The Abrams Clean Tech Report

Co-Combustion of Biosolids Reviewed…

Sunday, February 26th, 2006

ACS has published a peer review of biosolids, appraising 5 co-combustion scenarios with policy analysis.

http://pubs.acs.org/cgi-bin/sample.cgi/esthag/2006/40/i03/html/es052181g.html 

Here’s a major part of the piece, minus that tables.  It’s worth the read.

Environ. Sci. Technol., 40 (3), 649 -658, 2006. 10.1021/es052181g S0013-936X(05)02181-4
Web Release Date: December 20, 2005

Copyright © 2005 American Chemical Society

Biosolids-A Fuel or a Waste? An Integrated Appraisal of Five Co-combustion Scenarios with Policy Analysis

Elise Cartmell, Peter Gostelow, Drusilla Riddell-Black, Nigel Simms, John Oakey, Joe Morris, Paul Jeffrey, Peter Howsam, and Simon J. Pollard*

Sustainable Systems Department, School of Industrial and Manufacturing Science, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, U.K., Lupus Science, Huntercombe, Nuffield, Oxfordshire, RG9 5SG, U.K., and Institute of Water and Environment, Cranfield University, Silsoe, Bedfordshire, MK45 4DT, U.K.

Received for review November 1, 2005

Accepted November 9, 2005

Abstract:

An integrated appraisal of five technology scenarios for the co-combustion of biosolids in the UK energy and waste management policy context is presented. Co-combustion scenarios with coal, municipal solid waste, wood, and for cement manufacture were subject to thermodynamic and materials flow modeling and evaluated by 19 stakeholder representatives. All scenarios provided a net energy gain (0.58-5.0 kWh/kg dry solids), having accounted for the energy required for transportation and sludge drying. Co-combustion within the power generation and industrial (e.g., cement) sectors is most readily implemented but provides poor water utility control, and it suffers from poor public perception. Co-combustion with wastes or biomass appears more sustainable but requires greater investment and presents significant risks to water utilities. Incongruities within current energy and waste management policy are discussed and conclusions for improved understanding are drawn.

Introduction

Sewage sludge (biosolids) is an unavoidable byproduct of wastewater treatment. It has valuable attributes but can be odorous and often contains persistent environmental pol lutants that restrict its further use. Applying it to land as a nutrient and a source of organic matter has been the dominant reuse outlet in the UK, absorbing about 50% w/w of total production (1), and this is often the best practical environmental option for reuse (2). However, the reduced availability of land, the increased public concerns over food chain safety, and the associated uncertainties and costs of reuse have required water utilities to explore alternative management options that can contribute to a more sustain able biosolids strategy.

Policy Context. Co-combusting biosolids with fuels or wastes could provide one secure outlet, among others, and generate income through energy recovery. The changing UK energy policy climate (3) lends support, in principle, to the use of biomass and locally generated waste as fuel, as part of a move toward the low carbon economy. New energy legislation and policy initiatives, such as the renewables obligation (4) and the climate change levy (CCL) (5), are stimulating renewables in the UK, offering, in principle, a more favorable environment for biosolids to be considered for use as fuel. However, the classification of biosolids as "waste" imparts legislative demands that constrain its onward use (6, 7). Further, the introduction of a new fuel stream to an industrial facility can require operators to comply with additional legislation, a perceived barrier to using biosolids, unless there are significant economic or operational benefits. Enright (7) collates European case law on the Waste Framework Directive (8) as it relates to the use of wastes as secondary products (e.g., as a fuel), and has suggested that the absence of legal clarity on the definition of waste remains a critical barrier.

This policy incongruity between renewable fuel incentives and environmental protection presents an obstacle to the development and implementation of biosolids strategies. Given the multitude of influences including technical, sociopolitical, operational, and economic, a central question can be posed: Are biosolids a fuel or a waste? This paper adopts an integrated approach, examining the environmental impact, energy balance, risks, costs, and sustainability of biosolids’ co-combustion with coal, municipal solid waste (MSW), and biomass co-fuels and as a supplementary fuel in other industrial processes (e.g., cement manufacture). While similar approaches have been adopted for wastewater treatment (9, 10), this analysis has been historically lacking in biosolids management, although often called for as a prerequisite to project-based environmental assessment (11, 12). The aim was to undertake a generic assessment and explore, with data, the complexities of the tradeoffs required and the implications these might have for an improved policy context.

Experimental Methods

Approach. Five biosolids co-combustion scenarios were selected, with the assistance of UK water utility biosolids managers, reflecting a range of operational scales, project party structures, technologies, and growth potential feasible in the UK. Assessments of the prior art, and a series of 1.5 h structured interviews with representatives of the water industry, the power sector, regulatory bodies, and the nongovernmental (NGO) sector elicited views on the risks, benefits, barriers, and opportunities for biosolids co-combustion for each scenario. The technical and environmental feasibility of each scenario was assessed using a materials flow and energy model. An economic analysis compared indicative costs and income potential for the five scenarios.

Co-combustion Scenarios. The principal factors defining biosolids co-combustion are the thermal capacity (MWth) of the combustion process, the type of materials co-combusted, and the extent of organizational involvement that a utility might have in financing and operating a facility. Secondary factors include the conversion technology employed, the supply chain security, and the type and amount of biosolids available. Potential co-fuels include fossil fuel, plant biomass (i.e., wood, such as coppiced willow), and MSW. The operating options open to a water utility include the following: supply to third party only, with no involvement in the combustion of the biosolids into energy, the biosolids being delivered to the combustion facility and a gate fee possibly imposed on the water utility, or a fuel income received by the water utility; a joint venture with a third party, where the water utility is technically and financially involved in the operation; and stand alone, where the water utility operates a facility without the involvement of a third party. There are five representative scenarios (Figure 1) adopted in this study:

Figure 1 Five co-combustion scenarios: (1a) a pulverized fuel coal power station boiler, biosolids dried at wastewater treatment works (WwTW), (1b) a pulverized fuel coal power station boiler, biosolids supplied as cake and dried at power station, (2a) with MSW at third party facilities, biosolids dried at WwTW, (2b) with MSW at third party facilities, biosolids dried at combustion premises, (3) with wastes at water utility facilities; (4) with wood chips at a water utility facility, and (5) in cement kilns.

(i) Scenario 1, co-combustion with coal in a third party large scale combustion facility with no participation of the water utility in power generation or sale.

(ii) Scenario 2, co-combustion with wastes at a third party energy-from-waste (EfW) facility.

(iii) Scenario 3, co-combustion with wastes at an EfW facility operated by the water utility.

(iv) Scenario 4, co-combustion with supplementary fuel (plant biomass) at a water utility combustion facility with possible third party involvement.

(v) Scenario 5, co-combustion in a cement kiln or other industrial user of power and/or heat (CHP).

Theoretical Basis of a Mass and Energy Flow Model. A thermodynamic model was constructed to perform mass and energy balances for transportation, drying, incineration, steam generation, and electricity generation. A unit process approach allowed scenarios 1-5 to be simulated. The potential combinations of co-fuel, energy market, partner organization, and scale (Table 1) provided 14 combinations. The system boundaries for the model were similar to those from Hellweg et al. (13). Scenarios were modeled using digested (d) and raw (r) biosolids as fuel input. In addition, scenarios 1 and 2 considered the delivery of biosolids to the combustion unit either as dried product (95% w/w dry solids; DS) or as dewatered cake (25% w/w DS). Where biosolids were dried, off-site drying was assumed to be performed by a gas-powered dryer (a). Where delivered as cake, on-site drying was assumed to be performed by a theoretical flue gas dryer (b) to allow a combustion temperature of >900 C to be achieved, compliant with the EC Waste Incineration Directive (WID) (14).

Thermodynamic Modeling. Inputs to the thermodynamic model are the biosolids and supplementary fuel mass flow rates (kg DS/h), moisture contents (%w/w), ash content (%w/w DS) and their ultimate composition [C, H, O, S, N, Cl, and F as % DS Table 2), and the higher heating value, (MJ/kg DS)]. From these parameters, a mass and flow balance (Figure 2) was performed for fuel consumed, energy converted, air consumed, and the fractional composition of the flue gas (N2, CO2, SO2, and NOx). The model derives the quantities of steam produced and electricity generated on the basis of the mass flow of fuel input (Figure 2). Efficiency values were assumed or determined from industrial plant suppliers. All efficiency values are presented in Figure 2 or in the Supporting Information.

Figure 2 Input/output worksheet of the mass/energy balance model for scenario 1a (a pulverized fuel coal power station boiler, digested biosolids dried at sewage treatment works premises using a gas-powered dryer).

Environmental Impact Modeling. Compounds of environmental concern include heavy metals, dioxins and furans, ash, and other pollutants (NOx, SOx, HCl, and HF). The formation of CO2, NOx, N2O, and SOx was estimated from the model data, though the formation of NOx and N2O is not straightforward and requires high temperatures. Estimation of HCl and HF was performed on the basis of proximate, elemental, and heavy metal content. Emission of dioxins and furans is not generally a problem for biosolids combustion, provided temperatures above 600 C are maintained and flue gases are rapidly cooled through the 250-400 C range (15, 16). For each scenario, a combustion temperature of >900 C was assumed. The potential for acidification and winter smog was estimated on the basis of empirically derived fractionations, by considering whether the contaminants were oxidized or partitioned to the ash, water (in the scrubber), or air (17).

Qualitative Risk Assessment. A series of 19 structured interviews with 11 industry, 6 regulatory, and 2 NGO stakeholders was used to build a characterization of the key risks posed from each of the five scenarios and combinations of management control (first party, water utility, or third party), co-fuel type, and technology type. Interviews lasted ca. 1.5 h with interviewees invited to score the relative magnitude of the key risks and comment on the benefits, barriers, and opportunities for the biosolids co-combustion for the five scenarios using a common questionnaire. Risks associated with each scenario were presented on a semiquantitative, Likert-type (18) 1-5 scale, distinguishing between the likelihood and consequences common to each scenario. Combined rankings were banded as negligible (scores 1-5), low (scores 6-10), medium (scores 11-15), high (scores 16-20), and very high (scores 21-25).

Economic Analysis. The economic analysis assessed the incremental costs and revenues to a water utility, comparing the five scenarios and identifying where there were additional costs for unit operations (e.g., biosolids drying and emission control) and whether these could be offset through additional income streams (e.g., energy sale and gate fees). The analysis assumed that the initial feedstock was dewatered at 25% w/w DS and that co-combustion requires precombustion drying to 95% w/w DS, unless otherwise stated. The analysis assumed costs associated with taking raw biosolids at 2-4% w/w DS through to 95% w/w DS for co-combustion, but that dewatering to 25% w/w DS was performed at the wastewater treatment works. While generic cost estimates can be derived, actual costs and revenues are site- and context-specific.

Sustainability Appraisal. Government guidance in England and Wales (26) provides 20 UK framework indicators to evaluate the progress of projects against sustainability objectives. Four broad indicators were constructed for this project to capture the main dimensions of sustainability with the respect to co-combustion: economic performance, based on the analysis of costs of the co-combustion options; social impacts, based on the impacts on public health risk and the acceptability of the options to communities; environmental performance, based on emissions and net energy consump tion of the options; and flexibility, based on the robustness and adaptability of the options. A simple scoring system (Table 3) was used to assess the relative performance of the options drawing on the preceding analysis. Options were scored against each indicator on a scale -2 (very negative), through 0 (neutral, or balance of negative and positive), to +2 (very positive).

Results

Technical Appraisal. Model output (Figure 3) is represented in terms of the net biosolids energy provided by co-combustion after we accounted for transport and drying, the electricity generation, and the incremental environmental impacts of biosolids combustion, representing the difference in model output for the scenarios operating with biosolids and with the co-fuel only.

The net biosolids energy from supplying sludge to the combustion facility, estimated by subtracting the energy required for transporting and drying the sludge from the gross energy supplied by the sludge, is presented in Figure 3. In each case, net sludge energy is positive, though for scenarios with longer transport distances and higher drying requirements (scenarios 1 and 5, Table 1) it is reduced. Raw sludge scenarios present a higher net energy than digested sludge scenarios because of the higher calorific value of raw sludge.

Figure 3 Environmental impacts of five generalized co-combustion scenarios for biosolids normalized to unit/kg DS.

Incremental environmental impacts of combustion were estimated by comparing outputs with and without sludge, allowing the incremental impacts of using sludge to be modeled directly. Most scenarios produce less electricity when sludge is substituted as a co-fuel, as expected; the exception is dried sludge when compared to MSW (scenario 2a). Scenarios 1 and 5 have reduced global warming potential due to the replacement of nonrenewable fuels with carbon neutral sludge. Effects on global warming potential for the remaining scenarios, each using a carbon neutral co-fuel, are negligible, with the exception of scenario 2a, where a gas-powered dryer is assumed. In these generalized scenarios, theoretical acidification and winter smog potential are both increased by burning sludge, due to the higher sulfur and ash content compared to the co-fuels. Digested sludge is preferable in these categories owing to its lower sulfur content. Sludge produces an increase in heavy metals where it offsets coal (scenarios 1 and 5), and a reduction in heavy metals where it offsets MSW. The effects where sludge offsets wood chips are negligible. Our analysis of WID (12) compli ance for these generalized scenarios indicates that WID emission criteria appear to be achievable for all scenarios, with the exception of mercury, which is difficult to remove using conventional flue gas treatment techniques. Overall, the use of sludge in the scenarios examined is energy positive although less electricity is produced with sludge than with fuel alone, except for MSW.

Risk Analysis. A collated summary of the risk rankings provided by the structured interviews is presented in Figure 4. Stakeholder views on the combustion of wastes in the UK are mature and highly developed. Interview responses on the business risks to water utilities for the five scenarios showed considerable agreement, even between what one might consider as opposing parties. These parties recognize the influence that various stakeholder perspectives might have on the significance of risks to a water utility considering the scenarios examined in this work, with respect to the potential for campaigning opposition, for example (Figure 4). One should not infer, however, that such agreement extends to the perceived public health risks posed by co-combustion. To avoid over-reaching, scores were banded with lower risk scenarios and key attributes contributing to high risk shaded for illustrative purposes. The collective analysis across all risk categories is of most value. There are important risks for water utilities adopting a first party role as the operator of co-combustion facilities, in terms of the uncertainty around returns on investment and technological risk, and as potential recipients of campaigning opposition to the siting and operation of the plant. Security of the waste supply chain where used as a co-fuel with biosolids and the risks of regulatory delay are also issues for first parties. Generally, it would be better to contract out (risk transfer) co-combustion to third parties (scenarios 1 and 5, shaded) unless the water utility has a partner waste company, but even here, security of the waste supply chain may be a concern.

Figure 4 Risk characterization for scenarios 1-5.

Economic Analysis. Estimates of the costs to the water utility of co-combustion scenarios are presented in Table 4. Costs range from ca. 296 Euro/t DS for co-combustion in coal fired power stations to ca. 390 Euro/t DS for co-combustion with MSW or biomass, inclusive of revenues to the water utility from power generation or environmental credits. The gate fees paid by water utilities to third parties reflect dry matter content and pretreatment costs for the biosolids, such as pelleting, which can determine the suitability for co-combustion. The revenue accruing to third parties taking biosolids for co-combustion is also outlined in Table 4. These users would most likely incur additional capital and operating costs associated with co-combusting biosolids. For illustration, the costs for the land application of sludge cake are ca. 56 Euro/t at 25% w/w DS, that is ca. 144 Euro/t DS, the benchmark against which to compare the cost of other options. Application to land is the most financially attractive option where land is available. Co-combus tion by third parties appears more financially attractive than water utility (first party) co-combustion. There is a tradeoff between transporting dried cake to a co-combustion unit where it can be further dried using flue gases, and drying at the wastewater treatment works. It makes sense to use cheap heat for drying at the point of combustion, but this requires the expensive transport of wet (heavy) cake. Third party interest in co-combustion depends on the availability of gate fees and access to renewable and climate change credits and a company commitment to reduce fossil fuel use.

Sustainability Appraisal. The relative performance of the options against the criteria is mapped in Figure 5. On sustainability grounds, scenario 2 appears to perform best, assuming equal weights are given to sustainability criteria, followed by scenario 4. In practice, the relative sustainability of the options will vary according to local circumstances and the weights given to the criteria used to guide preferences. The rationale guiding this analysis includes the following considerations:

Figure 5 Sustainability appraisal of biosolids co-combustion scenarios 1-5.

(i) Scenarios 1 and 2 appear to cost the least, allowing for the "credits" attributable to renewable obligation caretificates and climate change levy exemption. These credits are indicative of environmental economic benefits.

(ii) Long distance transport of biosolids is perceived to be less socially acceptable, and local community and smaller scale options may be more acceptable (scenarios 3 and 4), depending on how they are designed and operated.

(iii) In environmental terms, scenarios 2 and 3 appear to have a relatively low environmental footprint. However, relative emissions depend on the fuel being substituted, the degree of abatement being greatest where fossil fuels are replaced. Net energy gain (Figure 3) is sensitive to transport distance, and it is greatest for options that offer economies of scale.

(iv) Scenarios 1, 2, and 5 provide greater relative flexibility. These are existing facilities that take biosolids if there are incentives to do so, whether through environmental credits or gate fees. Scenario 3 could offer considerable flexibility in terms of locally relevant solutions, but it is dependent on the development of appropriate technologies and management regimes that are reliable and efficient and that can gain public assurance.

Discussion

The co-combustion of biosolids with other fuels, as a component of a sustainable biosolids strategy, is receiving substantial attention (1). However, this has not translated into many projects on the ground in the UK in contrast with other countries, Germany, for example, where co-combustion projects have been in operation for over 10 years. The Heilbronn power plant (750 MWe) uses raw and digested dewatered sludge at 25% DS (at a rate of approximately 60 000 tonnes of sludge cake per year) in addition to pulverized coal. No additional drying process is employed by the WSP since drying occurs within the coal mills. The station is equipped with a "SCR-DeNox" plant using electric precipita tion technology and a desulfurization plant based on the limestone/gypsum method for the treatment of flue gases (27).

Even though there are limited co-combustion projects in the UK at present, the opportunities for biosolids co-combustion in the UK are regarded as potentially substantial. They vary in the level of investment required and the time frame over which they can be implemented. This study has illustrated the tradeoffs involved in making these decisions for biosolids, discussed by others (9, 10) for wastewater treatment, and suggests that the scenarios that can be most rapidly implemented (1 and 5) are those suffering from poor water utility control and public perception, while those with potentially greater security and sustainability (2, 3, and 4) require a greater level of investment and incur greater financial risk by the water utilities.

Barriers to the development of co-combustion include an absence of policy and legal clarity, the legal classification of biosolids as a waste when used as a fuel, and the uncertainties surrounding continued eligibility of energy produced from biosolids for renewable and climate change credits. Many stakeholders perceive these barriers to be problematic at present, and the market is insufficiently mature for biosolids co-combustion to proceed with confidence.

The generic risk issues that arise include the poor ready availability of alternative outlets for biosolids, potential exposure to long-term contract liability (third party capture), the likelihood of organized campaigning opposition to a new plant, an absence of market maturity for advanced thermal technologies, and the possibilities of reputation damage through contentious developments. Policy interventions are not required here because conventional means exist for managing contract liabilities and potential reputation issues associated with new business ventures such as biosolids co-combustion. That said, market maturity with respect to perceived new technology, the possibility of organized campaigning, and water utility sector risks from being in a low state of preparedness in the event of the land route being closed are issues beyond the immediate control of the utilities alone. These require multi-stakeholder action.

The key policy issue relating to this study is the need for coherence in the signals sent to industry of the use of wastes as fuel. Wastes are increasingly viewed as resources and as replacements for fossil fuel within a low-carbon economy. Progress, however, can be constrained by historical policy and legislative structures that have yet to gain pace. The co-combustion of biosolids with wastes is an example. The legal scrutiny of recovered materials and their candidacy as co-fuels has focused on whether they remain "waste" (i.e., their position in the chain of utility, refs 6, 7) and if they do remain waste, the applicability of emissions regulations to facilities that co-combust. A recent judgment in the UK regarding a power station co-firing dried sewage sludge has reinforced the view that use of a waste as a fuel constitutes discarding the waste, even if it had an economic value (6). Materials that are discarded are subject to the full weight of waste management legislation that may extend to the residues (ash) from facilities co-combusting wastes.

Within this policy and legal climate, water utilities are having to develop long-term "mixed outlet" reuse strategies for biosolids so as not to be held captive to a single route that might come under pressure at short notice (28). This study illustrates the complexities and tradeoffs involved and has shown, for five co-combustion scenarios, that net sludge energy is positive, although less electricity is produced with sludge than with fuel alone (except for MSW). There appears to be considerable agreement among stakeholder communi ties with respect to the business risks faced by water utilities seeking to co-combust biosolids. Supply chain security (wastes or biosolids), organizational capacity (technology availability), and market maturity (risk aversion) are the key business risks and, in the current climate, are best managed by water utilities through risk transfer to third parties, either to power companies or, preferably, to industrial users (e.g., cement kilns) already using wastes (e.g., solvents, tires) as a supplementary fuel.

The economics of co-combustion encompass a complex set of volatile revenue and cost streams including gate fees, power production revenue, renewable and carbon credits, and cost savings whether more costly fuels are displaced. Transport costs and the wet/dry weight of biosolids play an essential role. Again, environmental permitting issues are critical because of the regulatory requirements on sludge driers, which third (nonutility) parties are reluctant to install and operate.

Current hopes for addressing these conflicting issues rely on a review of European waste policy and the definition of waste under a revised European thematic strategy. The UK will become a net importer of gas by 2006 and of oil by 2010, and greater emphasis can be expected on the diversity and security of energy generation and supply. This, along with an emissions reduction target for carbon dioxide (CO2) of 60% of current levels by 2050, means that, although there are no support mechanisms specifically for biosolids co-combustion currently, all fuel sources, including waste biomass, that are domestically derived, should be regarded more favorably in the future (3).

Are biosolids a fuel resource or a waste? Our work has shown that the use of biosolids in all scenarios considered results in a positive energy balance, and it can be considered as a fuel. When thermally dried and granulated, it is a premium product that, providing suitable health and safety controls are in place, is easy to transport and feed directly into combustion units without loss of performance. Water utility costs are sensitive to whether biosolids are classified as a waste or a resource, and their eligibility for credits. Three things need to take place for biosolids to be more widely regarded as a fuel:

(a) Consistency in the classification of biosolids between the waste and energy policy regimes is called for.

(b) WSPs must view co-combustion as a power production activity and produce and promote biosolids that are suitable for co-combustion applications.

(c) There would appear to be a strong case for the commencement of biosolids co-combustion demonstration projects, with active and early stakeholder involvement, to reduce project risk.

The integrated appraisal methodology adopted here has wider utility for the strategic analysis of technology options for integrated waste management and would benefit from a more quantitative platform. Issues of supply chain security, waste logistics, the "bankability" of technologies, and market maturity are receiving growing attention in the UK as it responds to the demands of European and domestic waste legislation. We are developing this research by undertaking quantitative technology appraisals that critically evaluate the compliance of emerging waste technologies with newly introduced policy and regulatory targets for landfill diversion and recovery.

Acknowledgment

This work was funded by UKWIR Ltd., under contract SL/13 (2003). The authors thank Gordon Wheale (UKWIR Ltd., project manager), John Spence (Southern Water and UKWIR client director), and the UKWIR Project Board for their inputs. The opinions expressed are the authors’ alone.

 

Leave a Reply

You must be logged in to post a comment.

^ back to top

 

Contact Lara Abrams

To contact Lara, please email her at lara@laraabrams.com or call 415 613 1704.

 

Subscribe

Your email:  
Subscribe Unsubscribe  

 

Archives

 

Blogroll

 

Sustainable Green Web Hosting