Ash Accumulation in Diesel Particulate Filters

Alexander Sappok

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Abstract: The accumulation of ash in diesel particulate filters is an important factor limiting the filter’s service life, increasing its pressure drop and having an adverse effect on fuel economy. The main sources of ash are engine lube oil additives, as well as fuels and engine wear and corrosion. A number of studies have been conducted and test methods have been developed to investigate the properties and morphology of ash, as well as its impact on DPF flow restriction. The composition and properties of ash can also be affected by the lubricant chemistry, the exhaust gas conditions and filter regeneration strategy.

Introduction

The accumulation of ash in diesel particulate filters is one of the most important factors limiting the filter’s service life and has been described as one of the most important problems facing diesel engine manufacturers [2665][469]. Despite considerable emphasis and work in understanding and optimizing DPF performance for soot accumulation alone, the reality is quite different. Unlike these idealized cases, the DPF always contains some amount of ash in real-world operation. In fact, more often than not, the amount of ash in the filter can significantly exceed the amount of soot the DPF was initially designed to trap. Figure 1 best illustrates the magnitude of the problem, as it presents the ash fraction of the total mass of material accumulated in the DPF (ash and soot), assuming a 6 g/L maximum soot load limit [2667].

[SVG image]

Figure 1. Ash Accumulation As a Function of Filter Cleaning Interval [2667]

Estimated for a typical heavy-duty vehicle. Ash fraction = ash/(ash + soot) at a soot load of 6 g/L.

Based on Figure 1, after only 33,000 miles (53,000 km) of on-road use, approximately 50% of the material accumulated in the DPF is ash. In other words, the amount of ash equals the amount of soot at the maximum allowable soot load limit of 6 g/L. Further, after 150,000 miles (241,000 km) of operation—equivalent to the minimum EPA ash cleaning interval—ash comprises over 80% of the material trapped in the DPF, with the minority being soot.

Conceptual Description. Ash accumulates in the DPF over extended use, as the incombustible material left behind following filter regeneration and soot oxidation. The ash consists of various metallic compounds originating from lubricant additives, trace elements in the fuel, and engine wear and corrosion products. Ash accumulation in the DPF alters the filter geometry as shown in Figure 2, which illustrates the differences between a filter containing no ash and a filter containing significant amounts of ash.

[schematic]

Figure 2. Effect of Ash Accumulation on Particle Filter Geometry and Soot Distribution

(Image: A. Sappok, MIT)

As shown in Figure 2, the ash can occupy a large portion of the filter volume, as it may accumulate in a thin layer along the channel walls or pack in plugs towards the back of the filter channels. One effect of the ash is to decrease the effective filter volume or filtration area and reduce the filter’s soot storage capacity. The ash deposition also alters the distribution of the accumulated soot, generally shifting it toward the front of the filter. These combined effects serve to restrict the channel diameter and reduce the effective filter length. As a result, the ash contributes to increased exhaust flow restriction.

In addition, the reduction in channel diameter and filter length, due to the ash accumulation, result in an increase in the DPF channel and wall velocities, which may further alter the properties of the accumulated soot and affect the filter’s pressure drop sensitivity. Given the reliance on filter pressure drop measurements in filter soot load estimation, a thorough understanding of these ash effects is required to compensate for ash-induced variations in the filter’s pressure drop response over time.

Figure 2 also shows the ash layer forming a barrier, physically separating the soot from the channel walls. This is important for two reasons. First, after extended aging and with some level of ash accumulation, it is the ash that is doing the majority, if not all, of the soot filtering. In this sense, the filter substrate is acting as a support for the “new” filter medium, which is essentially composed of the ash. Given the small pore size of the ash layer, an increase in filtration efficiency is generally observed in particulate filters with even a low level (< 2 g/L) of ash loading [2668]. Second, the ash layer also physically separates the accumulated soot from the catalyst which may be deposited on the surface of a catalyzed DPF. This not only prevents any contact between the soot and catalyst particles, but further increases the required diffusion length for NO2 assisted soot oxidation.

Impact on Performance. Due to the long time scales over which the ash builds up in the DPF, several thousand hours and tens-to-hundreds of thousands of miles, significant progress in understanding ash impacts on filter performance was limited prior to the widespread introduction of DPFs in 2007. Much of the early research into ash effects prior to 2007 utilized various approaches to accelerate filter aging and ash build up in an effort to identify the various ash sources and means by which ash may affect diesel aftertreatment system performance. This initial work resulted in the following generally-accepted observations and conclusions:

A detailed review of the literature in 2007 was conducted by Bodek which provides additional details of the impact of ash on diesel aftertreatment system components including DOC, SCR, and LNT technologies, in addition to DPFs [2679]. Figure 3 presents a summary of the heretofore known impact of ash accumulation in the DPF on exhaust backpressure increase for various lubricants, filter technologies, and drive cycles. More recent results show lubricant-derived ash from CJ-4 specification oils, containing no more than 1.0% sulfated ash, resulting in an approximately doubling of the DPF pressure drop after 4,680 hours or 188,000 miles (303,000 km) of equivalent on-road use [2680].

[schematic]

Figure 3. Impact of Ash on Measured Backpressure Increase As a Function of Simulated Driving Distance [2679]

Data from SAE papers: (1) 2004-01-3013, (2) 2004-01-1955, (3) 2003-01-0408, (4) 910131.

Fuel Economy Effects. Ash build up in the DPF directly affects fuel consumption through two pathways: (1) increased exhaust flow restriction and backpressure, and (2) decreased filter regeneration intervals (increased regeneration frequency) through a reduction in filter soot storage capacity. Furthermore, the ash may also reduce the regeneration efficiency in catalyzed systems, requiring an increased reliance on active regeneration or higher temperature operation for successful passive soot oxidation.

While several studies have quantified the increase in vehicle fuel consumption attributed to the DPF, most consider only the effects of soot accumulation on exhaust backpressure and regeneration intervals. Depending on the regeneration frequency and soot level, the DPF-related increase in fuel consumption has been reported to range from 4.5% to 7.0% [2681]. In reality, however, the DPF-related increase in fuel consumption may be greater, as all of these studies fail to consider the additional increase in exhaust flow restriction and regeneration frequency due to the build-up of ash over the life of the filter.

The contribution of the ash-related backpressure increase to an increase in the overall fuel consumption is estimated from 2% to 3%, which includes the compounding impact of the ash to increase the filter’s pressure drop sensitivity to soot accumulation [2682]. Specific to the increase in regeneration frequency, other studies have shown an increase in regeneration frequency by up to a factor of two following approximately 240,000 miles of ash accumulation, if ash effects are not properly accounted for in pressure-based regeneration control schemes. However, even assuming perfect knowledge of the amount and distribution of the ash in the DPF, an increase in regeneration frequency by a factor of 1.6 over 240,000 miles is unavoidable, in the best case, due to the significant filter volume occupied by the ash and reduction in the DPF’s soot storage capacity [2667].

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