Pyrolysis, Gasification, and Incineration--How They Compare

Introduction Residual solids from wastewater treatment often require further processing prior to end use, beneficially or otherwise. Historically called sewage sludge, the industry refers to these residuals as biosolids if the final product meets specific federal and state requirements (WEF 2021). Historical applications for these materials face mounting pressures from cost and decreasing site options (Collins, 2019; Slaughter, 2013; USEPA, 1994; EREF, 2021). Another pressure emerging in the industry are per- and polyfluoroalkyl substances (PFAS) (Boxall et al., 2012; Kinney et al., 2006; Navarro et al., 2018; Sepulvado et al., 2011; Walters et al., 2010; Winchell, Wells, et al., 2022). Incineration of sludge and biosolids is a widely applied and accepted technology for addressing this suite of challenges because it provides a high level of control for municipalities. However, obtaining and maintaining permits to construct and operate facilities comes at a significant cost and resource commitments in addition to relatively complex operation and maintenance requirements (WEF, 2009). These complications create an opportunity for alternative technologies to enter the industry. Pyrolysis and gasification systems offer incineration alternatives while achieving similar objectives for mass and handling cost reductions. This paper will provide the Water Environment Federation (WEF) audience with an objective comparison of incineration, pyrolysis, and gasification technologies. Basic principles, process outputs, permitting, and equipment capital costs will get covered. The information will largely reflect the recent publication by the authors in WEF's peer-reviewed Water Environment Research journal (Winchell, Ross, et al., 2022). Basic Principles The primary differentiator, outside of temperature and products, between pyrolysis, gasification, and incineration processes is the amount of oxygen introduced into the high-temperature reactor (Figure 1). Pyrolysis thermochemically converts organic feedstock to carbon-rich char and a hydrocarbon-rich off-gas in the absence of oxygen. Gasification processes further refine the char and off-gas from a pyrolysis step using a gasifying medium (such as air, oxygen, or steam). In the presence of adequate oxygen, the primary constituents of sludge and biosolids carbon, hydrogen, oxygen, nitrogen, and sulfur are combusted to thermodynamically stable end products, such is the goal of incineration. The general arrangement of the process train for pyrolysis, gasification, and incineration is similar. Figure 2 depicts the sub-processes for thermally treating sludge or biosolids and indicates the processing train is categorically similar among the three technologies, with a few exceptions. The common input to all three technologies is dewatered sludge or biosolids. Typical outputs include gas-phase emissions from the stack and solid residuals (i.e., char or ash). Table 1 summarizes the fundamental differences among these three processes. Process Outputs Regardless of the thermal technology or site-specific considerations, each process results in solid and gas-phase emissions. The solid ash or char is collected from the thermal reactor or air pollution control equipment, while gas-phase emissions are ultimately emitted to the atmosphere through the process stack. During pyrolysis and gasification, off-gas production increases as temperatures increase, while char yields decrease due to the loss of volatile components (Chen et al., 2014; Yuan et al., 2015). Nearly all carbon is combusted, and volatile components are lost during incineration, resulting in ash comprised of inorganic minerals dominated by silicon, aluminum, iron, and phosphorus oxides along with alkaline earth minerals (Li et al., 2018; Tempest, 2013). Summaries of the ash and char characteristics and air emissions, including examples of allowed levels from operating facilities, from Winchell, Ross, et al (2022) will be presented. Permitting Permitting a gasification or pyrolysis facility differs significantly from an incinerator. The Clean Water Act and Clean Air Act have sections specific to 'incineration' of 'sewage sludge.' Further, both Acts include definitions specifying combustion of the sludge or biosolids as a critical characteristic of incineration, 40 C.F.R. § 60 (USEPA, 2011). However, there are no specific references to pyrolysis, gasification, or associated thermal oxidizers, and permitting requirements are subject to a case-by-case determination (Hambrick, 2021). To date, pyrolysis and gasification facilities have avoided the 'incineration' designation (Aries Clean Technologies, 2021; USEPA, 2013, 2016) and the associated regulatory requirements. Pyrolysis and gasification systems still must undergo an air permitting process similar to that required for an incinerator as outlined in WEF (2009). In general, the current regulatory framework provides pyrolysis and gasification systems a distinct advantage over sewage sludge incinerators if they can continue avoiding the 'incineration' designation. Equipment Capital Costs Several high-temperature equipment suppliers provided planning level capital costs for a single train of equipment from the point of dewatered sludge or biosolids delivery to the stack, including a handling system for residual solids. Figure 3 displays the cost information in cost curve fashion. The pyrolysis and gasification technologies offer a cost-benefit at lower processing capacities but become comparable to incineration at the 110 dry tonnes/day capacity. Given the cost advantages of pyrolysis and gasification technologies in small to mid-size treatment facilities, this is likely where these technologies will enter the wastewater market. Conclusions Primary advantages of pyrolysis and gasification systems versus a contemporary incinerator identified include operating characteristics, equipment sizing, air emissions, emissions regulations, potential char applications, and equipment costs. These advantages need to be weighed against the added complexity of equipment required for these processes. These advantages and disadvantages will be discussed and detailed in the full paper. Altogether, pyrolysis and gasification show promise as alternatives to incineration for some facilities, particularly small to mid-sized facilities, and the current and contracted-to-build installations will, over time, provide valuable information on real-world operating and maintenance requirements.