市政污泥能源化应用的方案比选研究

论文总字数：87762字

Abstract

With the development of the industry and the growth of the population, the energy demand of the society is growing rapidly. At the same time, the environmental issues are becoming more and more serious. Under this background, the treatment of sewage sludge with energy recovery is gaining attentions around the world. Anaerobic digestion, production of biofuels, microbial fuel cells, incineration, co-incineration, gasification and pyrolysis, supercritical wet oxidation of sewage sludge, all these methods were assessed and discussed in this paper. Although all these methods are at different development stages and are being used under different regulations, which made them hard tom compare, but finally it can be decided that in the future, anaerobic digestion of the sewage sludge can be expected to be the most widely applied method.

Table of Content

Abstract…………………………………………………………………………………..2

Chapter 1. Introduction………………………………………………………..….4

1.1 Energy and environment background………………………………..…..…4

1.2 Sewage sludge…………………………………………………..…………….…..5

Chapter 2. Overview of all the methods…………………………………….8

Chapter 3. Detail of each sewage sludge handling option……….....9

3.1 Anaerobic Digestion of Sewage Sludge………………………………….……9

3.2 MFCs…….…………………………….………………………………………..…..13

3.3 Incineration of Sewage Sludge with Energy Recovery……………………18

3.4 Co-incineration of sewage sludge with Energy Recovery……………..…22

3.5 Pyrolysis of Sewage Sludge………………………………………………………26

3.6 Gasification of Sewage Sludge……………………………………………....…29

3.7 Supercritical oxidation of sewage sludge……………………………..….….31

Chapter 4 Comparisons Between options………………………………..…32

Chapter 5 Case Study……….……………………………………..…………….……35

5.1.1 Valorga Technology………………………………………………………….…..36

5.1.2 The Valorga plant at Tilburg, Netherlands………………………………....37

5.2.1 The DRANCO Process…………………………………………………………….39

5.2.2 The Brecht Plant…………………………………………………………………..39

5.3.1 The low-solids, multi-stage BTA Process……………………………………..41

5.3.2 The BTA Newmarket Plant…………………………………………………..…..41

Chapter 6 The Situations of China and UK …………………………………….42

Chapter 7 The Conclusion……………………………………………………………..45

Chapter 1 Introduction

• 1. Energy and environment background

According to the World Energy Outlook of IEA (2015), fossil fuels keep meeting almost 80% of total basic energy demand and at the same time, more than 90% of energy consume-related emissions are CO2 from fossil fuel combustion. [1] If governments around the world do not start to change something about the existing policies, basic energy demand all over the world will increase by around1.5% per year between 2007-2030. [2]

Fossil fuels now are remaining persistently as a dominant energy source, it is fair to say that currently fossil fuels-dependent energy consumption and production have caused a wide range of impacts both on the environment public health. [3-7] The most well-known environmental impact is the greenhouse effect. In all the emissions of the greenhouse gases, the part that came from of the energy usage will take up about 2/3, since the recent century, the emission of the carbon dioxide started to grow sharply. After 1995, the first Conference of the Parties (COP), the emission of the greenhouse-gases have already increased more then 1/4 of the sum of the past years [8]. The world is at a very critical point to combat climate change, just as the IPCC (The international Panel on Climate Change) said, “In the absence of fully committed and urgent action, climate change will have severe and irreversible impacts across the world.” [9]

How to make sure that there will be enough affordable and reliable energy for ourselves and our future generations is a big challenge that needs to be dealt with urgently. Sustainable development will be the solution to address and unlock the energy-related issues, one small step to this direction is re-produce energy through waste-to-energy approach. It is commonly proved that organic waste (waste biomass, sewage, animal manure, etc.) contain a certain amount of energy. And at the same time, if treat these waste matter un-appropriately or leave them untreated, environmental pollution and health risk could happen, under this circumstance, the waste-to-energy strategy seems to be more attractive. [9]

1.2 Sewage sludge

Sewage sludge is the left over of organic matter and dead bacteria that came from wastewater treatment process or the bio-solids removed treatment process. Generally, most of the sewage come from the preliminary and the secondary treatment of wastewater treatment plants. Biologic, physical, and physicochemical reactions would happen during the treatment process. [9]

The main objective of the waste water treatment is the reducing of toxic matters and the returning of cleaner waste water to the environment. However, the consequence of waste water treatment is that a large amount of sewage will be produced, and now across the world, the waste water treatment plants are requiring higher standards of the wastewater treatment, as a result, greater quantities of sewage is being produced. Yet in the past, in the UK, about a 1/4 of sewage sludge generated across country was either discharged to water surface through pipes or disposed to sea from ships. [10] Only until the end of the 1990s, the the UK government required the cessation of these practices and alternative re-use or disposal methods have had to be used. The changes to re-use of sewage sludge are shown in Table 1.

Table 1 The usage of sewage sludge [10]

It is clear that for the environment and the public health, the sewage sludge need to be treated and disposed effectively, it is very important to understand the characteristics of the sludge that is going to be treated. [11] At a very rough specification, the composition of sewage can be characterized by six groups of components [12]:

A typical chemical composition and properties of untreated and digested sludge reported in Table 2.

Table 2 Typical chemical composition and properties of untreated/digested sludge [13]

The main reason that makes the treatment of sewage sludge difficult is all the groups of sewage motioned above are present in one mixture. First of all, in order to dispose and transport and storage the sewage easily, water in the sludge need to be removed. And then organic carbon-, nitrogen-, phosphorous-containing compounds are considered as useful compounds, most of the recovery of energy will involve these compounds. However, as motioned above, sewage sludge contains pathogens, heavy metals etc. So the treatment of sewage will also involve the handling of the un-favoured compounds, to reduce the possibilities of the adverse impact of sewage or residues from sewage treatment on the environment and public health.

Chapter 2 Overview of all the methods

Very briefly, the various options for the recovery of energy from sewage sludge, in fact, from the organic compounds in the sludge, can be subdivided into nine groups:

Several of these treatment options are already applied in practice; others are still in the research phase. In the next few paragraphs, (1), (2), (3), (4), (5), (6), (8) will be further discussed and assessed.

Chapter 3 Detail of each sewage sludge handling option

3.1 Anaerobic Digestion of Sewage Sludge

Anaerobic digestion is capable of converting bio-degradable substance to biogas (60–70vol% of methane, CH4) in the absence of molecular oxygen, it is an anaerobe-based biological process. [14] The bio-gas that is produced by anaerobic digestion can be used as an energy resource at the waste water treatment plant itself and elsewhere if possible. Right now, anaerobic digestion of sewage mostly is applied at large and medium size waste water treatment plants. Yet, also, in small sized waste water treatment plants, a growing interest of the application of anaerobic digestion is also observed. The map below shows the AD plants across UK. Green flags mark “‘On-Farm Site”, red “Industrial Site” and yellow “Demonstration Site”, blue “Commercial Site”.

Fig. 1 AD plants across the UK [15]

The first step of anaerobic digestion is the thickening of sewage by gravity, flotation or belt filtration. By this very first step, the volume of the sewage sludge can be reduced to as a third of its initial volume, and the separated water can be recycled to the influent of the waste water treatment plant. After the anaerobic digestion is finished, a further treatment is required, the dewatering and thickening of settled sludge. The goal is to separate as much water as possible to decrease the volume of material. And finally, the sewage sludge can be put under a certain form of biochemical stabilization. The sludge stabilization reduces the concentration of pathogens in the residual solids, reduces offensive odors and the putrefaction potential.

A process flowchart of the sludge-processing steps is shown in Fig. 2.

Fig. 2. process flowchart of the sludge-processing steps [16]

More specifically, the digest of sewage sludge consists of two main phases, as it shown below in Fig. 3. In the first tank, all the incoming flows of sludge are combined together, and the mixture of the sludge will be heated to a mild temperature, about 35^oC, this is for the acceleration of the biological conversion. And the residence time of this phase will range from 10 to 20 days. And then in the second tank, the mixture of sludge will undergo further digestion, and in this step, there should be no active mixing for the promoting of separation, and there is no need to heat as the first step of this process will generate its own heat.

Fig. 3 the digest of sludge [17]

As a matter of fact, the anaerobic digestion of municipal sewage sludge has been widely practiced since the early 1900s and it is fair to say that among all the handling option of sewage sludge the most widely used is anaerobic digestion. The anaerobic digestion treatment can convert approximately 40% to 60% of the organic matter to Methane and carbon dioxide (CH₄ and CO₂). The composition of the bio-gas that been produced is 30-35% carbon dioxide,60-65% methane, and along with small amount of H2, H2S, N2 and H2O. In all the matters been produced, the methane is the most valuable production for it is a very effective hydrocarbon fuel (36.5MJ/m3 in combustion). After the digestion, there is certain amount residual organic matter, and this matter is nearly odorless and quite stable on the chemical level and contains largely reduced levels of pathogens.

Recently, scientists around the world are giving an increasing attention to enhancing the bio-gas production and quality of the anaerobic digestion process. All the efforts been made basically involve the below area:

1. Try to optimize the conditions of the process, such as the sludge retention time and the sludge loading rate [18,19]
2. Try to deal with the sludge with multi-stage process, such as microorganism community-phased and temperature-phased [20,21]
3. Try to apply pre-treatment process to increase the biodegradability of the sludge, including chemical, thermal, biological, ultrasound, microwave irradiation and pulse power [22,23,24]. For the chemical pre-treatment, such as the application of acid, alkali, Fenton and ozone. [25,26,27,28]; and as for the thermal pre-treatment, heating, freezing, thawing and hydrothermal method have been applied. [29,30,31]

All the aspects motioned above have been looked by researchers cross the world, there is a positive outlook for anaerobic digestion to be more effective and more environmental friendly. As conclusion, several main advantages of the anaerobic digestion includes [32]:

1. Anaerobic digestion is a well developed and practiced process, it has a very long history since the 1900s;
2. Anaerobic digestion can handle the sludge came from high load wastewater at a loading rate of 10-20kg COD/(m3 day);
3. Anaerobic can reduce the pathogens, eliminates offensive odors and reduce the potential for putrefaction;
4. Methane can be applied as combustion material or be converted to useful electricity.

And at the same time, there are certain disadvantages about anaerobic digestion [32]:

1. Anaerobic digestion needs to be operated at a relatively high temperature about 35oC, so it might require additional energy input;
2. The bio-gas produced through the anaerobic digestion is difficult to store and it need to be treated because the bio-gas contain unwanted component such as H2S;

3.2 Microbial Fuel Cells of Sewage sludge

By breaking the inner chemical bonds of matters, the energy that stored in that bond can be released, and the Microbial fuel cells can break the bonds by different kind of micro-organisms, and the energy released can be transferred into electricity. And MFCs has generated considerable interests around the world, appealed academic researchers looking into it recently [33,34,35,36]. As a matter of fact, the idea of gaining electricity by breaking chemical bonds is not advanced, this process have been studied since the 1970s [37,38], and in 1991, treat the domestic wastewater with microbial fuel cells was presented [39]. However, it is only recently the MFCs embraced an improvement of power output [40,41,42,43,44], and for this improvement of the energy output by the MFCs, it can be expected that the microbial fuel cells will be applied largely in the future.

In the MFCs’ chamber, the microbes will oxidize the substrates and generate electrons and protons during the reaction. And at the same time, carbon dioxide will be produced as an oxidation product. The reaction of the MFCs is quite different from the combustion reaction, through an external circuit, the electrons that been produced in the chamber are absorbed by the anode and then been transported to the cathode. And after crossing the salt bridge, the protons will enter the cathodic chamber and in this chamber the protons will combine with the oxygen to form water. [45] and as long as the microbes in the chamber are kept separated from other end terminal acceptor like oxygen except the anode, we can make the electric current generation possible, to do this, an anaerobic anodic anodic chamber is required. (Figure 3)

Figure 4 Schematic of a microbial fuel cell. [46]

So, the overall reaction of the MFCs is the breaking of chemical bonds of the substrates, electricity, carbon dioxide and water will be produced. Just as the reaction stated above, in the outside circuit of MFCs bio-reactor, electricity from the electron from anode to cathode can be generated.

At a theoretical level, most of the microbes can be used as biocatalyst in the microbial fuel cell chamber. In 1910, the earliest MFC concept was presented by Potter. [47] And electrical energy was produced from living cultures of Escherichia coli and Saccharomyces by using platinum electrodes [48]. However, this did not generate a lot of interest until the 1980s, it was when people discovered that by the addition of electron mediators, the electrical current density and the power output could be largely enhanced. The microbes do not have the ability to transfer electrons to the anode directly unless the species in the anodic chamber are anodophiles. The outer layers of the most microbial species are composed of the non-conductive lipid membrane, so the peptidoglycans and lipopolysaccharides will influence the direct electron transfer to the anode negatively. However the electron mediators can accelerate the transfer [49]. Under the state of oxidization, the mediators can be easily reduced by capturing the electrons from within the membrane. Then the mediators will move across the membrane and release the electrons to the anode and then become oxidized again in the solution of the anodic chamber. This process will accelerate the electron transfer rate, and so the power output of the microbial fuel cell increases at the same time. We can choose the mediators by the good features below [50]:

1. Can travel the cell membrane easily;
2. Can capture electrons;
3. The mediators should possess a relatively high electrode reaction rate;
4. Can easily solute in the anolyte;
5. The mediators can not be biodegradable and can not be toxic to the microbes;
6. Cheap to obtain

It is common recognized that the lower the redox of the mediator the lower the anodic redox the mediator can give, and then the redox difference between the anode and the cathode will be maximized. Yet it will not be an efficient condition for the capturing of the electrons inside the microbes, on the opposite, the higher the redox of the mediator the higher the total power of the mediator will be given [51]. Like thionine, methylene blue (MB), neutral red (NR), all these synthetic exogenous mediators have quite high capturing power [52,53,54,55,56], but unfortunately, most of the synthetic mediators are toxic and unstable, so the application of the synthetic mediators in the microbial fuel cells is limited. And later it was found out that besides all the synthetic mediators, some naturally compounds like microbial metabolites can be used as the mediators for certain types of microbes [57]. It is quite a breakthrough for the development of the MFCs when there are some microbes have the ability to move the electrons to the anode directly [58,59]. They are very stable and the transfer is very efficient [60,61]. An biofilm can be formed on the surface of the anode, and the electrons and be transported directly be the conductance in the biofilm. Microbes like like Geobacteraceae sulferreducens, Rhodoferax ferrireducens, Shewanella putrefaciens and Geobacter metallireducens all can achieve this [62,63,64,65]. Besides this, the biofilm also can improve the transportation of the electrons between the electrodes and the microbes itself, in the MFCs that applied these kind of microbes mentioned above, the anode can act as the final acceptor of the electrons, only by this, electricity can be generated [66,67,68]. After this breakthrough mentioned above, the cost of mediators is largely reduced, the microbial fuel cells systems are becoming more advantageous in the wastewater treatment and sewage sludge treatment [69].

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