In September 1998 Vincent Corporation gave a presentation to a group of over five hundred citrus processors. The subject was using citrus waste as a boiler fuel.

It was years after this presentation that a fundamental observation was made. In order to burn biomass such as orange peel, the moisture content must first be reduced to zero. That is, as long as moisture is present, the temperature cannot rise above 212 F (at atmospheric pressure). Since combustion cannot occur until the mass reaches a much higher temperature, any burning system has an absolute requirement of first reducing the moisture content to 10%. If the system is frozen (on paper) at that point, it is easy to calculate if the mass now has more value as animal feed or as the fuel. Invariably, the feed value is greater than the fuel value.

There was interest in the paper for special reasons. The price of orange peel pellets, which are used as cattle feed, had been a very depressed $40/ton the previous season. Furthermore, dioxin had been found in Brazilian pellets, resulting in a European embargo. In the end the interest was academic because of the large capital requirements.

The price of pellets has currently (2008) gone to $170 per short ton, further reducing the financial justification of such a project.

On a dry solids basis, orange peel has a BTU content of 7,500 BTU/pound. This compares quite favorably with material like wood. It is enough energy value to satisfy several needs:

1. Dry the wet peel sufficiently so that it will burn.
2. Release energy to generate steam in a boiler.
3. Generate enough steam to run the juice evaporators in an orange juice concentrate plant.
4. Generate enough steam to produce electricity to run the citrus plant.

Several equipment alternatives were evaluated. A practical configuration recommended was to first use a screw press to remove moisture from the peel. Solids from high Brix citrus molasses could be diffused into the press cake until the moisture content was reduced to 60%. Press cake (peel) at this level of dryness will burn in either a fluid bed or stoke grate boiler. The high-pressure steam generated in the boiler would be used in a turbine to generate electricity. The low-pressure steam extracted from the steam turbine would be used both to (a) produce high Brix molasses from the press liquor and to (b) drive the TASTE evaporators that are used to produce orange juice concentrate.

The adoption of this system is attractive where fuel costs are high and the value of citrus pellet animal feed is low. Unfortunately, as is the case with all steam based electricity generators, the capital costs are extremely high. This high investment requires a long amortization period.

Copies of the paper that was presented are available from Vincent Corporation.

September 2010:  Alex Andreassen of Danisco has pointed out that  burning citrus pellets will lead to slagging of the boiler tubes because of the alkali-metals (Na, K) content of the peel.  Thus applications would be limited to using pellets as dryer fuel.

Issue 89