Sulfur Contaminants in Bioderived Products
By Pam Buzzetta and Ryan Caulfield
Many biosolids feedstocks contain sulfur naturally. Unfortunately, sulfur is no friend to emissions permits, waste handling, or many catalysts used in industrial biotechnologies. There are three options for removal: pre-treatment of feedstock, treatment of an intermediate product, or post-treatment once waste streams exit the process.
Sulfur is often part of complex molecular structures in the feedstock, such as amino acids, which makes it very difficult and costly to try to remove in a pre-treatment step. The biomass must be broken down first into its constituent parts, though the sulfur can be converted into toxic forms by the reaction chemistry of the process.
Every new installation and operational change should be carefully monitored and tested to ensure safety of the operation. Toxic sulfur content can increase with slight differences in processing due to scale-up, or changes in feedstock composition from similar sources.
An intermediate step is often going to be more viable than pre-treatment. For example, if the technology uses a pyrolyzer to produce a liquid or gas intermediate, consider a fluid phase removal process.
However, the next challenge is removing sulfur in its many forms: H2S, mercaptans, COS, CS2, sulfates, etc. Each species may require its own filter bed or process for removal, which quickly increases capital costs and operational complexity! Frequent maintenance or disposal of media is not only costly, but also presents increased safety issues. Testing is essential to identifying which sulfur species are present.
Small quantities of vapor production from the process may make post-treatment through thermal oxidation a viable solution. Or, process off-gases may be mixed with natural gas for use as fuel in process heaters or furnaces. Burning the vapors will convert all sulfur species into SO2, which can be scrubbed from the vapor through well-known technologies.
If solid wastes with high sulfur content exit the process, it may or may not be considered hazardous waste. Ideally, the content would be low enough that the waste could be landfilled, but high cost waste handling may be required if the sulfur cannot be removed earlier in the process.
Another point of post-treatment may be in waste water discharge. If the content of the various sulfur species is too high to discharge directly from the facility (driven by permitting), there are a variety of treatments that may be applied depending on the sulfur species.
Clearly, many factors drive the optimized design to manage sulfur content in biosolids feedstocks. Ensure your design has carefully evaluated the tradeoffs!