Closing the Loop - Utilization of Secondary Resources by Low Temperature Thermal Gasification

This study addresses certain issues related to unsustainable management of secondary resources like organic waste, sewage sludge and residues from agriculture and industry with a focus on losses of nonfossil energy potential and valuable elements. In this context it is investigated how suitable application of low temperature thermal gasification could be applied to reduce the environmental impact of such management systems and increase the value and positive awareness of the resources in question.

In the first part of this study, the Low Temperature Circulating Fluidized Bed (LT‐CFB gasifier) is described.The LT‐CFB gasifier is a technology originally developed for pre‐processing of biomass fuels like cereal straw. In popular terms, the LT‐CFB gasification process separates the inorganic and organic fractions of the straw.

The majority of the inorganic material is extracted in one or several different ash fractions and the organic material is converted into a hot combustible gas product, which is subsequently combusted in an adjacent boiler. This substantially reduces the influence of the fuels inorganic composition on thecombustion properties.

When combining LT‐CFB gasification with existing dust‐fired coal boilers, fossil fuels can be directly substituted with renewable fuels while reusing existing energy infrastructure. Currently, two operational LT‐CFB gasifiers exist: A pilot scale facility with a thermal capacity (TH) of 100 kW and a demonstration unit of 6 MWTH.

Both units are involved in the present study. Many different fuels have previously been tested in LT‐CFB gasifiers and previous results from these experiments are described and evaluated with focus on the energy efficiency of the process and the quality of LT‐CFB ashes for use as fertilizers. The general benefits and drawbacks of low temperature gasification compared to anaerobic digestion and incineration are briefly discussed in this regard.

Development and implementation of a method to screen for new fuel candidates for LT‐CFB gasification is conducted, and 22 new potential fuel candidates are characterized and compared to 4 previously proven LT‐CFB fuels. The investigated fuel candidates are categorized by their apparent suitability as LT‐CFB fuels and various positive characteristics as well as potentially problematic issues are discussed.

The overall conclusion from the fuel screening is that in a Danish context it is highly relevant to consider low temperature gasification of especially sewage sludge and manure fibers while the international perspective includes especially sugarcane bagasse, various residues from olive oil production and rice husks.

Only five fuel candidates are considered as potentially very problematic for single fuel LT‐CFB gasification: Fat separated from wastewater treatment, palm kernel shell residues, two animal meat and bone meal samples and wood pruning from Italian vineyards. The problems mainly relate to the proximate composition, ash sintering, char deposit formation and corrosion of steel surfaces during thermal tests.

The fuel screening also includes a screening of P fertilizer quality of ashes and chars produced from thermal treatment of the different fuels, and significant differences were identified between the P fertilizer quality of ashes and chars. The fuel screening also involves an investigation of how analytically determined characteristics of three fuel mixes differ from the expected linear sum of the involved fuels’ individual characteristics.

The results indicate profound possibilities for optimizing fuel and ash characteristics by fuel mixing with regard to ash deposit formation and sintering as well as ash and char P fertilizer quality. Of the 5 best candidates identified in the fuel screening, sewage sludge is found to be one of the most interesting as it is a locally as well as globally available resource with a large potential for optimized management compared to several of the currently applied management options.

Proper management of sewage sludge holds a substantial potential for recovery of highly concentrated phosphorus (P) with good plant availability in ashes and chars from the thermal conversion. It is therefore decided to progress with sewage sludge in a series of experimental campaigns to provide a detailed investigation of potential benefits and problematic issues related to sewage sludge management by LT‐CFB gasification.

Four experimental campaigns with gasification and co‐gasification of sewage sludge in LT‐CFB gasifiers are conducted and the results on process performance and the quality of the gas product are compared to results from other studies on thermal gasification of sludge. The overall conclusion is that many different gasifier designs have been proven successfully on sewage sludge fuels and LT‐CFB gasification is very well suited for gasification of sewage sludge as well as co‐gasification of sewage sludge and cereal straw.

The LTCFB gasifier is found to yield the highest hot gas efficiency, carbon conversion rate and total system electrical efficiency of the assessed systems. Examination of the fertilizer quality of ash substrates from thermal conversion of sewage sludge is a central part of this study. Fertilizer quality is addressed by comparing the elemental composition, PAH content and P plant availability of LT‐CFB ashes from different gasification and co‐gasification campaigns to ash and char samples from incineration and pyrolysis of sewage sludge as well as to their respective untreated sludge samples and a mineral P reference.

In addition to the conventional thermal platforms, a process for postoxidation of pyrolysis chars and gasification ashes has been developed and the oxidized substrates are also included in the investigation. From the investigation of ash fertilizer quality it is concluded that all of the investigated thermal platforms are applicable for production of P fertilizers by conversion of sewage sludge with the proper design and operational settings.

Post‐oxidation of pyrolysis chars and gasification ashes is found to have a remarkable effect on P fertilizer quality while co‐gasification of sludge and straw in LT‐CFB gasifiers in general seem to provide a better ash fertilizer than mono‐sludge gasification. Assessment of the influence of the thermal process on the fertilizer quality of the ashes is studied with chemical sequential extraction and scanning electron microscopy to identify changes in P association induced by different thermal treatments.

Changes in P fertilizer quality as function of incubation time and as function of changes in the particle size distribution of the ash substrate is also discussed. In the last part of the study, the results from the previous chapters are combined in an assessment of the possibilities to produce controlled release fertilizers and context‐specific designer fertilizers in systems encompassing thermal conversion of secondary resources.

A discussion about burden shifting in such management systems is also introduced and results are analyzed from two life cycle assessment case studies comparing sewage sludge gasification in centralized LT‐CFB gasifiers with the current practice of direct application of sludge on farm soil. The results indicate a substantial improvement of the LT‐CFB scenario compared to the reference case with regard to a reduced impact on climate change and reduced toxicity of the P fertilizer.

Based on this work it is concluded that there is a profound potential for optimizing the management of sewage sludge – and most likely also many other secondary resources, by applying the proper thermal processes. With a good match between fuel characteristics, process design and end use of the produce dash and gas products, such a system can be setup to encompass full utilization of the energy potential in the resource and simultaneously produce high quality fertilizers.

LT‐CFB gasification is in many respects a very promising platform for this purpose combining flexibility in fuels and products and high energy efficiency. Co‐gasification of sewage sludge and cereal straw is found to produce very high quality thermally purified P fertilizers, and the potential benefits of fuel mixing needs to be further examined.