1. Introduction
Because of high population growth and urban planning in Taiwan, the prevalence of public sewage systems reached 30% of the population in 2012, and is expected to increase to 36% by 2014 [
1]. Thus, sewage sludge production will increase with the expansion of the sewage treatment system, and should reach up to 1040 t/day by 2014 [
2]. Sewage sludge generally contains pollutants such as human pathogenic organisms, and must be disposed of in ways that reduce environmental and public health effects.
Most sewage sludge in Taiwan is currently disposed of in landfills, with the remainder being co-incinerated with municipal solid waste (MSW). Existing crane and grapple-feeding devices have difficulty handling the pasty sludge cake with MSW, and the sludge degrades combustion efficiency. Thus, the co-incineration ratio is limited. Some MSW incineration plants even ban sewage sludge. The scarcity of available landfills and limited capacity of co-incineration are pressing problems. Other solutions for handling sewage sludge in more environmentally friendly ways, and recovering its energy, have caused great concern in recent years.
The imported energy ratio in Taiwan is as high as 99.4%, and energy security is unfavorable [
3]. Finding alternative energy sources, such as bioenergy, is necessary. Carbonization technology can transform sludge into a carbon-containing product that can be used as biocoal and co-fired with fossil coal to generate electricity in power plants [
4,
5,
6,
7,
8]. Carbonizing sludge reduces its volume to approximately one-eighth of the sludge cake, increases its calorific value, removes its odor, and improves its combustibility and grindability, making it a better co-firing material for pulverized coal power plants [
4,
5,
9]. Reference plants applying sewage sludge carbonization technology in Japan and North America have successfully demonstrated its feasibility [
8,
9,
10,
11]. These applications advance the goals of using sewage sludge as an energy resource and simultaneously reducing greenhouse gas emissions and coal extraction.
Life cycle assessment (LCA) is a method of evaluating the environmental effects associated with a product, process, or service throughout its life cycle. The LCA method is generally performed according to ISO14040 standards, which define the principles and framework of LCA [
12]. Researchers have also applied LCA to sewage sludge management. Hospido
et al. [
13] compared three alternative sewage sludge post-treatments (agricultural use, incineration, and pyrolysis) and then assessed the energy reuse strategy used in pyrolysis. Hong
et al. [
14] combined LCA and LCC (
i.e., life cycle cost) to estimate the environmental and economic effects of six alternative sewage sludge treatments. Their results indicate that dewatered sludge combined with electric melting is an environmentally optimal and economically affordable method. Murray
et al. [
15] also applied life cycle environmental and life cycle cost assessments to nine alternative sewage sludge treatments. Their results indicate that coal-fired incineration is the most environmentally and economically costly of all treatments. However, no study has presented the LCA of sewage sludge carbonization. Because carbonization is increasingly being adopted in several countries and is a candidate for sludge treatment in Taiwan, understanding the potential effects of this biomass usage method is necessary.
The objectives of this study are to simulate the sewage sludge carbonization process, using local sludge properties, to evaluate the environmental effects and benefits of the carbonization process using LCA. This study also uses LCA to investigate current approaches for sewage sludge treatment, including direct landfills, co-incineration with MSW, and mono-incineration for comparison.
2. Simulation of the Sewage Sludge Carbonization Process
Because no inventory is available within the existing LCA database applicable to carbonization, this study adopts an energy model developed by Maski
et al. for biomass pretreatment [
16] and previous research results of biomass torrefaction [
17,
18,
19]. This study also refers to a batch-type carbonation experiment, conducted by Park and Jang [
4], specific to dried sewage sludge at 300–500 °C for 30 min. Koga
et al. [
8] also reported a sewage sludge carbonization system handling 40–60 kg/h of dewatered sludge at a 500 °C carbonation temperature to produce biocoal. Therefore, the simulated carbonization process in this study assumed dewatered sludge to be bone dried at 100 °C, and subsequently carbonized at 450 °C for 30 min in the absence of oxygen. Carbonized liquid and volatile gases were collected and recovered for their heat energy [
20,
21], which was supplied to the drying and carbonization units through a combustor and heat exchanger. The final carbonized product or biocoal, which had properties similar to fossil coal, can generate carbon neutral bioenergy at a pulverized coal power plant.
The following equations were used to simulate the sewage sludge carbonization process:
(1) The energy use of a drying unit (
ER,D, MJ/kg) is represented by:
where
Mwet (wt %) is the moisture content of sewage sludge;
DBwet (wt %) is the percentage of dry solid in sewage sludge (note:
Mwet + DBwet = 100 wt %);
Cp,w (MJ/kg K) is the specific heat of water = 0.004187 MJ/kg K; T
i (K) is the initial temperature of sewage sludge = 298 K;
Lv,w (MJ/kg) is the latent heat of water at its boiling point = 2.27 MJ/kg; C
p,b (MJ/kg K) is the specific heat of sewage sludge = 0.001763 MJ/kg K [
18]; and
ef,D is the efficiency of the drying unit (assumed to be 0.85 in this study, a relatively high efficiency).
(2) The energy use of a carbonization unit
(ER,C, MJ/kg) is represented by:
where
TC (K) is the carbonization temperature, and e
f,C is the efficiency of the carbonization unit, set to 0.85 in this study.
(3) In a combustor and heat exchanger, available energy is derived from the combustion of volatile gas and carbonized liquid, where available energy from volatile gas (
EA,CG, MJ/kg) and available energy from carbonized liquid (
EA,CL, MJ/kg) are represented by:
and:
The terms
LHVvolatile (MJ/kg) and
LHVliquid (MJ/kg) represent the heating value of the volatile gas and carbonized liquid generated by the carbonization unit, respectively, and
DB (kg) is the weight of dried sludge,
yMG is the volatile gas yield,
yML is the carbonized liquid yield [calculated by Equation (5)],
ef,C is the efficiency of the combustor unit (assumed to be 0.85), and
HL is the heat loss of the heat exchanger (assumed to be 0.5%).
(4) The product yield (
yM) is defined according to mass by [
22]:
where
min is the mass of the biomass input, and
mout is the mass of product output of a carbonization unit (note:
daf = dry and ash free).
Figure 1 shows the simulation results of sewage sludge carbonization based on the properties of dewatered sewage sludge after anaerobic digestion from a local sewage treatment plant and experimental results of carbonization yield [
23,
24] (
Table 1). Mass and energy balance calculations show a 0.08 (
daf) product yield (
yM), 3.19 MJ/kg total energy required of units, 0.14 MJ/kg available energy from volatile gas (
EA,CG), and 0.5 MJ/kg available energy from carbonized liquid (
EA,CL). The supplementary information (
Table A1) explains the nomenclature and provides parameter values. Although the simulation process is a simplified form, this approach presents a feasible method to access the LCA of sewage sludge carbonization. By changing the efficiencies of the drying unit and carbonization unit from 0.85 to 0.65 (decreasing 20%), the overall required energy increases 25% and the available energy from volatile gas and carbonized liquid decreases 19%. The influence of the assumed efficiency of each unit on the LCA results can be estimated accordingly.
Figure 1.
Mass and energy balances of the sewage sludge carbonization process.
Figure 1.
Mass and energy balances of the sewage sludge carbonization process.
Table 1.
Sewage sludge characteristics.
Table 1.
Sewage sludge characteristics.
Dewatered sludge characteristics* | Value |
---|
Moisture (wt %) | 80 |
High heating value dry (MJ/kg) | 15.18 |
Proximate analysis (dry basis, wt %) | |
Ash content | 35.2 |
Volatile matter | 64.8 |
Elemental analysis (dry and ash free basis, wt %) | |
C | 54.60 |
H | 7.69 |
N | 4.52 |
O | 30.29 |
S | 2.52 |
Experimental results of carbonization yield (dry and ash free basis, wt %)** | |
Solid yield | 39.14 |
Liquid yield | 34.09 |
Volatile gas | 26.77 |
5. Conclusions
This study presents an assessment of the environmental effects of four sewage sludge treatment options: carbonization, mono-incineration, direct landfills, and co-incineration with municipal solid waste. This study uses an energy model to simulate the process of sewage sludge carbonization and produces theoretical energy and mass balance data for conducting the LCA. The results of the four treatment scenarios show that carbonization was the most preferable sludge-handling option overall, followed by co-incineration, landfills, and mono-incineration in descending order.
However, the co-incineration option emitted less greenhouse gases than carbonization because the overall energy recovery ratio of electricity was higher during the incineration process than during carbonization. Although this analysis considers heat recovery during carbonization, electricity generation, and coal substitution during co-firing, the energy used in drying the dewatered sludge emitted more greenhouse gases, contributing greatly to the damage category of climate change. However, changing both the feeding water content after the drying process and the carbonization temperature may mitigate the energy use of the carbonization scenario.
The aspect of cost must also be considered in the assessment and selection of sewage sludge treatment options. Because the application of sewage sludge carbonization is currently receiving great attention from municipal authorities, the significance of sewage sludge as a valuable energy source may increase even more.