushrooms have been grown on hardwoods such as Alder, Oak, Birch, etc. However, the forest fire fuels reduction programs in Northeastern Washington State remove overgrown Douglas fir, Western larch, and various species of pine. The challenge facing Mountain Mushroom Farm was to develop commercially viable gourmet mushroom varieties which were suited to these conifers. Mountain Mushroom Farm has spent the last few years developing and testing hundreds of gourmet mushroom strains, always selecting for those which showed suitability to conifers, and then further crossing until strains were developed which would perform on conifers with the same BE as on hardwoods. The result of those efforts has been the achievement of 100% BE on aged conifer sawdust, which happens to be the industry standard for hardwoods. No other use for this woody-biomass has as much potential to ameliorate hunger and bring financial security to the economically depressed areas of Eastern Washington State.

However, the final step in achieving a zero carbon footprint for Mountain Mushroom Farm was to develop and build a highly efficient, clean-burning wood fired steam boiler. On mushroom farms, the steam boiler, often called a steam generator is used to sterilize the mushroom substrates prior to inoculation with sterile culture media. It is also used to heat the growing rooms, which must be ventilated with a constant supply of fresh air to prevent unwanted molds. The steam generator at Mountain Mushroom Farm supplies steam to a heat exchanger in the intake air duct, heating the outside air which in the winter months can be as cold as -30F/-22C to a comfortable 60F/15C, the temperature preferred by gourmet mushrooms.

Construction of the airtight wood fired steam boiler

A major problem with most wood-fired steam boilers is their horrible efficiency. Homeowners in the US Midwest with hydronic heating systems often burn 20 cords or more per year, a horrible biomass to heat conversion ratio. The same home could be heated with a super efficient airtight wood stove on just three to four cords per year, if the stove were located in a central part of the house.

This boiler design utilizes the efficient engineering of an airtight wood stove, except with a steam generator tank inside, and a very large thermal mass of solid concrete surrounding the combustion area. The result is a highly efficient steam generator which uses only a fraction of the wood of most outdoor steam boilers. It is also relatively inexpensive to build, using a food grade 55 gallon drum as the boiler tank. A 5 PSI mechanical relief valve prevents overpressure, should the normal operating controls fail.

The first step in the construction process was the installation of a water line to the site of the future boiler. In our area, the water line has to be buried at least 48" deep to prevent freezing during the winter months.

All images on this page can be clicked to see full size. A 1" line feeds two freeze proof faucets, the one on the right at the location of the soon-to-be-built boiler.

Next, the concrete pad had to be built. The dimension of the concrete pad depends on the layout of the boiler. For a do-it-yourself type person wishing to duplicate this project, first measure your intended boiler tank, and then design the structure around it, finally pouring the concrete slab to match.

The outside dimensions of our boiler were calculated to be 44" X 48", thus the slab was poured 44" X 72" to provide for a 'porch' in front of the future boiler. After pouring, but before the concrete is fully cured, a broom was raked across the slab to provide traction during the wet months.

Page under construction: To be continued. . .