Heat production from biological wood oxidation

Biological wood oxidation (BWO) is an alternative to wood combustion for heat production. So far, wood degradation has mostly been studied at room temperature. There is little information about BWO at elevated temperatures, especially in mixed culture. Thus, the general objective of this PhD project is to explore the possibility of BWO as a promising technology for sustainable heat production. To understand the mechanisms of BWO better, we choose nutrients and temperature as the main studying factors of this dissertation. In this PhD thesis, we first review the literatures connecting heat and composting and propose proper methods to calculate the heat production of BWO (Chapter 2). The methods are used in the following experiments. Second, we explore the effect of nutrients and temperature on BWO. For the nutrients, we do experiment to explore the optimal concentration of nitrogen, phosphorus under different pH condition (Chapter 3); to reduce the cost and environmental pressure, we do experiment to explore the possibility of using human urine as an alternative to chemically synthesized nutrients (Chapter 4). For the temperature, we do experiment to find out the preferred temperature of BWO at batch scale and its response to temperature change (Chapter 5). Based on the results of Chapter 5, we do experiment at a larger scale to figure out if BWO at larger scale can get the similar BWO performance as at batch scale. The detailed outline of each chapter is:

In Chapter 2, we firstly point out the types of organic solids wastes that are amenable to compositing. Next, we summarize the methods to estimate the potential heating value of organic solid wastes. Furthermore, we discuss the advantage and disadvantages of different methods that are used for calculating the actual heat production from composting. In addition, we summarize and evaluate the different heat recovery methods that have been used for composting. Moreover, we list the potential applications to the generated heat. Finally, we give an outlook about studies relating to composting heat.

Chapter 3 and Chapter 4 of this thesis present an investigation into the effect of nutrients addition on BWO. In Chapter 3, we investigate the optimal addition amount of NH4Cl addition and KH2PO4 addition under different pH values on BWO. We evaluate their performance by monitoring the oxygen consumption and weight loss of dry wood. To reduce the cost and environmental impacts of chemical nutrients, we therefore choose urine as an alternative source of nutrients.

The effect of urine addition on BWO will be discussed in Chapter 4. In Chapter 4, we investigate the possibility of human urine as nutrients of BWO by determining the oxygen consumption and weight loss of dry wood. We first determine the optimal dilution ratio of synthetic human urine by assessing the oxygen consumption and weight loss of dry wood. Thereafter, we investigate the effect of fresh human urine addition on BWO. To explore the relationship between nutrient availability and degradation rate, we re-add the synthesis human urine to the BWO system and continue the incubation process.

Chapter 5 and Chapter 6 of this thesis present the effect of temperature on BWO. In Chapter 5, we investigate the effect of different temperatures (30, 40, and 50 °C) on BWO in a bottle-scale experiment. We also change the temperature when the degradation rate becomes stable to explore if temperature can stimulate the degradation rate. Moreover, we also track the ratio between fungi and bacteria during the BWO process. In Chapter 6, we investigate effect of two temperatures (40 and 50 °C) on BWO by running a continuous bench-scale reactor for 150 days. We evaluate the BWO process by measuring the oxygen consumption and weight loss of dry wood during incubation. Besides, we also monitor the ratio between fungi and bacteria.

In Chapter 7, we address the research questions and discuss the bottlenecks of BWO for sustainable heat production based on the results of this PhD thesis. First of all, we calculate how much heat can be recovered from a scaling-up BWO system. Secondly, we analyze the potential challenges to the scaling-up application and come up with possible solutions based on this PhD dissertation. In the last, we discuss the future perspectives on research and application of BWO.